Typing Chapter 12: The Respiratory System PDF

**Chapter 12: The Respiratory System** **12.1 Word Roots and Combining Forms** Alveol/o: alveolus, air sac Bonch/o: bronchial tube Bronchi/o: bronchus Bronchiol/o: bronchiole Capn/o: carbon dioxide Cyan/o: blue Laryng/o: larynx Lob/o: lobe Nas/o: nose Pharyng/o: pharynx Phren/o: diaphragm Pneum/o: air Pneumon/o: air Pulmon/o: lung Rhin/o: nose Sinus/o: sinus Spri/o: breathing Thorac/o: chest Trache/o: trachea **12.2 Overview** In this chapter, you will study **respiration** first as breathing **([ventilation]),** which is the movement of air into **([inspiration)]** and out of the lungs **([expiration]).** Then you will explore respiration as the exchange of gases in two areas---between the air in the lungs and the blood in capillaries and between the blood in the capillaries and the tissues out in the body. Once you understand how the exchange of gases takes place, you will be prepared to investigate how gases are transported in the blood. **12.2 Anatomy of the Respiratory System** The entire respiratory system's anatomy is housed in the head, neck, and thorax. In general, the anatomy in the head and neck is the **upper respiratory tract,** while the anatomy from the trachea through the lungs is the **lower respiratory tract.** You have already studied some of this anatomy, such as the pleurae (serous membrane), To refresh your memory, a serous membrane is a double-walled, fluid filled membrane. In case of the pleurae, the visceral pleura is in contact with the lung's surface, while the parietal pleura is not. The parietal pleura lines the thoracic cavity and covers the diaphragm's superior surface. Fluid exists between the visceral and parietal pleurae. This anatomy will be important when you study the mechanics of breathing later in the chapter. Before you get started on the rest of the anatomy, consider the way air enters moves through the body. Take a deep breath now with your mouth closed, and trace the air that breath as it travels on its route. The air enters the **nasal cavity** through the **nose.** From there is goes the **[pharynx]** (FAIR-inks), the **larynx** (LAIR-inks), to the **trachea** (TRAY-kee-ah), to the **bronchi** (BRONG-kye) (where it enters the lungs), to the **[bronchial tree],** to the **bronchioles,** and finally to the tiny air sacs called **[alveoli]** (al-VEE-oh-lye). At the alveoli, the second part of respiration---the exchange of gases---takes place. Now you are ready to zoom in on all of the respiratory system's specific anatomy (that we have just mentioned) in the order that the air traveled through it in your deep breath. You will need to become familiar with the gross and microscopic anatomy along the way because this is important in understanding precisely how the anatomy functions. **Disease Point:** **Pleurisy** is characterized by inflammation of the pleurae. This condition has a variety of causes, including respiratory infections such as tuberculosis or pneumonia, cancer, trauma or injury to the chest, inflammatory conditions such as lupus or rheumatoid arthritis, pulmonary embolus, or conditions related to asbestos exposure. Pleurisy causes pain in the chest, especially when taking a deep breath or coughing. The pain is caused by the friction of the two inflamed membranes rubbing against each other. Breathing can become difficult, and this could lead to additional symptoms such as cyanosis, tachypnea, and shortness of breath. To diagnose pleurisy, physicians listen to breath sounds and may order a series of tests including a blood test, CT scan, chest X-ray, and ultrasound. **Nose** Air enters the nasal cavity through the nose's two **nares** (NAH-reez) (nostrils). The nasal bones superiorly and the plates of hyaline cartilage at the end of the nose are responsible for the nose's shape. You can feel where the nasal bone ends and cartilage begins at the bridge of the nose. **Nasal Cavity** A septum divides the nasal cavity into right and left sides. The ethmoid bone (superiorly), the vomer (inferiorly), and a septal cartilage anteriorly form the septum. The anterior part of the nasal cavity (the **vestibule**) is lined by stratified squamous epithelial tissue with stiff **guard hairs** to block debris from entering the respiratory tract. The nasal cavity widens posterior to the vestibule to make room for three bony, lateral ridges called the **nasal conchae** (KON-kee). The ethmoid bone forms the superior and middle nasal conchae, while the inferior nasal concha is a separate bone. This portion of the nasal cavity is lined by mucous membranes that trap debris and warm and moisturize the incoming air. The nasal conchae provide extra surface area for the mucous membranes to function. The mucous membranes are composed of ciliated pseudostratified columnar epithelial tissue. The cilia move mucus and any trapped debris posteriorly so that it can be swallowed. Olfactory neurons located in the roof of the posterior nasal cavity detect odors and provide the sense of smell. *Sinuses* You studied the sinuses of the frontal, ethmoid, sphenoid, and maxilla bones in the skeletal system chapter. Theses cavities within the bones are also lined with respiratory epithelial tissue to warm and moisturize the air. The mucus produced in the sinuses is drained to the nasal cavity through small openings. At this point your deep breath, the inspired air leaving the nasal cavity has been partially warmed and moistened, and some of its debris has been trapped. The structure the air encounters next---the pharynx---is explained now. **Disease Point:** Inflammation of the epithelium in the sinuses **(Sinusitis)** causes increased mucus production, and the accompanying swelling may block its drainage to the nasal cavity. The pressure within the sinuses created by the buildup of mucus causes a *sinus headache*. Decongestants (vasoconstrictors) help reduce the swelling, thereby improving mucus drainage, which reduces the increased pressure. **Pharynx** The **[pharynx],** commonly called the *throat,* is divided into three regions based on location and anatomy---the **nasopharynx,** the **oropharynx,** and the **laryngopharynx** (lah-RING-oh-FAIR-inks). You will explore these in the paragraphs that follow. *Nasopharynx* The nasopharynx is located posterior to the nasal cavity and the soft palate. This passageway is also lined by ciliated pseudostratified columnar epithelial tissue whose cilia move mucus and trapped debris to the next region of the pharynx so that they can be swallowed. The pharyngeal tonsils and the opening to the auditory tube (eustachian tube) are located in this region. *Oropharynx* This region of the pharynx is inferior to the nasopharynx. The oropharynx is common to the respiratory and digestive systems as a passageway for air, food, and drink. For the oropharynx to withstand the possible abrasions caused by the passage of solid food, it must be lined with a more durable tissue---stratified squamous epithelial tissue. In addition, the palatine tonsils are located in this region to deal with any incoming pathogens. *Laryngopharynx* This region of the pharynx extends from the level of the epiglottis to the beginning of the esophagus. Like the oropharynx, the laryngopharynx is lined by stratified squamous epithelial tissue to handle the passage of air, food, and drink. Solids and liquids continue on from the laryngopharynx to the esophagus, but inspired air moves through an opening **(glottis)** to the larynx, the next structure in the respiratory pathway. **Larynx** The Larynx is a cartilage box (voice box) of nine separate cartilages, eight of which are composed of hyaline cartilage connective tissue. The **epiglottis** (the ninth cartilage of the larynx) is composed of elastic cartilage connective tissue. The epiglottis stands almost vertically over the glottis. Its function is to fold over the glottis during swallowing to prevent solids and liquids from entering the larynx. You will learn more about how this works in the digestive system chapter. The epiglottis remains in its vertical position at all other times to ensure the easy passage of air from the laryngopharynx through the glottis to the larynx. A closer look at the larynx. Here you can see the **laryngeal prominence** ("Adam's apple") of the **thyroid cartilage.** It enlarges to be more visible in men than women due to the presence of testosterone. You can also see (in this figure) the two **arytenoid cartilages** (ah-RIT-end-oyd) and the two **corniculate cartilages** (kor-NIK-you-late) that operate the vocal cords. *Vocal Cords* The walls of the larynx are muscular to operate the **vocal cords.** There are two sets of floods in the inner wall of the larynx---the **vestibular folds** and the vocal cords. The vestibular folds have no function in speech. They are important in closing the larynx during swallowing. The vocal cords are abducted (spread apart) and *adducted* (brought closer together) by muscles pulling on the arytenoid and corniculate cartilages. The opening formed by *adducted* vocal cords is the glottis. Air passing through *adducted* vocal cords causes them to vibrate to make sounds of varying pitch depending on the tautness of the cords. So speech is a very active process, which involves muscles pulling on cartilages of the larynx to operate the vocal cords. The larynx at the vocal cords is lined with stratified squamous epithelial tissue to withstand the vibrations. **Trachea** From the larynx, inspired air travels to the trachea, a rigid tube with 18 to 20 C-shaped cartilages composed of hyaline cartilage connective tissue. These cartilages hold the trachea open for the easy flow of air. The C-shaped cartilages are open posteriorly with smooth muscle bridging the gap. This feature allows the esophagus (directly posterior to the trachea) room to expand into the tracheal space when swallowed food passes on its way to the stomach. If the cartilages were circular instead of C-shaped, a swallowed piece of meat could get hung up on each cartilage as it passed down the esophagus. Like the nasal cavity and the nasopharynx, the trachea is lined with ciliated pseudostratified columnar epithelial tissue with goblet cells that secrete mucus. The air you breathe is full of particles, such as dust, pollen, and smoke particles. You may have seen the dust in the air as the sun shines through a window. Even during sleep, the cilia of the trachea move mucus and any trapped debris up (like an escalator) toward the pharynx to be swallowed. This prevents the accumulation of debris in the lungs. The trachea splits to become the right and left main bronchi, each of which enters its respective lung. You will explore the lungs and bronchial tree together, looking first at their gross anatomy. **Disease Point:** The smoke inhaled with each drag on a cigarette contains a lot of particles, but the respiratory anatomy is designed to prevent this debris from accumulating in the lungs. However, the increased amount of debris may, over time, cause the lining of a habitual smoker's trachea to go through metaplasia, changing from ciliated epithelial tissue to a more durable, nonciliated tissue. Without the ciliated escalator, the respiratory system resorts to coughing up the debris. As a result, the long-term smoker develops the smoker's hack each morning to move the debris inspired each night. **Lungs and the Bronchial Tree** The right and left main bronchi each enters its respective lung at an area on the medial surface of the lung called the **hilum.** This is the same location used by pulmonary arteries and veins to enter and leave the lung. The left **bronchus** (BRONG-kuss) is slightly more horizontal than the right bronchus due to the location of the heart. The main bronchi and all of their further branches make up the bronchial tree. Upon entering the lung, each main bronchus branches to become the **lobar bronchi,** each going to a separate lobe of the lung. The left lung has fewer lobes (two) than the right, again because of the position of the heart. The right lung has three lobes and, therefore, three lobar bronchi. Lobar bronchi further divide to smaller and smaller bronchi that branch to form the bronchial tree. All of the bronchi are supported by cartilage plates, which hold them open for the easy passage of air. The smallest bronchi further branch to form bronchioles. These small tubes do not have cartilage in their walls. Instead, their walls have smooth muscle that allows them to dilate or constrict to adjust airflow. You will learn more about this later in the chapter. Each bronchiole supplies air to a **lobule** (subsection of a lobe) of the lung composed of tiny air sacs called alveoli. **Alveoli** The alveoli are clustered like grapes at the end of the bronchiole. A network of capillaries covers the alveoli. This is vital for gas exchange, as you will read shortly. The histology of the alveoli with respect to the bronchioles and blood supply to the capillaries. There are approximately 150 million alveoli in each human lung. Each alveolus is a tiny air sac with two types of cells in its walls---simple squamous cells and **great (type II) alveolar cells.** Most of the alveolar wall is composed of one layer of thin squamous cells that allow for rapid gas exchange across their surface. The great alveolar cells are important because they secrete a fluid called **[surfactant].** Next, you will find out why this fluid is so important. *Surfactant* To understand the importance of surfactant, you must first understand a property of water: high surface tension. This basically means that water will always try to have the smallest surface are to volume ratio possible. In other words, water forms beads or drops because a sphere has a smaller surface area to volume ratio than a flat sheet. This is why water forms beads or drops on smooth surfaces such as glassware in your dishwasher. Surfactant reduces the surface tension of water much like the rinse agent you may add to your dishwasher to avoid water spots. The rinse agent reduces the surface tension of water (sheeting action), so water sheets off your glassware instead of forming beads that leave water spots as the glasses dry. Surfactant also causes water to form a thin sheet instead of a bead. Why is this important? By the time air has entered the alveoli, it has been thoroughly moisturized by all the mucous membranes it has passed along the respiratory route. If a bead of water were to form inside the tiny alveoli, the plump bead might touch the wall on opposite side of the air sac and cause the thin, delicate walls of the alveoli to stick together, and this would cause the alveoli to collapse. A thin sheet of water in the alveoli (instead of a plump bead) reduces the chance of the alveoli walls collapsing on each other. Collapsed alveoli do not easily fill with air. **Disease Point:** A fetal respiratory system does not mature until late in pregnancy. The alveoli in infants born before the lungs are mature often collapse because of the lack of sufficient surfactant. This condition, called **respiratory distress syndrome (hyaline membrane disease),** is a common cause of neonatal death. Oxygen under positive pressure can be administered along with surfactant to keep the lungs (alveoli) inflated between breaths. *Respiratory Membrane* The relationship of the alveoli of the alveoli to the bronchioles and the cells that make up the alveoli. You can see the structure formed by the capillary network adjacent to the alveoli---the respiratory membrane. This is a very important structure because it is the location of gas exchange in the lung. Take a closer look at this figure. The respiratory membrane is composed of the thin layer of water with surfactant in the alveoli, the single squamous cell alveolar wall, and the single cell capillary wall. If all of the respiratory membrane in one lung were laid out in a single layer, it would cover approximately 70 square meteres [(*m*^2^)]{.math.inline}, equivalent to the floor of a room 25 feet by 30 feet. **12.4 Physiology of the Respiratory System** **Mechanics of Taking a Breath** Take a deep breath. How did you do that? Air moves (but is not bushed) along the respiratory passageways on its way to the lungs because of pressure differences within the chest. This much like the syringe example you became familiar with while studying blood flow through the heart. The syringe example explained the relationship between volume, pressure, and flow. If the volume of space in the syringe is increased, the pressure inside the syringe is decreased, so air flows into the syringe to equalize the pressures inside the outside the syringe. Likewise, if the volume of space in the syringe is decreased, the pressure inside the syringe is increased, so air flows out of the syringe to equalize the pressures. As a result, pushing or pulling on the plunger changes the volume of the syringe. How does the body change the volume of the chest? Concentrate on the major muscles for breathing in this figure. As you can see during inspiration the external intercostal, pectoralis minor, and sternocleidomastoid muscles contract to expand the rib cage and diaphragm contracts to flatten its dome shape. The combined effect of these contractions is an increase in the size (volume) of the chest cavity. All that needs to be done for normal expiration is to have the same muscles relax. Then the rib cage returns to its normal position and the diaphragm becomes dome-shaped again due to the recoil of abdominal organs. The volume of the chest is decreased, and air flows from the body. Expiration during normal breathing is a passive process (no energy required) involving the relaxation of muscles. But you can also see that forced expiration involves the contraction of muscles too. The internal intercostals and abdominal wall muscles do contract in *forced expiration;* however, these muscles are not used for expiration during normal breathing. Forced expiration is intentionally forcing air out of the lungs, which happens when blowing out a candle or inflating a balloon. In this case, energy for muscle contraction is required. So far in this explanation of the mechanics of breathing, you have seen how the muscles of the chest can increase the volume of the chest, but what about the volume of each lung? How is the volume of the lungs increased? This involves the pleural membranes and the pleural fluid. The parietal pleura is attached to the thoracic wall and diaphragm, while the visceral pleura is attached to the lung. The pleural fluid between the parietal and visceral pleurae cause the two pleurae to stick together and move as one. As the respiratory muscles expand the thoracic wall and flatten the diaphragm, the parietal pleura moves with the wall and diaphragm, the visceral pleura and the lung move with it---expanding the lung along with the thoracic cavity. As the lung expands, the pressure within the lung (intrapulmonary pressure) decreases, so air moves in until the pressure inside the lung is equal to the pressure outside the body. The intrapulmonary and atmospheric pressure are then equal. When inspiration ends, the thoracic wall returns to its original position and its volume is diminished. The pressure is now greater in the lung than outside the body, so air flows from the body until the intrapulmonary and atmospheric pressures are again equal. The muscle action and the pressure changes during inspiration and expiration. **Disease Point:** A **[pneumothorax]** (collapsed lung) occurs if air is introduced in the pleural cavity between the pleural membranes. Just as fluid holds the parietal and visceral pleurae together, air between the pleurae allows them to separate. Normally, there is tension on the lung, keeping it partially inflated at all times. However, in a pneumothorax, the pleurae separate, so the lung may recoil and separate from the thoracic wall. The air in a pneumothorax may be introduced by a penetrating trauma such as a knife wound or broken rib, medical procedures such as inserting and needle to withdraw pleural fluid, or even a disease such as emphysema (covered later in this chapter). Diagnosis of a pneumothorax involves listening to breath sounds to determine if they are absent or decreased, doing a chest X-ray and testing arterial blood gases to determine if there is an adequate ratio of O2 and CO2 in the blood. In mild cases, the pneumothorax may correct itself without medical intervention. In more severe cases, a chest tube may need to be introduced into the pleural space to remove the air to inflated the lung, and surgery may be required to repair the opening into the pleural space. In a **hemothorax**, blood is introduced into the pleural cavity. This can be caused by blood clotting defects, thoracic surgery, severe respiratory infections, or lung cancer. If the hemothorax is not treated, it can lead to pneumothorax, among other severe complications. Diagnosis of a hemothorax includes listening to breath sounds to determine if they are absent or decreased, doing a chest X-ray and a CT scan of the lungs, and analyzing pleural fluid to determine the presence of blood. Treatment involves controlling the cause of the bleeding and inserting and chest tube to remove the blood from the pleural cavity. In severe cases, surgery may be necessary. **Mechanics of Taking a Breath** How well the respiratory system functions to move air into and out of the lungs can be measured in the pulmonary function **([spirometry])** tests. A **[spirometer]** is a device used to measure the volume of air moved/ Breathing into the spirometer to determine his various **lung volumes** and **lung capacities** (capacities are determined by adding two volumes). Exercise may temporarily increase the tidal volume for an individual, but this does not mean that all of the other values will increase. The maximum amount of air the respiratory system can move (vital capacity) does not change on a temporary (minute-by0minute) basis. So if there is an increase in tidal volume during a workout, there must be a decrease in the inspiratory and expiratory reserve volumes. Lung volumes and capacitates vary from on individual to another due to gender, size, age, and physical condition. In general, a woman's vital capacity is less than a man's; tall, thin person has a greater vital capacity than someone short and obese; and a trained athlete has a greater vital capacity than someone who has a sedentary lifestyle. **[Compliance]** is another measurement of pulmonary function. It measures how well the lung can expand and return to shape (elasticity). It is harder to expand the lungs and the thorax if there is decreased compliance. This may be due to the buildup of scar tissue in the lung (pulmonary fibrosis), collapse of the alveoli (respiratory distress syndrome), skeletal disorders (scoliosis or kyphosis), or **[chronic obstructive pulmonary disorders (COPDS)]** such as asthma, chronic bronchitis, emphysema, and lung cancer (discussed later in the chapter). **Lung Volumes and capacities:** **[Tidal Volume (TV):]** The tidal volume is the amount of air moved in a normal breath (inspired or expired) at rest - Typical Value: 500 mL **[Inspiratory reserve volume (IRV]):** The inspiratory reserve volume is the amount of air that can be forcefully inspired beyond the amount inspired in a normal breath at rest - Typical Value: 3,000 mL **Expiratory reserve volume (ERV):** The expiratory reserve volume is the amount of air that can be forcefully expired beyond the amount expired in a normal breath at rest - Typical Value: 1,100 mL **Residual volume (RV):** The residual volume is the amount of air in the lungs that cannot be moved. - Typical Value: 1,200 mL **[Functional residual capacity (FRC)]:** The functional residual capacity is the amount of air remaining in the lungs after the expiration of a normal breath at rest. FRC=ERV + RV - Typical Value: 2,300 mL **Inspiratory capacity (IC):** The inspiratory capacity is the maximum amount of air that can be inspired after expiration of a normal breath at rest. IC = TV + IRV - Typical Value: 3,500 mL **[Vital capacity (VC)]:** Vital capacity is the maximum amount of air that can be moved. VC = IC + FRC - Typical Value: 4,600 mL **Total lung capacity (TLC):** The total lung capacity is the maximum amount of air the lung can hold. TLC = VC + RV - Typical Value: 5,800 mL **Composition of Air** Gases diffuse across membranes from high concentration to low concentration until the concentrations are equal. So it is important to be able to talk about quantities of gases. The air you breathe is a mixture of gases---78.6% nitrogen, 20.9% oxygen, 0.04% carbon dioxide, and variable amounts of water vapor depending on humidity levels. Gases fill whatever space is available to them and can be compressed, so volume is not a good measure of the amount of a gas. For example, an open scuba tank (of a given volume) will fill with air, but more air can be pumped under pressure into the same tank before it is sealed (compressed air). Therefore, the amount of a gas is expressed not as volume but in terms of the pressure a gas exerts. In the case of a mixture of gases, such as air, the amount of each gas is expressed as a **[partial pressure]---**the amount of pressure an individual gas contributes to the total pressure of the mixture. So, if the total pressure of the air (atmospheric pressure) is 760 mmHg, then the partial pressure of nitrogen (PN2) is 78.6% of 760, or 597 mmHg; the partial pressure of oxygen (PO2) is 20.9% of 760, or 159 mmHg; the partial pressure of carbon dioxide (PCO2) is 0.04% of 760, or 0.3 mmHg; and the remainder, 3.7 mmHg, is the partial pressure of water vapor. All of the partial pressures of the gases added together equal the total pressure of the air (760 mmHg). You will need to understand partial pressures as a measurement of the amount a gas when you study gas exchange in the lung and out at the tissues in the next section of this chapter. **Gas Exchange** Before studying **[Gas exchange],** it will be helpful for you to keep these two facts in mind: (1) Carbon dioxide is a waste product produced in the tissues through cellular respiration, and (2) blood travels to the lungs to be oxygenated. With that stated, we begin by explaining gas exchange at the **[respiratory membrane]** between an alveolus and a capillary in the lung. In this discussion, we use general symbols─greater than (\>), less than (\[ ]{.math.inline}PO2 tissues so oxygen diffuses to the tissues until PO2 tissues = PO2 capillary. The blood containing deoxyhemoglobin and bicarbonate ions continues to the right side of the heart and on to the alveoli of the lung. You will now learn what happens in alveolar gas exchange and transport. *Alveolar Gas Exchange and Transport* Again, you should focus on the largest red and blue arrows representing oxygen and carbon dioxide. In the alveolus, the PO2 alveolus = PO2 capillary so oxygen diffuses into the capillaries. When it does, deoxyhemoglobin reacts with oxygen to release hydrogen ions and form oxyhemoglobin: HHb + O2 [→ ]{.math.inline}HbO2 + H+ The now free hydrogen ions (H+) in the capillary at the alveolus bind to the bicarbonate ions (HCO3-) to form carbonic acid (H2CO3) in the blood. This results in carbon dioxide and water. Notice that this is the reverse of the reaction happening for carbon dioxide at the tissues. PCO2 capillary \> PCO2 alveolus so carbon dioxide diffuses across the respiratory membrane to the alveolus until PCO2 capillary = PCO2 alveolus H+ + HCO3- [→ ]{.math.inline}H2CO3 [→ ]{.math.inline}CO2 + H2O Basically, most of the oxygen is transported in the blood by hemoglobin as oxyhemoglobin, and most of the carbon dioxide is transported in the blood as bicarbonate ions. Hemoglobin functions to carry oxygen from the lungs to the tissues and hydrogen ions from the tissues to the lungs. **Clinical Point** You have already read about hemoglobin carrying carbon monoxide (CO) in the cardiovascular system chapter on blood. Hemoglobin binds to carbon monoxide 210 times more tightly than to oxygen, and it does not carry oxygen as long as it is bound to carbon monoxide. CO is produced during combustion, so it can be emitted from improperly vented furnaces, cars (exhaust), and even cigarettes as they are smoked. Typically, less than 1.5% of hemoglobin is bound to carbon monoxide in nonsmokers, while 10% of a heavysmoker's hemoglobin may be bound to carbon monoxide. Mechanics need to ventilate their garages while they work because even just a 0.1% concentration of CO in the air can bind to 50% of the worker's hemoglobin, and a 0.2% atmospheric concentration can be lethal. **Regulation of Respiration** Now that you have become familiar with how oxygen and carbon dioxide are transported in the blood, you are ready to examine how the respiratory system is regulated to homeostatically control blood gases and pH. The main control centers for respiration are located in the medulla oblongata. From there, messages to stimulate inspiration travel through inspiratory (I) neurons that go to the spinal cord and then out to the diaphragm and intercostal muscles (by way of the phrenic and intercostal nerves). Expiratory (E) neurons in the medulla oblongata send signals only for forced expiration. As you have previously learned, increasing the frequency of nerve impulses causes longer muscle contractions and, therefore, deeper inspirations. If the length of time (duration is increased for each stimulus, the inspiration is prolonged and the breathing is slower. When nerve impulses from the inspiratory neurons end, muscles relax and expiration takes place. Information concerning the need for regulation comes to the respiratory centers in the medulla oblongata from several sources. These sources are explained in the following list: - Stretch receptors located in the walls of bronchi and bronchioles of the lung send signals to the medulla oblongata as to the degree of the chest's expansion. When maximum expansion has been reached, the medulla oblongata stops sending inspiratory messages. This prevents overinflation of the lungs, and it is most important in infants. This action is called the **Hearing-Breuer reflex.** - Proprioceptors in the muscles and joints send signals to respiratory centers during exercise so that ventilation is increased. The respiratory centers in the medulla oblongata can increase the depth and rate of respiration. - The **pontine respiratory group** in the pons receives input from the hypothalamus, limbic system, and cerebral cortex. It then sends signals to the medulla oblongata to adjust the transitions from inspiration to expiration. In the way, the breaths become shorter and shallower or longer and deeper. This center helps adjust respirations to special circumstances, such as sleep, exercise, or emotional responses such as crying. - The cerebral cortex can exert voluntary control over the respiratory system, but this is limited control. For example, a stubborn child may threaten to hold his breath to get his way. However, if he is allowed to do so, he will eventually pass out and will start breathing again. - **Peripheral chemoreceptors** in the aortic arch and carotid arteries (discussed in the cardiovascular system chapter on the heart and vessels) and **central chemoreceptors** in the medulla oblongata send information to the respiratory centers in the medulla oblongata concerning pH, CO2, and O2. The peripheral chemorecptors monitor the blood, while the central chemoreceptors monitor cerebrospinal fluid (CSF). Why monitor both fluids? Both fluids must be monitored in order to maintain homeostasis. Hydrogen ions in the blood cannot pass the blood-brain barrier, but carbon dioxide does cross the barrier. When it does, it reacts with the water in the CSF to from H+, just as it does at the tissues when mixing with the water in the blood. An increased concentration of hydrogen ions in either fluid means reduced pH. The most important driver of respiration is pH, the next is CO2, and the driver of minor importance is O2. Respiration is adjusted by the medulla oblongata to maintain pH in the homeostasis range for blood of 7.32 to 7.45. **[Acidosis]** occurs if the pH of the blood is less than 7.34. The medulla oblongata then stimulates **hyperventilation** (decreased respiratory rate) to keep CO2 in the blood to lower the pH. **Hypercapnia,** increased carbon dioxide in the blood, causes the pH to fall in both fluids. Oxygen is of minor importance as a driver of respiration because the blood's hemoglobin is usually 97% saturated with oxygen during normal breathing. Oxygen drives respiration only during extreme conditions, such as mountain climbing at high altitudes, and when it does, this is called **hypoxic drive.** You have now explored all the respiratory anatomy and the physiology of this system. It is time to put that information together to see how the functions of this system are carried out for Carol, who is on a break from her job in the business office at the hospital. **Functions of the Respiratory System** Carol appears to be a relatively young, healthy woman who is a smoker. Although her respiratory system functions normally at present, her lifestyle choice to continue to smoke may have harmful, long-term effects on her respiratory system. In the following list, we explain the effects on each of the functions of this system: - Gas exchange. Carol's respiratory system functions to exchange carbon dioxide and oxygen across the respiratory membranes of her lungs and out to the tissues of her body. However, each time she inspires through a lit cigarette, she also exchanges carbon monoxide across her respiratory membrane, which then binds to the hemoglobin in her blood. Hemoglobin bound to CO no longer functions to carry oxygen, so her levels of O2 in the blood will fall (hypoxia). Her kidneys notice the decreased O2 levels and secrete erythropoietin (EPO, discussed in the cardiovascular system chapter on blood) to stimulate erythropoiesis to increase the RBC count and the available hemoglobin to carry O2. - Acid-base balance. Carol's respiratory system regulates her acid-base balance by increasing respirations whenever the pH of her blood begins to fall below homeostasis. Hyperventilation gets rid of more CO2, so the pH of the blood increase. If Carol's blood pH rises above homeostasis, her respiratory rate will slow (hypoventilation), so any CO2 in her system remains in the blood longer, thus lowering her pH to normal levels. - Speech. The muscles of Carol's larynx contract to move the arytenoid and corniculate cartilages that operate her vocal cords and cause them to vibrate to produce sound. Even the sinuses connect to her nasal cavity will aid in her speech by giving resonance to her voice. However, her smoking tends to irritate and dry the lining of her larynx. This may lead to laryngitis and a scratchy voice. - Sense of smell. Olfactory neurons in the epithelium of the roof of Carol's nasal cavity detect odors for her sense of smell (covered in detail in the chapter on the senses). Carol's smoking mya cause these receptors to become less sensitive, limiting her ability to sense odors and appreciate flavors. - Creation of pressure gradients necessary to circulate blood and lymph. The muscles used during respiration increase the volume of the thoracic cavity, causing the pressure inside the cavity to fall. As you have learned in this chapter, this pressure decrease will cause air to flow into Carol's lungs. However, you have also studied this mechanism in the circulatory lymphatic systems as the thoracic pump. This pressure decrease in the thorax also helps Carol's blood return (through the inferior vena cava) to her heart and helps her lymph return (through the thoracic duct) to her left subclavian vein. The thoracic pump created by this system will be less effective if Carol's blood becomes thicker due to smoking-related polycythemia (discussed in the chapter on blood). Carol's respiratory system is functioning now, but what can she expect to be the effects of growing older even if she decides to stop smoking? **12.5 Effects of Aging on the respiratory System** Aging has many effects on the respiratory system, which basically lead to a decline in maximum function. These effects are as follows: - With age, more mucus accumulates in the respiratory tract because the ciliated escalator becomes less efficient. The inability to efficiently clear debris leaves older people open to more respiratory infections. So vaccines to prevent infections, such as flu and pneumonia, are highly recommended for older individuals. - Thoracic wall compliance decreases due to the diminished ability to expand the chest that comes with age. This can be due to weakened respiratory muscles, stiffening of the cartilages in the rib cage, decreased height of the vertebrae from age-related osteoporosis, and kyphosis. The net effect of the reduced compliance is reduced vital capacity because the ability to fill the lungs (inspiratory reserve volume) and the ability to empty the lungs (expiratory reserve volume) are both decreased. - Some of the alveolar walls may break down, and this reduces the area of the respiratory membrane. The remaining membrane thickens with age, reducing alveolar gas exchange. Tidal volume gradually increases with age to compensate for the reduced are and thickening of the respiratory membrane. - Obstructive sleep **apnea** (breathing repeatedly stops and starts during sleep) may develop in older people as the pharyngeal muscles intermittently relax and block the airway during sleep. This form of apnea may or may not be accompanied by snoring, and it is more prevalent in people who are overnight. The effects of aging may not be readily apparent in a healthy individual, but they may diminish even the healthy individual's ability to perform vigorous exercise. After exploring the relevant diagnostic tests, you will complete your study of this system by examining what can go wrong---respiratory disorders. **12.6 Effects of Aging on the respiratory System** Common diagnostic tests used for respiratory system disorders. A lot of the tests may be familiar to you as they have been mentioned in previous chapters, but their specific relation to the respiratory system is explained **Diagnostic Tests for Respiratory System Disorders:** **Arterial blood gas:** A test of the arterial blood that determines the level of O2 and CO2 in the blood **Biopsy:** A procedure in which tissue is collected and examined for the presence of abnormal cells **Chest X-ray:** The use of electromagnetic radiation that sends photons through the body to create a visual image of dense structures such as the lungs **Complete blood count (CBC):** A series of blood tests including hematocrit, hemoglobin, red blood cell count, white blood cell differential, white blood cell count, and platelet count **Computed tomography (CT):** An imaging technique used to visualize internal structures. The scan produces images in "slices" of areas throughout the body. In regard to disorders o9f the respiratory system, CT can be used to determine changes in organs of the respiratory system located in the head, neck, and chest **Cultures and sputum analysis:** A procedure that involves collecting a culture or sputum from a patient and performing various tests to identify the microorganism causing an infection **Mantoux test for TB:** A test that determines whether a person has developed an immune response to the bacterium that causes tuberculosis **Monospot test:** A test used to determine the presence of antibodies to infectious mononucleosis **Oxygen saturation test:** A test that measures the amount of oxygen being carried by red blood cells **Peak flow meter:** A test that measures the rate at which a person can exhale air **Pulmonary angiogram:** An X-ray of the blood vessels in the lungs **Pulse oximetry:** The use of infrared light to determine the amount oxygenated hemoglobin in the blood **Rapid influenza test:** A test used to determine the presence of influenza antigens **Rapid strep test:** A test used to determine whether strep bacteria are present in the patient's throat **[Spirometry]:** A test that measures the respiratory system functions of moving air into the lungs and moving air out of the lungs **Thoracentesis:** A procedure in which fluid is removed from the chest through a needle or tube **Ultrasound:** An imaging technique in which sound waves create visual images of internal structures **12.7 Respiratory System Disorders** The respiratory disorders discussed in this section fall into three categories: infections, chronic obstructive pulmonary diseases (COPDs), and lung cancer. **Functions of the Respiratory System** *Cold* The most common respiratory infection is the common cold, which is commonly caused by a rhinovirus. Its symptoms include congestion, sneezing, and increased mucus production. This infection can easily spread to the sinuses, throat, and middle ear. Typically, a cold runs its course in about a week. **Applied Genetics** Cystic fibrosis is the most common fatal genetic disease in the United States. As you read in "Levels of Organization of the Human Body," cystic fibrosis is caused by a single gene in the human DNA that codes for a faulty chloride channel on cell membranes. People with a faulty cystic fibrosis transmembrane regulator (CFTD) gene produce a sticky mucus that cannot be easily moved by the respiratory epithelium's ciliated escalator. As a result, the sticky mucus accumulates in the lungs and airways, and this then leads to infection. Gene therapy for cystic fibrosis began in 1990 when scientists were successful in introducing correct copies of the gene to cells in laboratory cultures. In 1993, common rhinoviruses were tried as a delivery mechanism (vector) to deliver the correct gene. These viruses were tried as vectors because rhinoviruses specifically invade respiratory cells and deliver a piece of nucleic acid to the invaded cell. If the rhinovirus could be modified to carry and insert the correct copy of the CFTR gene, it would deliver it to the appropriate type of cell in a patient with cystic fibrosis. Since then, other vectors have been tried in an effort to find the most efficient way of introducing the correct gene to the affected cells. Life span of the respiratory cells also needs to be considered to determine the correct vector and treatment schedule. The research into gene therapy for this disease continues. *Influenza* Flu is a respiratory---not digestive---illness caused by a virus. In addition to its cold symptoms, flue is characterized by fever, chills, and muscle aches. Flu can be diagnosed by physical examination. A rapid flu test can also be performed to test for the presence of influenza viral antigens. The mortality rate for influenza is approximately 1%, with most of the deaths occurring in very young children and older people. Influenza viruses mutate and change often, so vaccines are created each year to protect against the expected viral flu strains for that year. *Pharyngitis* This condition is commonly known as a sore throat. Pharyngitis is caused by inflammation of the pharynx, which usually results from respiratory infections such as the cold or flu. Other symptoms associated with pharyngitis are fever, headache, rash, swollen lymph nodes in the neck, and muscle aches. Physical examination of the throat and laboratory tests such as a rapid test or throat culture may be done to determine which type of organism (bacterium or virus) is responsible for the infection. **Disease Point:** **Strep throat** is a common bacterial infection that causes pharyngitis. *Streptococcus* comprises the bacteria responsible for this infection. Symptoms of strep throat can include fever, sore throat, malaise, painful swallowing, headache, rash, and loss of appetite. Health care providers can perform a rapid strep test or a throat culture to determine whether the *streptococcus* bacteria are present. If the test shows the presence of *streptococcus* bacteria, then antibiotics can be prescribed for treatment. **Mononucleosis (mono)** is a viral infection that causes pharyngitis.it is known as the *kissing disease* because it is often transmitted through saliva and close contact with an infected person. Symptoms of mono include fatigue, fever, a sore throat that gets progressively worse, swollen tonsils that become covered with white or yellow patches, loss appetite, muscle aches, rash, swollen lymph nodes, and a swollen spleen. Similar to the test for strep throat, a monospot test is a rapid test that can be performed to determine whether a patient is positive for mononucleosis. Because mono is caused by a virus, antibiotics will not treat the disease; therefore, treatment is usually geared toward relieving the symptoms *Laryngitis* This condition is characterized by the inflammation of the larynx accompanied by the loss of the voice (hoarseness). Usually, a respiratory viral infection such as a cold or flu causes laryngitis. Other causes include pneumonia, croup, bronchitis, gastroesophageal reflux (which is covered in the digestive system chapter), allergies, or exposure to irritants and chemicals. Because viral infections commonly cause laryngitis, antibiotics may not be helpful in treating this condition. Resting the voice and using certain medications can help with the discomfort associated with laryngitis. *Croup* **Croup** is characterized by a loud, seal-like, barking cough and difficulty breathing. It is common in infants and children and is usually caused by a viral or bacterial respiratory infection that results in inflammation of the larynx. In severe cases, the airway can become so inflamed that it swells shut, cutting off the air supply and possibly resulting in death if not treated. Croup is diagnosed by listening to characteristic cough associated with the disease. Physicians will also listen to breath sounds to determine whether they are decreased or accompanied by wheezing or whether expiration and inspiration are prolonged. Visual examination of the chest during respiration may also indicate difficulty breathing. Treatment depends on the cause of the disease, but medications are used to manage the symptoms and inflammation associated with croup. *Tuberculosis* This infection is caused by the *Mycobacterium tuberculosis* bacterium that enters the lungs by way of air, blood, or lymph. The lungs react to the infection by walling off bacterial lesions with scar tissue that diminishes lung compliance. Health care workers are tested for exposure to the bacteria with a Mantoux test. *Pertussis* This highly contagious bacterial infection causes the paralysis of cilia in the respiratory epithelium. The accumulation of mucus and debris results in a *whooping cough,* which gives this disorder its common name. Pertussis can be diagnosed by the symptoms present (the whooping cough), a CBC (which would show elevated lymphocytes), and sputum cultures (which would determine the presence of the bacterium that causes pertussis). Pertussis vaccine is one part of the DPT shot routinely given in the United States to children (D stands for *diphtheria,* P stands for *pertussis,* and T stands for *tetanus*). *Acute Bronchitis* Sometimes, inflammation of the bronchial tubes follows a respiratory infection, such as the ones already discussed. This is known as **acute bronchitis.** The bacterium or virus responsible for the respiratory infection may travel to the bronchi and the lungs, causing a productive cough (cough with mucus), wheezing, shortness of breath, and pain in the chest. Other symptoms can include fatigue and fever. To determine whether a patient has acute bronchitis, a health care provide listens to the lungs and may perform the following diagnostic test: chest X-ray, pulse oximetry, and/or examination of sputum samples for evidence of bacteria. Treatment for chronic bronchitis is based on the cause of the disease. Antibiotics may be given if the cause is a bacterial infection. Other treatments may be geared toward relieving the symptoms associated with acute bronchitis. *Pneumonia* This infection can be caused by bacteria, a virus, a fungus, or even a protozoan. Symptoms include fever, difficulty breathing, and chest pain. In this type of infection, fluid accumulates in the alveoli (pulmonary edema), and inflammation causes the respiratory membrane to thicken, thereby reducing gas exchange. Diagnosis of pneumonia occurs through a variety of tests such as arterial blood gas, CT scan, CBC, and sputum cultures. **Chronic Obstructive Pulmonary Disorders** Chronic obstructive pulmonary disorders (COPDs) cause the long-term decrease in ventilation of the lungs. Many COPDs are the result of cigarette smoking. *Chronic Bronchitis* You have previously covered acute bronchitis and its relation to respiratory infections. Chronic bronchitis often results from long-term irritation of the epithelium of the bronchial tree. With the subsequent inflammation, cilia are lost and mucus is overproduced. Without the cilia escalator, mucus and debris accumulate, leading to further chronic inflammation and infections. The long-term effect is a decrease in the diameter of the bronchioles, which reduces ventilation of the alveoli. Chronic bronchitis often leads to emphysema. *Emphysema* In this disorder, constant inflammation from irritants narrows bronchioles, reducing the airflow to the lungs. The respiratory system tries to clear built-up mucus and debris---often from chronic bronchitis---by coughing. The coughing causes increased pressure in the alveoli that results in rupturing of the alveolar walls. This loss of respiratory membrane reduces gas exchange and reduces the recoil of the lung (compliance). Symptoms of emphysema include shortness of breath and an enlargement of the thoracic cavity (barrel chest). *Asthma* This disorder involves increased constriction of the lower respiratory tract due to a variety of stimuli. The symptoms include wheezing, coughing, and shortness of breath. Although no definitive cause has been found, asthma and allergies often go together. Whatever the cause, asthma is characterized by chronic airway inflammation, airway obstruction, and airway hyperreactivity in which the smooth muscle overreacts to a stimulus by constricting the bronchioles. Treatment involves avoiding the stimulus---if it can be determined---and drug therapy. **Respiratory Distress Syndrome** Respiratory distress syndrome is a condition of the lungs that results in a lack of oxygen in the blood. **Acute respiratory distress syndrome (ARDS)** typically occurs in patients who are already experiencing chronic illness or have had major trauma. Infection of an injury causes inflammation of the lung tissue, which results in collapsed alveoli and excess fluid accumulation in the alveoli. This in turn affects gas exchange in the following ways: - By interfering with membrane area (collapsed alveoli reduce the amount of membrane area available for gas exchange.) - By interfering with membrane thickness (Inflammation of lung tissue increases respiratory membrane thickness, which makes it harder for gas to diffuse across the respiratory membrane.) - By interfering with ventilation-perfusion coupling (Damaged parts of the lung receive less air and less blood flow.) **Cancers of the Respiratory System** Two types of respiratory system cancers are laryngeal cancer and lung cancer. *Laryngeal Cancer* **Laryngeal cancer** (cancer of the larynx) is commonly associated with the use of tobacco products or excessive alcohol consumption. The cancer starts in the larynx and can metastasize to other areas of the head and neck. Symptoms of laryngeal cancer include a hoarse voice, a lump in the neck, and difficulty swallowing or breathing. Laryngeal cancer is diagnosed by biopsy to determine whether cancerous cells are present. If cancer is present, the physician may also perform CT scans and chest X-rays to determine whether the cancer has spread to other areas. Treatment of laryngeal cancer involves removal of the cancerous tumor by surgery, radiation, or chemotherapy. Depending on the extent of the tumor, a laryngectomy (removal of the larynx and vocal cords) may be performed. It is likely that treatment for laryngeal cancer will affect a person's ability to speak and swallow, and rehabilitation may be necessary. *Lung Cancer* The are three different types of lung cancers: - Squamous cell carcinoma originates in the bronchial epithelium. In this form of lung cancer, the ciliated epithelium undergoes metaplasia first, changing to stratified epithelial tissue. Cancerous cells further divide, invading tissues of the bronchial walls and forming tumors that can block airways. - Adenocarcinoma originates in the mucous glands of the bronchial tree in the lung. Like squamous cell carcinoma, it also invades other tissues of the bronchial tree and lung. - Oat cell carcinoma is the least common form of lung cancer, but is the most deadly because it easily metastasizes to other tissues. It usually begins in a main bronchus and then invades the mediastinum and travels to other organs. **Diagnostic Tests for Respiratory System Disorders:** **Infections** **Acute bronchitis:** Inflammation of the bronchial tubes following a respiratory infection **Cold:** The most common respiratory infection, which is caused by a rhinovirus **Croup:** A viral or bacterial respiratory infection that results in inflammation of the larynx; characterized by a loud, seal-like, barking cough and difficulty breathing **Flu:** A respiratory infection caused by an influenza virus **Laryngitis:** Inflammation of the larynx accompanied by loss of the voice or hoarseness of the voice **Mononucleosis:** A viral infection that causes pharyngitis **Pertussis:** A bacterial respiratory infection that causes the paralysis of cilia in the respiratory epithelium. The resulting accumulation of mucus and debris causes a distinctive whooping cough. **Pharyngitis:** Inflammation of the pharynx, which usually results from respiratory infections such as a cold or flu **Pneumonia:** An infection resulting in pulmonary edema and inflammation, which causes the respiratory membrane to thicken, thereby reducing gas exchange **Strep throat:** A common bacterial infection caused by *streptococcus* **Tuberculosis:** A respiratory infection caused by the *Mycobacterium tuberculosis* bacterium **Chronic obstructive pulmonary disorders (COPDs)** **Asthma:** Increased constriction of the lower respiratory tract due to a variety of stimuli **Chronic bronchitis:** Long-term irritation of the bronchial tree's epithelium, resulting in inflammation, loss of cilia, and overproduction of mucus **Emphysema:** Loss of respiratory membrane caused by chronic coughing, which results in increased pressure in the alveoli that leads to rupturing of the alveolar walls **Respiratory distress** **Acute respiratory distress syndrome (ARDS):** Respiratory distress in patients who are already experiencing illness or have had major trauma **Hyaline membrane disease:** Respiratory distress in premature infants de to the collapse of alveoli from the lack of surfactant **Cancers** **Laryngeal cancer:** Cancer of the larynx, which is commonly associated with the use of tobacco products or excessive alcohol consumption **Lung cancer:** Cancer of the lungs. There are three types: squamous cell carcinoma, adenocarcinoma, and oat cell carcinoma. **Other respiratory disorders** **Acidosis:** Condition in which the pH of the blood is less than 7.35 **Alkalosis:** Condition in which the pH of the blood greater than 7.45 **Cystic fibrosis:** A genetic disease that causes the production of a sticky mucus that cannot be easily moved by the respiratory epithelium's ciliated escalator. The mucus accumulation in the lungs and airways leads to infection. **Hemothorax:** Blood in the pleural cavity **Hypercapnia:** Increased carbon dioxide in the blood **Pleurisy:** Condition characterized by inflammation of the pleurae **Pneumothorax:** A collapsed lung, which occurs if air is introduced in the pleural cavity between the pleural membranes **Sinusitis:** Inflammation of the epithelium in the sinuses, which results in increased mucus production **Key Words for Review** *The following terms are defined in the glossary.* **[Alveoli]:** Tiny air sacs in the lung at which gas exchange takes place. **[Bronchial tree]:** Series of ever-decreasing-size tubes branching from the bronchi and ending with bronchioles in the lungs. **[Chronic obstructive pulmonary disorders (COPDs)]:** Disorders that cause a long-term decrease in ventilation of the lungs. **[Compliance]:** Measurements of how well the lungs can expand and return to shape. **[Expiration]:** Movement of air out of the lungs. **[Functional residual capacity (FRC)]:** The amount of air remaining in the lungs after the expiration of a normal breath at rest. **[Gas exchange]:** The movement of oxygen and carbon dioxide that occurs between capillary blood and the alveoli in the lungs and between capillary blood and the tissues of the body. **[Gas transport]:** The movement of gases in the blood to and from the lungs and tissues. **[Inspiration]:** Movement of air into the lungs. **[Inspiratory reserve volume (IRV)]:** The amount of air that can be forcefully inspired beyond the amount inspired in a normal breath at rest. **[Partial pressure]: The amount of pressure an individual gas contributes to the total pressure of a mixture of gases.** **[Pharynx]:** An area---commonly called the *throat---*that is divided into three sections based on location and anatomy---the nasopharynx, the oropharynx, and the laryngopharynx. **[Pneumothorax]:** A condition in which air is introduced in the pleural cavity between the pleural membranes; commonly called a *collapsed lung.* **[Respiratory membrane]:** A membrane that is composed of a thin layer of water with surfactant in the alveoli, a single squamous cell alveolar wall, and a single cell capillary wall and across which gas exchange takes place in the lung. **[Spirometry]:** Measurement of lung volumes and capacities. **[Surfactant]:** Fluid secreted by great alveolar cells in the lungs that reduces the surface tension of water. **[Tidal volume (TV)]:** The amount of air moved in a normal breath (inspired or expired) at rest. **[Ventilation]:** airflow to the lungs. **[Ventilation-perfusion coupling]:** The matching of airflow to blood flow in the lungs. **[Vital capacity (VC)]:** The maximum amount of air that can be moved. **Chapter 13: The Digestive System** **13.1 Word Roots and Combining Forms** Chol/o: gall, bile Col/o: colon Cyst/o: bladder, sac Duoden/o: duodenum Emet/o: vomit Enter/o: intestine Esophag/o: esophagus Gastr/o: stomach Gingiv/o: gums Gloss/o: tongue Hepat/o: liver Peps/o: digestion Rect/o: rectum Sigmoid/o: sigmoid colon **13.2 Overview** The anatomy of the digestive system is like a complicated tube─one end of the tube is the mouth and the other end is the anus. This tube─called the **[alimentary canal]** or **gastrointestinal (GI) tract**─may be as long as 8 meters. It twists and turns, enlarges and narrows as it uses muscle contractions to push its contents toward its end. You insert food in the mouth but see a very different product emerge at the anus. In this chapter, you will be exploring the anatomy of the digestive system and what happens to food (a cheeseburger) along its journey through each section of the alimentary canal. You will also look at the way digestion is regulated, the effects of aging on the system, and digestive system disorders. The cheeseburger's journey begins in the mouth and continues to the pharynx, esophagus, stomach, small intestine, large intestine, and anus, where the journey ends. All along the pathway, mucous membranes and mucosa-associated lymphatic tissue (MALT) line the alimentary canal to fight any foreign invaders─such as bacteria─that may enter with the food (discussed in the lymphatic system chapter). Along with studying the structures of the alimentary canal, you will need to study the accessory structures in each section of the tract. These accessory structures─including the teeth, tongue, salivary glands, liver, gallbladder, bile ducts, and pancreas─are necessary for the system to carry out its functions. Before you get started, it is important to understand the two types of digestion─mechanical and chemical. **[Mechanical digestion]** is the breakdown of large pieces of complex molecules to smaller pieces of complex molecules. On the other hand, **[chemical digestion]** is the breakdown of complex molecules to their building blocks so that they can absorbed. As a result of chemical digestion, proteins are broken down to their amino acids, carbohydrates to their monosaccharides, and lipids to their fatty acids and glycerol. Mechanical digestion must happen first for chemical digestion to take place. Next, you will learn about the structures of the mouth and how they help mechanically and chemical digest a cheeseburger. **13.3 Anatomy and Physiology of the Digestive System** **Anatomy in the Mouth** *Oral Cavity* The mouth can also be called the *oral cavity.* The roof of the oral cavity consists of the **hard palate,** formed by the maxilla and palatine bones, and the **soft palate,** composed of soft tissue. The soft palate ends with the **uvula,** a posterior projection that directs materials downward to the pharynx so that they do not travel to the nose. The sidewalls of the oral cavity are the cheeks, and the floor of the cavity is where the tongue is attached. The oral cavity is lined by stratified squamous epithelial tissue, which is a very durable epithelial tissue that can withstand the abrasions of manipulating solid food in the mouth. *Teeth* A baby is not born with teeth but will develop two sets of teeth over its lifetime─a **deciduous** (primary) set and a **permanent** (secondary) set. The baby's first set of teeth─the deciduous teeth─begin to erupt, or grow in, at approximately 6 months and will be complete by the age of 2 years. This primary set consists of 10 teeth in each jaw. Later, as the permanent teeth erupt, they push out the deciduous teeth. This secondary set begins to erupt at 6 years of age and will not be fully complete, with 16 teeth in each jaw, until the individual reaches 17 to 25 years of age. A tooth is held in its bony socket **([alveolus])** in the jaw by **periodontal ligaments.** The tissue surrounding a tooth is the **gingiva** (JIN-jih-vah), commonly called the *gum.* The portion of the tooth emerging from the gingiva is called the **[crown].** The crown is covered by a very hard, smooth, white layer called **enamel.** The enamel's functions is to protect the underlying layer, the **dentin.** The **root** of the tooth, below the gum line, is not covered by enamel. Deep to the dentin is a **pulp cavity** that contains the blood vessels and nerve for the tooth. *Tongue* The tongue is composed of skeletal muscle tissue anchored to the floor of the oral cavity by a medial fold called the **lingual frenulum.** On the tongue's superior surface, stratified squamous epithelial tissue covers the lingual papillae, which house the taste buds. As you will recall from the nervous system chapter on senses, taste buds contain nerve endings that sense sweet, sour, salt, bitter and umami. The purpose of the tongue it to manipulate what is **[ingested]** (eaten and to provide the sense of taste. *Salivary Glands* The salivary glands, which produces about 1.0 to 1.5 liters (L) of saliva a day, consist of the **parotid glands** (anterior to the ears), the **submandibular glands** (inferior to the angle of the mandible on each side), and the **sublingual glands** (below the tongue). Each gland produces saliva that travels to the oral cavity through ducts. The saliva, which is mostly water, also contains the enzymes **amylase** and **lingual lipase,** along with mucus, lysozymes, and antibodies. Saliva secretion is initiated by taste receptors that send signals by way of the facial and glossopharyngeal nerves to centers in the medulla oblongata and pons. These control centers also receive other stimuli so that odors, sight, or even the thought of food may stimulate saliva secretion. **Disease Point:** A tooth's enamel wears down and thins with age. A **dental carles,** commonly called a cavity, is an erosion through the enamel into the dentin. If the erosion continues to the pulp cavity, bacteria may gain access and travel beyond the tooth's root. This infection is called an **abscess.** The mouth is full of bacteria. Every time you eat, bits of food are wedged between the teeth and between each tooth and the gingiva. Bacteria feed on this buffet left out for them digest the food, and then excrete their acidic waste in the same location. This waste erodes the enamel to form a caries and irritates the gingiva, causing it to become inflamed and infected **(gingivitis).** As gingivitis progresses, the gingiva pulls away from the tooth and recedes. THis allows more food to become wedged between the tooth and the gingiva and more unprotected dentin to be exposed. The daily bacteria buildup that forms on the tooth is **plaque.** Plaque can be flossed and brushed away, but if it is allowed to remain, it hardens to form **tartar,** which must be removed by a dental professional. If gingivitis goes untreated, the inflammation and infection can spread to the ligaments and bone that hold the teeth in place **(periodontitis).** This causes the teeth to become loose and eventually fall out. Periodontitis is the most common cause of tooth loss in adults. **Physiology of Digestion in the Mouth** Think again of the example of the cheeseburger. You bite into with your teeth. The process of chewing, called **[mastication],** uses the masseter and temporalis muscles to move the jaw in a crushing motion, while the tongue, orbicularis oris, and buccinator muscles work to keep the food between the teeth. This begins mechanical digestion---breaking the bite of cheeseburger into smaller pieces. Saliva from the salivary glands mixes with the bite of cheeseburger in the mouth. The saliva's pH is 6.8 to 7.0. At this pH, amylase *partially* breaks down the carbohydrate from the bun. This is the beginning of chemical digestion. Lingual lipase does nothing at this pH, but it is activated later by the lower pH of the stomach. Amylase, however, will no longer function at the lower pH in the stomach. Thus, it is important to masticate the bite of cheeseburger thoroughly because doing so allows for mechanical digestion and gives amylase sufficient time to partially break down the carbohydrates in chemical digestion before the food is swallowed. The mucus in the saliva moistens the bite of food (now called a **[bolus]**), making it easier to swallow. The lysozymes and antibodies in the saliva are not used for digestion. They destroy and inhibit the growth of bacteria that may have entered with the bite. Digestion is finished in the mouth when the tongue pushes the bolus to the pharynx. **Anatomy of the Mouth to the Stomach** *Pharynx* The parts of the pharynx were covered in the respiratory system chapter, but it will be helpful to review them here. The nasopharynx leads from the nasal cavity to the oropharynx. The oropharynx is a funnel leading from the oral cavity to the laryngopharynx. This funnel is lined by stratified squamous epithelial tissue and has smooth muscle in its walls. The laryngopharynx leads to the trachea and the esophagus. The respiratory and digestive pathways intersect in the pharynx. *Epiglottis* The epiglottis is made of elastic cartilage connective tissue. It is one of the cartilages of the larynx. It stands guard over the glottis, which is the opening of the larynx. *Esophagus* The esophagus is a straight, muscular tube that extends from the laryngopharynx, travels through the mediastinum, penetrates the diaphragm, and connects to the stomach. It is lined by stratified squamous epithelial tissue. Deep to the epithelial lining is submucosa of connective tissue containing esophageal glands that secrete protective mucus of the esophagus. The upper one-third of the esophagus has skeletal muscles in its walls, while the middle one-third has a mixture of skeletal and smooth muscle and the lower one-third has just smooth muscles in the walls of the esophagus. Unlike the trachea that is held open by C-shaped cartilages, the esophagus is normal collapsed. **Physiology of Digestion from the Mouth to the stomach** Once the bolus has been sufficiently masticated in the mouth. It is time to swallow. Swallowing, called **[deglutition]** (dee-glue-TISH-un), is a very complex process controlled by the medulla oblongata. It requires four cranial nerves (V, VII, IX, and XII) to stimulate the muscle contractions necessary to move the bolus from the pharynx to the esophagus. Swallowing begins with the tongue pushing the bolus back to the pharynx (1). The larynx pushes up, causing the epiglottis to close over the glottis (2). This ensures that bolus moves into the esophagus and *not* into the larynx as the pharyngeal muscles push the bolus down (3). Once in the esophagus, the muscular walls move the bolus along its length in wavelike contractions called **[peristalsis]** (per-ih-STAL-sis) (4). Gravity aids in the movement towards the stomach if the individual is in an upright position, but being upright is not necessary. The bolus can still move to the stomach even if the individual is upside down. **Anatomy of the Stomach** The stomach is a J-shaped organ found in the upper left quadrant of the abdomen, immediately inferior to the diaphragm. It is a muscular sac capable of holding 1.0 to 1.5 L after a meal, but it can stretch to hold up to 4 L when extremely full. The **cardiac sphincter (lower esophageal sphincter)** controls the opening to the stomach from the esophagus. This circular muscle's purpose is to allow food to enter the stomach and make sure it does not return to the esophagus. The stomach can be described in terms of the following areas: the **lesser curvature** on the inside of the **J**; the **greater curvature** on the other side of **J**; the **fundus,** superior to the cardiac sphincter; the **body,** making up the majority of the stomach; and the **pyloric region** leading to the smooth muscle **pyloric sphincter,** which regulates the passage of materials to the duodenum. There are three layers of smooth muscle in the walls of the stomach: outer longitudinal muscles, middle circular muscles, and inner oblique muscles. Having muscles oriented in different directions allows for maximum churning of the stomach's contents. Longitudinal wrinkles called **gastric rugae** (ROO-guy) can be seen inside the stomach when the stomach is empty. These wrinkles become less apparent as the stomach stretches. They also allow for more surface area to accommodate microscopic depressions in the lining, called **gastric pits,** that extend to form **gastric glands.** Five different types of cells line the gastric pits and gastric glands. **Disease Point:** It is crucial that the cardiac sphincter closes tightly after the bolus has entered the stomach, as the mucosa lining the esophagus provides insufficient protection from the gastric juices produced in the stomach. Irritation, creating a burning sensation, results if gastric juices leak back to the esophagus. This is commonly called **heartburn** because of the close proximity of the end of the esophagus to the heart. Chronic leakage of gastric juices back to the esophagus is called **gastroesophageal reflux disease (GERD).** **Gastric Juices: Chemicals Produced in the Gastric Pits and Gastric Glands of the Stomach:** **Endocrine Cells** **Gastrin:** Tells chief and parietal cells to produce their products **Parietal cells** **Hydrochloric acid:** - Converts pepsinogen to **pepsin,** which *partially* breaks down proteins through chemical digestion - Activates lingual lipase, which, along with gastric lipases, *partially* breaks down lipids through chemical digestion - Converts iron in the diet to a usable form that can be absorbed - Destroys some bacteria **Intrinsic factor:** Allows vitamin B12 to be absorbed **Chief cells** **Pepsinogen:** Changes to pepsin to *partially* break down proteins **Gastric lipase:** *Partially* breaks down lipids **Mucous cells** **Mucus:** Protects the stomach walls - Mucous cells secrete a highly alkaline mucus to protect the stomach walls from the hostile environment caused by the acid and digestive enzymes produced in the stomach. - Endocrine cells secrete many hormones, but we will focus on the hormone gastrin. - Parietal cells produce and secrete hydrochloric acid and intrinsic factor. - Chief cells secrete pepsinogen and gastric lipase. - Regenerative cells are stem cells that divide and differentiate to replace any of the other cells of the gastric pits and gastric glands. Regenerative cells are very necessary because the cells lining the stomach are short-lived, lasting only 3 to 6 days due to the stomach's harsh, acidic environment. The gastric pits' cells must be continually replaced. The stomach has several mechanisms it uses to protect itself from the harsh environment created by the cells of the gastric pits and gastric glands. These mechanisms include the following: 1. The lining has the highly alkaline mucous coat that resists the hydrochloric acid and digestive enzymes. 2. There is epithelial cell replacement of the lining by the regenerative cells. 3. There are tight junctions between epithelial cells, so acid and enzymes cannot get to the submucosa and smooth muscle walls made of mostly protein. **Physiology of Digestion in the Stomach** In this section, you will continue to trace the cheeseburger on its journey through the digestive system. During swallowing, the medulla oblongata sends signals to the stomach telling it to relax. As the bolus is moved down the esophagus by peristalsis to the stomach, the stomach's cardiac sphincter opens to allows the bolus to enter. This relaxation of the stomach and the opening of the cardiac sphincter allow the stomach to fill. As the stomach fills, the three layers of smooth muscle in the stomach's walls stretch, causing the muscular walls to contract. These contractions result in peristaltic waves in the direction of the pyloric canal. The pyloric sphincter remains closed, however, making sure the contents stay in the stomach. As the bolus enters the stomach, the endocrine cells of the gastric pits produce the hormone gastrin. Gastrin targets chief cells and parietal cells, telling the chief cells to produce pepsinogen and gastric lipase and telling the parietal cells to produce hydrochloric acid and intrinsic factor. The hydrochloric acid (HCI) produced by the parietal cells has a pH of 0.8. It converts pepsinogen (produced by the chief cells) to pepsin, which partially breaks down proteins (the burger) in the bolus. This is the start of the chemical digestion of proteins in the bolus. The hydrochloric acid also activates lingual lipase from the saliva that mixed with the bolus in the mouth. The activated lingual lipase works together with the gastric lipase produced by the chief cells to *partially* break down the lipids in the bolus (the cheese). This is the start of chemical digestion of the lipids in the bolus. The intrinsic factor produced by parietal cells allows vitamin B12 to be absorbed later in the small intestine. The churning of the stomach continues mechanical digestion by mixing all the gastric juices with the bolus. This liquefies the contents of the stomach, now called **[chyme]** (KYME). At this point, the digested carbohydrates have been partially digested in the mouth by amylase, the lipids have been partially digested in the stomach by lingual lipase and gastric lipase, and the proteins have been partially digested in the stomach by pepsin. As the mixing continues, the pH of the chyme falls due to hydrochloric acid's low pH. As the pH of the stomach's contents approaches 2, the endocrine cells of the gastric pits are prevented from producing any more gastrin. With less and less gastrin, the chief and parietal cells are also prevented from producing their products. This is a good example of maintaining homeostasis using a negative feedback mechanism. This low pH also causes the pyloric sphincter to begin to open, allowing approximately 3 milliliters (mL) of chyme to leave the stomach at a time. Digestion in the stomach is complete when the chyme exits the pyloric sphincter. Chyme travels within the first part of the small intestine---the **duodenum** (du-oh-DEE-num). However, accessory structures play a large role in the digestion occurring in the duodenum. Therefore, we focus next on the anatomy of these accessory structures---the liver, the gallbladder, the pancreas, and the relevant ducts connecting these structures to the duodenum. **Anatomy of Digestive Accessory Structures** *Liver* The liver is a large, reddish-brown organ immediately inferior to the diaphragm on the right side of the abdominal cavity. It has four lobes: the **right and left lobes,** separated by the **falciform ligament;** the **quadrate lobe,** next to the gallbladder; and the **caudate lobe,** which is the most posterior lobe. The falciform ligament is a sheet of mesentery that suspends the liver from the diaphragm and anterior abdominal wall. The **round ligament** is a remnant, or leftover piece, of the umbilical vein, which had delivered blood from the mother's placenta to the liver in the fetus. The liver is a highly vascular organ that is arranged in **hepatic lobules.** Each hepatic lobule has a central vein as a hub and sheets over liver cells (hepatocytes) radiating out like spokes on a wheel. The liver receives oxygenated blood from the hepatic artery and nutrient-rich blood from the hepatic portal vein. The hepatic vein drains blood from the liver. The digestive function of the hepatocytes is to produce **bile.** Bile is a yellow-green fluid containing **bile acids,** synthesized from cholesterol (a steroid), and **lecithin** (a phospholipid). Both of these components function to aid in the digestion of lipids by **[emulsifying]** lipid droplets. Emulsification involves breaking the lipids into smaller droplets, a process much like the way detergents emulsify grease when you wash your dishes. Enzymes can then complete chemical digestion of lipids more efficiently. The other contents of bile are waste products that include bilirubin (from the breakdown of hemoglobin), cholesterol, neutral fats, bile pigments, and minerals. Bile travels within the liver from the hepatocytes, to pigments, and minerals. Bile travels within the liver from the hepatocytes, to hepatic ductulus, to the right and left hepatic ducts, and to the **common hepatic duct,** which exits the liver and leads to the **common bile duct.** The liver produces approximately 500 to 1,000 mL of bile per day, which is equivalent to one-quarter to one-half of a la4rge 2 L bottle of soda. *Common Bile Duct* The common bile duct is a tube running from the common hepatic duct to the duodenum. The **cystic duct** also feeds into the common bile duct. The **hepatopancreatic sphincter** at the opening to the duodenum regulates the passage of materials from the common bile duct and **pancreatic duct** into the duodenum. *Gallbladder* As you can see, the gallbladder is a pear-shaped sac on the inferior side of the liver. It stores and concentrates the bile produced by the liver. As the liver continually produces bile, it fills the common bile duct. Between meals, any overflow of bile in the common bile duct accumulates in the gallbladder through the cystic duct because the hepatopancreatic sphincter remains closed. The gallbladder then concentrates the bile by absorbing some of the water and electrolytes. When needed for digestion, the smooth muscle in the walls of the gallbladder contracts, squeezing the bile through the cystic duct to common bile duct through the relaxed hepatopancreatic sphincter to the duodenum. **Disease Point:** If the gallbladder concentrates the bile too much, the cholesterol in bile may precipitate (settle out as a solid), forming gallstones **(cholelithiasis).** When the gallbladder is directed to release its bile, the stones may block the cystic duct, causing pain and inflammation **(cholelithiasis).** Surgery---a **cholecystectomy**---may be necessary to remove the gallbladder and the gallstones within. *Pancreas* The ribbonlike pancreas has a pebbly appearance, and it is retroperitoneal, meaning it is posterior to the parietal peritoneum. The pancreas functions as two glands: (1) as an endocrine gland, because it produces the hormones insulin and glucagon secreted into the blood; and (2) as an exocrine gland, because it produces the bicarbonate ions and enzymes for protein, lipid, and carbohydrate digestion that are secreted into the **pancreatic duct.** The bicarbonate ions work to neutralize the low pH of the chyme entering the duodenum from the stomach. The pancreatic duct runs the length of the pancreas and joins with the common bile duct as it opens to the duodenum. Keep in mind that these organs---the liver, gallbladder, and pancreas---are not part of the cheeseburger's path. These organs secrete digestive chemicals that are delivered to the small intestine through ducts. Their digestive juices go to the duodenum; the cheeseburger does not go through these ducts to these accessory organs. **Digestive Juices from the Liver, Gallbladder, and Pancreas:** **Liver** **Chemical secreted:** Bile **Route Taken to the Duodenum:** Hepatic ductules to the hepatic ducts and then to the common bile duct; possibly overflows through the cystic duct into the gallbladder between meals **Function:** Emulsifies lipids **Gallbladder** **Chemical Secreted:** Bile **Route Taken to the Duodenum:** Cystic duct to the common bile duct **Function:** Emulsifies lipids **Mucous cells** Bicarbonate ions Enzymes for carbohydrates Enzymes for proteins Enzymes for lipids **Route Taken to the Duodenum:** Pancreatic duct to the duodenum **Function:** Bicarbonate ions: Neutralizes the acids of chyme Enzymes for carbohydrates: Chemically digests carbohydrates Enzymes for proteins: Chemically digests proteins Enzymes for lipids: Chemically digests lipids **Anatomy of the Small Intestine** The small intestine is composed of the duodenum, the jejunum (je-Jew-num), and the ileum. Digestion is completed in the duodenum, and absorption takes place throughout the small intestine, as you will read shortly. Although it may look as though the small intestine is very unorganized, the mesentery membranes neatly arrange the blood vessels and nerves traveling to and from each section of the small intestine. *Duodenum* The duodenum is the first 25 cm (10 inches) of the small intestine; it is located immediately after the stomach's pyloric sphincter. As with the entire small intestinal tract, there is smooth muscle in the duodenal walls, and the duodenal lining has circular folds with many tiny projections called **villi.** The villi are covered with simple columnar epithelial cells and mucus-producing goblet cells. The simple columnar epithelial cells have a brush border of microvilli to give these cells extra surface area for absorbing nutrients. Inside the villi are capillaries and small lymphatic vessels called **[lacteals]** (LAK-tee-alz). Absorption of nutrients takes place through the villi, either into the capillaries or into the lacteals. The lining of the duodenum also contains endocrine cells. These cells make two hormones---**secretin** and **cholecystokinin** (KOH-leh-sis-toe-KIE-nin)---that target the gallbladder and pancreas, telling them to release bicarbonate ions, digestive enzymes, and bile to be delivered to the duodenum. The duodenum, like the jejunum and ileum, has tight junctions between cells of the epithelial lining to protect itself from the acidic chyme. Only small amounts of chyme should enter the duodenum at any one time. This helps keep the mucous lining from becoming overwhelmed, and it give the duodenum time to neutralize the chyme. *Jejunum* The **jejunum**---the second part of the small intestine---has a very rich blood supply that gives it a pink appearance. The jejunum measures approximately 2.2 to 2.4 m in length, and its villi are slightly smaller than those in the duodenum. Circular folds in the lining allow extra surface area for absorption. Most of the absorption of nutrients takes place in the jejunum. *Ileum* The **ileum** is the last part of the small intestine, measuring 3.3 to 3.6 m in length. It walls are less muscular and thinner than the jejunum's. The ileum's lining is characterized by nodules of lymphocytes called *Peyer's patches.* These nodules increase in size as they approach the large intestine, and they function to destroy any bacteria or other pathogens entering the small intestine from the large intestine. The **ileocecal valve** (ILL-ee-oh-SEE-cal) is a sphincter muscle at the juncture of the ileum and the large intestine; it regulates the passage of materials from the ileum to the large intestine. **Physiology of Digestion in the Small Intestine** When the acidic chymes enter the duodenum, the endocrine cells of the duodenum begin to secrete their hormones---secretin and cholecystokinin. One minor role of these two hormones is to target the stomach's parietal and chief cells, telling them to stop producing hydrochloric acid and pepsinogen. If chyme is now entering the duodenum, there is no further need for digestion in the stomach. This is a second negative feedback mechanism to stop digestion in the stomach and maintain homeostasis. It complements the negative feedback mechanism of low pH in the stomach, mentioned earlier. Another role of these two hormones is to target the pancreas, telling it to release enzymes to complete carbohydrate, lipid, and protein digestion. These digestive enzymes travel through the pancreatic duct to the common bile duct and through the hepatopancreatic sphincter. Then they move into the duodenum to complete lipid, carbohydrate, and protein digestion. You will not investigate how that works. The low pH of the chyme entering the duodenum stimulates the duodenal endocrine cells to secrete secretin. This hormone mainly targets the pancreas, telling the pancreas to release bicarbonate ions to neutralize the acidic chyme. This bicarbonate solution from the pancreas carries pancreatic enzymes for lipid, protein, and carbohydrate digestion from the pancreas, through the pancreatic duct, to the common bile duct, and to the duodenum. The bicarbonate ions combine with the hydrogen ions of the hydrochloric acid to form carbon dioxide and water. The carbon dioxide is absorbed into the blood and carried to the lungs, where it is eventually expelled. All of these steps are necessary to help protect the duodenum from the low pH and maintain homeostasis. When partially digested lipids center the duodenum, the duodenum's endocrine cells release cholecystokinin, which travels through the blood to its main target tissues---the gallbladder and the hepatopancreatic sphincter. Cholecystokinin tells the gallbladder to squeeze (contract) and release its bile through the cystic duct to the common bile duct. Cholecystokinin also tells the hepatopancreatic sphincter to relax so that the bile in the common bile duct enter the duodenum. Bile helps complete lipid digestion by emulsifying (breaking up) the lipids to tiny droplets so that the lipases (enzymes) from the pancreas can break down the lipid to their building blocks---*fatty acids* and *glycerol.* Bile also helps activate some of the other digestive enzymes from the pancreas. The pancreatic enzymes complete protein digestion by breaking down the protein molecules to their building blocks, *amino acids,* while carbohydrate-digesting enzymes from the pancreas break down the chyme's carbohydrate to their building blocks, *monosaccharides* (simple sugars). In the small intestine, the mechanical and chemical digestion of the cheeseburger introduced in the beginning of the chapter is complete. The fats in the cheese are broken down to *fatty acids* and *glycerol.* The proteins of the burger are broken down to *amino acids.* And the carbohydrates of the bun are broken down to *monosaccharides.* The nutrients, waste products from the bile, and the indigestible materials continue on the jejunum and ileum, where nutrient absorption occurs through the villi. The chyme moves through the sections of the small intestine via two types of contractions. **[Segmentation]** is a stationary constriction of the smooth muscle in ringlike patterns. This type of contraction further churns the chyme, mixing the bile and digestive enzymes to finish chemical digestion. It also allows for maximum contact between chyme and the villi, facilitating maximum absorption of nutrients. Once the chyme has churned and mixed with the bile and digestive enzymes, it continues to move through the jejunum and ileum through peristalsis (wavelike contractions, mentioned earlier, during swallowing). **Absorption of Nutrients in the Small Intestine** Monosaccharides and amino acids are absorbed into capillaries through the epithelium of the villi by facilitated diffusion. Fatty acids and glycerol are absorbed across the epithelial membranes of the villi by diffusion. They are then coated with proteins and exocytosed to the lacteals, the small lymph vessels located in the villi. They will continue to travel through lymph vessels to the thoracic duct to the left subclavian vein, where they will enter the bloodstream. The ileum reabsorbs 80% of the bile acids in the chyme, while the other 20% will leave the body during **[defecation].** This is the body's way of removing cholesterol. The liver will make new bile acids from cholesterol to replace the lost 20% of bile acids. What was not absorbed moves through the ileocecal sphincter into the large intestine. **Anatomy of the Large Intestine** The large intestine **(colon)** is made up of six regions: the **cecum,** the **ascending colon,** the **transverse colon,** the **descending colon,** the **sigmoid colon,** and the **rectum.** Altogether, these regions measure about 1.5 m in length and 6.5 cm in diameter. Although the large intestine is shorter than the small intestine, it is termed *large* because its diameter is greater. *Cecum* The cecum is a blind pouch (does not lead anywhere) inferior to the juncture of the ileocecal valve in the lower right quadrant of the abdomen. The appendix is a dead-end tube extending approximately 7 cm from the inferior portion of the cecum. It contains many lymphocytes. **Disease Point:** Inflammation of the appendix---**Appendicitis**---can be extremely serious because the appendix can rupture and spill its contents into the abdominopelvic cavity. These contents are filled with bacteria, which may infect the entire abdominopelvic cavity if released. *Ascending Colon* The Ascending colon begins at the ileocecal valve and passes up the right side of the abdominal cavity toward the right lobe of the liver. As it approaches the liver, it forms a right-angle bend called the **right colic (hepatic) flexure.** *Transverse Colon* The transverse colon is a continuation of the large intestine that extends from the right colic flexure across the abdomen to the area of the spleen. There, the colon forms another right angle called the **left colic (splenic) flexure.** *Descending Colon* The descending colon is a continuation of the large intestine that extends from the left colic flexure down the left side of the abdominal cavity. *Sigmoid Colon* The sigmoid colon is a continuation of the large intestine that forms an S shape in the pelvic cavity. It connects to the last part of the large intestine, the rectum. *Rectum* The rectum is approximately 15 cm long, and it ends with the **anal canal.** The anus contains tow sphincter muscles: the smooth muscle **internal anal sphincter,** controlled by the autonomic nervous system, and the skeletal muscle **external anal sphincter,** controlled by the somatic nervous system. All of the large intestine's regions contain smooth muscle in the walls, but the ascending, transverse, descending, and sigmoid colons also contain longitudinal bands of smooth muscle called **taenia coli.** The taenia coli cause the large intestine's walls to bulge, forming pouches called **haustra** (HAW-stra). Unlike the small intestine, the large intestine does not contain villi. Instead, it is lined by simple columnar epithelial tissue, except for the lower part of the anal canal, which is stratified squamous epithelial tissue. This tissue needs to be stratified to withstand the abrasion of materials leaving the body. **Physiology of Digestion in the Large Intestine** Chyme (minus the absorbed nutrients) enters the large intestine in a very liquid state. The large intestine functions to absorb water, and this compacts its contents into **[feces]**. This process can take 12 to 24 hours. During that time, the large intestine also absorbs some electrolytes (especially sodium and chloride ions) and vitamin K produced by bacteria living in the large intestine. The large intestine then stores fecal matter until it is removed (defecation). The absorption of water in the large intestine is important to maintain homeostasis. Even after the water is absorbed and the feces have be compacted, feces are still typically composed of 75% water and 25% solid matter. The solid matter consists of bacteria that normally live in the colon, indigestible carbohydrates (dietary fiber), lipids, and a mixture of sloughed-off epithelial cells, digestive juices, mucus, and a small amount of protein. The lipids and proteins are not from the cheeseburger. They are from broken-down epithelial cells and bacteria, which normal live in the colon and have died. Indigestible carbohydrate from the cheeseburger feed the bacteria that reside in the large intestine. In return, the bacteria produce some B vitamins and vitamin K, a necessary vitamin for the production of clotting factors. Although these bacteria provide a very beneficial service, they also produce a gas called **[flatus],** which is not so desirable, as it can cause a bloated feeling and an unpleasant odor. The amount of flatus produced depends on the amount of bacteria present in the colon and the type of food ingested. The large intestine normally contains 7 to 10 L of gas. A typical human expels approximately 500 mL of flatus per day. How do materials move through the large intestine? Upon entering the large intestine, materials pass up the ascending colon by peristalsis to the transverse colon, where the materials stop. Distension (expansion) of the stomach and duodenum causes a **[mass movement],** which moves to the feces from the transverse colon to the descending colon, to the sigmoid colon, and on to the rectum. Distension of the walls of the rectum triggers the **defecation reflex.** This reflex drives the feces downward and relaxes the internal anal sphincter. Even though this is an involuntary reflex defecation occurs only if the external anal sphincter is voluntarily relaxed. **Disease Point:** If the large intestine absorbs too much water, the feces will become harder to move, leading to **[constipation].** Increased fluid intake, increased dietary fiber, and exercise can help move feces along. The increased pressure to push with constipation can cause **hemorrhoids,** which are bulging anal veins. They may be internal to the rectum or external to the anus. On the other hand, if the large intestine absorbs too little water, **[diarrhea]** may occur. A runny stool can result from irritation of the intestine caused by bacteria. In the case of diarrhea, the ileum's contents pass through the colon too quickly for adequate water absorption and compaction of feces to take place. **Types of Absorbed Nutrients** Until now, you have concentrated on the digestion and absorption of the major nutrients of a cheeseburger: proteins, carbohydrates, and lipids. To maintain homeostasis**,** the digestive system must absorb these nutrients from the diet. - The proteins in the burger were chemically digested to amino acids and were absorbed into the blood of capillaries in the small intestine. - The carbohydrates in the bun were chemically digested to monosaccharides and were also absorbed into the blood of capillaries in the small intestine. - The lipids of the cheese were chemically digested to fatty acids and glycerol and were absorbed into the lacteals in the small intestinal villi. Other nutrients, such as vitamins and minerals, are also absorbed by the digestive system. - Vitamins can be categorized as **fat soluble** or **water soluble.** Fat-soluble vitamins (A, D, E, and K) are absorbed along with the products of lipid digestion, so they must be ingested with fats to be absorbed. On the other hand, water-soluble vitamins (the B complex and C) are absorbed by simple diffusion. Vitamin B12 is an exception. It must first bind to intrinsic factor in the stomach and then be endocytosed by cells of the ileum for absorption. A list of vitamins and their RDAs can be found in Appendix B. - Minerals are electrolytes that are absorbed along the length of the small intestine, and some, such as sodium and chloride, can also be absorbed in the large intestine. Sodium is absorbed with sugars and amino acids. Chloride ions are mostly absorbed by active transport in the ileum. Potassium is absorbed by simple diffusion once water has been absorbed. Most minerals are absorbed at a constant rate. The kidneys excrete whatever excess may have been absorbed. Calcium and iron are an exception, as the body absorbs them to meet its level of need. You may recall from the skeletal system chapter that the hormone PTH regulates the absorption of calcium in the small intestine. A list of minerals and their RDAs can be found in Appendix B. **Circulation of Absorbed Nutrients** All blood from the capillaries in the stomach and intestines is circulated directly to the hepatic portal system so that it can be processed in the liver. The hepatic portal vein drains the

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