The Digestive System PDF

Prepared by: Danilo Gallardo Jr. , RN, MD HA11 Associate Professor CDU – CAMS OBJECTIVES  Identify the organs of the digestive system.  Describe the basic processes performed by the digestive system.  Describe the structure and function of the layers that form the wall of...

Prepared by: Danilo Gallardo Jr. , RN, MD HA11 Associate Professor CDU – CAMS OBJECTIVES  Identify the organs of the digestive system.  Describe the basic processes performed by the digestive system.  Describe the structure and function of the layers that form the wall of the gastrointestinal tract.  Describe the nerve supply of the GI tract.  Describe the peritoneum and its folds.  Identify the locations of the salivary glands, and describe the functions of their secretions. THE DIGESTIVE SYSTEM  Divided into 2 groups:  the gastrointestinal (GI) tract  accessory digestive organs  The gastrointestinal (GI) tract or alimentary canal is a continuous tube that extends from the mouth to the anus through the thoracic and abdominopelvic cavities.  Organs included:  mouth,  most of the pharynx,  esophagus,  stomach,  small intestine, and large intestine. GASTROINTESTINAL (GI) TRACT  The length of the GI tract is about 5–7 meters (16.5–23 ft) in a living person  It is longer in a cadaver (about 7–9 meters or 23–29.5 ft) because the muscles along the wall of the GI tract organs are in a state of tonus (sustained contraction) ACCESSORY DIGESTIVE ORGANS  Includes:  teeth,  aid in the physical breakdown of food  tongue,  assists in chewing and swallowing  salivary glands,  liver, other accessory digestive organs, however,  gallbladder, never come into direct contact with food  and pancreas.  They produce or store secretions that flow into the GI tract through ducts  The secretions aid in the chemical breakdown of food. THE DIGESTIVE SYSTEM  performs six basic processes:  Ingestion. This process involves taking foods and liquids into the mouth (eating).  Secretion. Each day, cells within the walls of the GI tract and accessory digestive organs secrete a total of about 7 liters of water, acid, buffers, and enzymes into the lumen (interior space) of the tract  Mixing and propulsion. Alternating contractions and relaxations of smooth muscle in the walls of the GI tract mix food and secretions and propel them toward the anus. This capability of the GI tract to mix and move material along its length is called motility. THE DIGESTIVE SYSTEM  Digestion. Mechanical and chemical processes break down ingested food into small molecules  Absorption. The entrance of ingested and secreted fluids, ions, and the products of digestion into the epithelial cells lining the lumen of the GI tract  Defecation. Wastes, indigestible substances, bacteria, cells sloughed from the lining of the GI tract, and digested materials that were not absorbed in their journey through the digestive tract leave the body through the anus. Eliminated material is termed feces. DIGESTION  mechanical digestion the teeth cut and grind food before it is swallowed, and then smooth muscles of the stomach and small intestine churn the food  In chemical digestion the large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis  Digestive enzymes produced by the salivary glands tongue, stomach, pancreas, and small intestine catalyze these catabolic reactions  A few substances in food can be absorbed without chemical digestion. These include vitamins, ions, cholesterol, and water. LAYERS OF THE GI TRACT  The wall of the GI tract from the lower esophagus to the anal canal has the same basic, four-layered arrangement of tissues. The four layers of the tract, from deep to superficial, are the:  mucosa,  submucosa,  muscularis,  serosa MUCOSA  inner lining of the GI tract, is a mucous membrane.  composed of  (1) a layer of epithelium in direct contact with the contents of the GI tract  (2) a layer of connective tissue called the lamina propria, and  (3) a thin layer of smooth muscle (muscularis mucosae). MUCOSA  The epithelium in the mouth, pharynx, esophagus, and anal canal is mainly nonkeratinized stratified squamous epithelium that serves a protective function  Simple columnar epithelium, which functions in secretion and absorption, lines the stomach and intestines.  The rate of renewal of GI tract epithelial cells is rapid: Every 5 to 7 days they slough off and are replaced by new cells  Located among the epithelial cells are exocrine cells that secrete mucus and fluid into the lumen of the tract, and several types of endocrine cells, collectively called enteroendocrine cells, that secrete hormones. MUCOSA  Lamina propria is areolar connective tissue containing many blood and lymphatic vessels, which are the routes by which nutrients absorbed into the GI tract reach the other tissues of the body  This layer supports the epithelium and binds it to the muscularis mucosae  also contains the majority of the cells of the mucosa-associated lymphatic tissue (MALT)  These prominent lymphatic nodules contain immune system cells that protect against disease  MALT is present all along the GI tract, especially in the tonsils, small intestine, appendix, and large intestine MUCOSA  A thin layer of smooth muscle fibers called the muscularis mucosae throws the mucous membrane of the stomach and small intestine into many small folds which increase the surface area for digestion and absorption.  Movements of the muscularis mucosae ensure that all absorptive cells are fully exposed to the contents of the GI tract. SUBMUCOSA  The submucosa consists of areolar connective tissue that binds the mucosa to the muscularis  contains many blood and lymphatic vessels that receive absorbed food molecules  Also located in the submucosa is an extensive network of neurons known as the submucosal plexus  may also contain glands and lymphatic tissue. MUSCULARIS  The muscularis of the mouth, pharynx, and superior and middle parts of the esophagus contains skeletal muscle that produces voluntary swallowing  Skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation MUSCULARIS  Throughout the rest of the tract, the muscularis consists of smooth muscle that is generally found in two sheets: an inner sheet of circular fibers and an outer sheet of longitudinal fibers  Involuntary contractions of the smooth muscle help break down food, mix it with digestive secretions, and propel it along the tract.  Between the layers of the muscularis is a second plexus of neurons—the myenteric plexus SEROSA  Those portions of the GI tract that are suspended in the abdominopelvic cavity have a superficial layer called the serosa  the serosa is a serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium)  also called the visceral peritoneum because it forms a portion of the peritoneum  The esophagus lacks a serosa; instead only a single layer of areolar connective tissue called the adventitia forms the superficial layer of this organ. NEURAL INNERVATION OF THE GI TRACT  The GI tract is regulated by an intrinsic set of nerves known as the enteric nervous system and by an extrinsic set of nerves that are part of the autonomic nervous system. NEURAL INNERVATION OF THE GI TRACT  Enteric Nervous System  consists of about 100 million neurons that extend from the esophagus to the anus  The neurons of the ENS are arranged into two plexuses: the myenteric plexus and submucosal plexus ENTERIC NERVOUS SYSTEM  The myenteric plexus (myo- muscle), or plexus of Auerbach, is located between the longitudinal and circular smooth muscle layers of the muscularis.  The submucosal plexus, or plexus of Meissner, is found within the submucosa ENTERIC NERVOUS SYSTEM  The plexuses of the ENS consist of motor neurons, interneurons, and sensory neurons  Because the motor neurons of the myenteric plexus supply the longitudinal and circular smooth muscle layers of the muscularis, this plexus mostly controls GI tract motility (movement)  The motor neurons of the submucosal plexus supply the secretory cells of the mucosal epithelium, controlling the secretions of the organs of the GI tract ENTERIC NERVOUS SYSTEM  The interneurons of the ENS interconnect the neurons of the myenteric and submucosal plexuses  The sensory neurons of the ENS supply the mucosal epithelium.  Some of these sensory neurons function as chemoreceptors, receptors that are activated by the presence of certain chemicals in food located in the lumen of a GI organ.  Other sensory neurons function as stretch receptors, receptors that are activated when food distends (stretches) the wall of a GI organ AUTONOMIC NERVOUS SYSTEM  Although the neurons of the ENS can function independently, they are subject to regulation by the neurons of the autonomic nervous system.  The vagus (X) nerves supply parasympathetic fibers to most parts of the GI tract, with the exception of the last half of the large intestine, which is supplied with parasympathetic fibers from the sacral spinal cord.  The parasympathetic nerves that supply the GI tract form neural connections with the ENS.  stimulation of the parasympathetic nerves that innervate the GI tract causes an increase in GI secretion and motility by increasing the activity of ENS neurons. AUTONOMIC NERVOUS SYSTEM  Sympathetic nerves that supply the GI tract arise from the thoracic and upper lumbar regions of the spinal cord.  Like the parasympathetic nerves, these sympathetic nerves form neural connections with the ENS  Sympathetic postganglionic neurons synapse with neurons located in the myenteric plexus and the submucosal plexus  the sympathetic nerves that supply the GI tract cause a decrease in GI secretion and motility by inhibiting the neurons of the ENS  Emotions such as anger, fear, and anxiety may slow digestion because they stimulate the sympathetic nerves that supply the GI tract GASTROINTESTINAL REFLEX PATHWAYS  Many neurons of the ENS are components of GI (gastrointestinal) reflex pathways that regulate GI secretion and motility in response to stimuli present in the lumen of the GI tract  The initial components of a typical GI reflex pathway are sensory receptors (such as chemoreceptors and stretch receptors) that are associated with the sensory neurons of the ENS.  The axons of these sensory neurons can synapse with other neurons located in the ENS, CNS, or ANS, informing these regions about the nature of the contents and the degree of distension (stretching) of the GI tract.  The neurons of the ENS, CNS, or ANS subsequently activate or inhibit GI glands and smooth muscle, altering GI secretion and motility. PERITONEUM  is the largest serous membrane of the body  it consists of a layer of simple squamous epithelium (mesothelium) with an underlying supporting layer of areolar connective tissue  The peritoneum is divided into the parietal peritoneum, which lines the wall of the abdominopelvic cavity, and the visceral peritoneum, which covers some of the organs in the cavity and is their serosa PERITONEUM  The slim space containing lubricating serous fluid that is between the parietal and visceral portions of the peritoneum is called the peritoneal cavity.  In certain diseases, the peritoneal cavity may become distended by the accumulation of several liters of fluid, a condition called ascites PERITONEUM  some organs lie on the posterior abdominal wall and are covered by peritoneum only on their anterior surfaces; they are not in the peritoneal cavity  Such organs, including the kidneys, ascending and descending colons of the large intestine, duodenum of the small intestine, and pancreas, are said to be retroperitoneal  Unlike the pericardium and pleurae, which smoothly cover the heart and lungs, the peritoneum contains large folds that weave between the viscera PERITONEUM  The folds bind the organs to one another and to the walls of the abdominal cavity  They also contain blood vessels, lymphatic vessels, and nerves that supply the abdominal organs.  There are five major peritoneal folds:  the greater omentum,  falciform ligament,  lesser omentum,  mesentery, and  mesocolon. THE GREATER OMENTUM  the largest peritoneal fold, drapes over the transverse colon and coils of the small intestine like a “fatty apron”  The greater omentum is a double sheet that folds back on itself, giving it a total of four layers.  extends downward anterior to the small intestine, then turns and extends upward and attaches to the transverse colon THE GREATER OMENTUM  The greater omentum normally contains a considerable amount of adipose tissue  Its adipose tissue content can greatly expand with weight gain, giving rise to the characteristic “beer belly” seen in some overweight individuals  The many lymph nodes of the greater omentum contribute macrophages and antibody producing plasma cells that help combat and contain infections of the GI tract. THE FALCIFORM LIGAMENT  attaches the liver to the anterior abdominal wall and diaphragm  The liver is the only digestive organ that is attached to the anterior abdominal wall THE LESSER OMENTUM  arises as an anterior fold in the serosa of the stomach and duodenum, and it suspends the stomach and duodenum from the liver  It is the pathway for blood vessels entering the liver and contains:  the hepatic portal vein  common hepatic artery  common bile duct,  along with some lymph nodes. THE MESENTERY  A fan-shaped fold of the peritoneum  binds the jejunum and ileum of the small intestine to the posterior abdominal wall  It extends from the posterior abdominal wall to wrap around the small intestine and then returns to its origin, forming a double-layered structure.  Between the two layers are blood and lymphatic vessels and lymph nodes. THE MESOCOLON  Two separate folds of peritoneum  binds the transverse colon (transverse mesocolon) and sigmoid colon (sigmoid mesocolon) of the large intestine to the posterior abdominal wall  It also carries blood and lymphatic vessels to the intestines  Together, the mesentery and mesocolon hold the intestines loosely in place, allowing movement as muscular contractions mix and move the luminal contents along the GI tract. MOUTH  referred to as the oral or buccal cavity  formed by the cheeks, hard and soft palates, and tongue  covered externally by skin and internally by a mucous membrane, which consists of  nonkeratinized stratified squamous epithelium.  lips or labia ( fleshy borders) are fleshy folds surrounding the opening of the mouth  The inner surface of each lip is attached to its corresponding gum by a midline fold of mucous membrane called the labial frenulum. MOUTH  The oral vestibule ( entrance to a canal) of the oral cavity is a space bounded externally by the cheeks and lips and internally by the gums and teeth.  The palate is a wall or septum that separates the oral cavity from the nasal cavity, forming the roof of the mouth.  This important structure makes it possible to chew and breathe at the same time.  The hard palate—the anterior portion of the roof of the mouth—is formed by the maxillae and palatine bones and is covered by a mucous membrane; it forms a bony partition between the oral and nasal cavities.  The soft palate, which forms the posterior portion of the roof of the mouth, is an arch-shaped muscular partition between the oropharynx and nasopharynx that is lined with mucous membrane. MOUTH  Hanging from the free border of the soft palate is a conical muscular process called the uvula (U¯ -vu¯ -la little grape). SALIVARY GLANDS  A salivary gland is a gland that releases a secretion called saliva into the oral cavity.  The mucous membrane of the mouth and tongue contains many small salivary glands that open directly, or indirectly via short ducts, to the oral cavity. These glands include labial, buccal, and palatal glands in the lips, cheeks, and palate, respectively, and lingual glands in the tongue, all of which make a small contribution to saliva. SALIVARY GLANDS  most saliva is secreted by the major salivary glands, which lie beyond the oral mucosa, into ducts that lead to the oral cavity  three pairs of major salivary glands:  the parotid,  Are located inferior and anterior to the ears, between the skin and the masseter muscle. Each secretes saliva into the oral cavity via a parotid duct that pierces the buccinator muscle to open into the vestibule opposite the second maxillary (upper) molar tooth  submandibular,  are found in the floor of the mouth; they are medial and partly inferior to the body of the mandible. Their ducts, the submandibular ducts, run under the mucosa on either side of the midline of the floor of the mouth and enter the oral cavity proper lateral to the lingual frenulum  and sublingual glands  are beneath the tongue and superior to the submandibular glands. Their ducts, the lesser sublingual ducts, open into the floor of the mouth in the oral cavity proper. SALIVARY GLANDS COMPOSITION AND FUNCTIONS OF SALIVA  saliva is 99.5% water and 0.5% solutes  Among the solutes are ions, including sodium, potassium, chloride, bicarbonate, and phosphate  Also present are some dissolved gases and various organic substances, including urea and uric acid, mucus, immunoglobulin A, the bacteriolytic enzyme lysozyme, and salivary amylase, a digestive enzyme that acts on starch  The parotid glands secrete a watery (serous) liquid containing salivary amylase  Because the submandibular glands contain cells similar to those found in the parotid glands, plus some mucous cells, they secrete a fluid that contains amylase but is thickened with mucus. COMPOSITION AND FUNCTIONS OF SALIVA  The sublingual glands contain mostly mucous cells, so they secrete a much thicker fluid that contributes only a small amount of salivary amylase.  The water in saliva provides a medium for dissolving foods so that they can be tasted by gustatory receptors and so that digestive reactions can begin  Chloride ions in the saliva activate salivary amylase, an enzyme that starts the breakdown of starch.  Bicarbonate and phosphate ions buffer acidic foods that enter the mouth, so saliva is only slightly acidic (pH 6.35–6.85). COMPOSITION AND FUNCTIONS OF SALIVA  Salivary glands (like the sweat glands of the skin) help remove waste molecules from the body, which accounts for the presence of urea and uric acid in saliva  Mucus lubricates food so it can be moved around easily in the mouth, formed into a ball, and swallowed  Immunoglobulin A (IgA) prevents attachment of microbes so they cannot penetrate the epithelium  the enzyme lysozyme kills bacteria; however, these substances are not present in large enough quantities to eliminate all oral bacteria SALIVATION  The secretion of saliva  controlled by the autonomic nervous system  Amounts of saliva secreted daily vary considerably but average 1000–1500 mL (1–1.6 qt)  Normally, parasympathetic stimulation promotes continuous secretion of a moderate amount of saliva, which keeps the mucous membranes moist and lubricates the movements of the tongue and lips during speech.  The saliva is then swallowed and helps moisten the esophagus.  Eventually, most components of saliva are reabsorbed, which prevents fluid loss SALIVATION  Sympathetic stimulation dominates during stress, resulting in dryness of the mouth  If the body becomes dehydrated, the salivary glands stop secreting saliva to conserve water  the resulting dryness in the mouth contributes to the sensation of thirst  Drinking not only restores the homeostasis of body water but also moistens the mouth.  The feel and taste of food also are potent stimulators of salivary gland secretions SALIVATION  Chemicals in the food stimulate receptors in taste buds on the tongue, and impulses are conveyed from the taste buds to two salivary nuclei in the brain stem (superior and inferior salivatory nuclei)  Returning parasympathetic impulses in fibers of the facial (VII) and glossopharyngeal (IX) nerves stimulate the secretion of saliva  Saliva continues to be secreted heavily for some time after food is swallowed; this flow of saliva washes out the mouth and dilutes and buffers the remnants of irritating chemicals such as that tasty (but hot!) salsa  The smell, sight, sound, or thought of food may also stimulate secretion of saliva OBJECTIVES  Describe the structure and functions of the tongue.  Identify the parts of a typical tooth, and compare deciduous and permanent dentitions.  Describe the location and function of the pharynx.  Describe the location, anatomy, histology, and functions of the esophagus.  Describe the three phases of deglutition.  Describe the location, anatomy, histology, and functions of the stomach. TONGUE  is an accessory digestive organ composed of skeletal muscle covered with mucous membrane  Together with its associated muscles, it forms the floor of the oral cavity  is divided into symmetrical lateral halves by a median septum  Each half of the tongue consists of an identical complement of extrinsic and intrinsic muscles TONGUE  The extrinsic muscles of the tongue, which originate outside the tongue (attach to bones in the area) and insert into connective tissues in the tongue, include the hyoglossus, genioglossus, and styloglossus muscles TONGUE  The extrinsic muscles move the tongue from side to side and in and out to maneuver food for chewing, shape the food into a rounded mass, and force the food to the back of the mouth for swallowing  They also form the floor of the mouth and hold the tongue in position  The intrinsic muscles originate in and insert into connective tissue within the tongue  They alter the shape and size of the tongue for speech and swallowing TONGUE  The intrinsic muscles include the longitudinalis superior, longitudinalis inferior, transversus linguae, and verticalis linguae muscles  The lingual frenulum, a fold of mucous membrane in the midline of the undersurface of the tongue, is attached to the floor of the mouth and aids in limiting the movement of the tongue posteriorly TONGUE If a person’s lingual frenulum is abnormally short or rigid—a condition called ankyloglossia-the person is said to be “tongue-tied” because of the resulting impairment to speech TONGUE  The dorsum (upper surface) and lateral surfaces of the tongue are covered with papillae, projections of the lamina propria covered with stratified squamous epithelium  Many papillae contain taste buds, the receptors for gustation TONGUE  Some papillae lack taste buds, but they contain receptors for touch and increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity  Lingual glands in the lamina propria of the tongue secrete both mucus and a watery serous fluid that contains the enzyme lingual lipase, which acts on triglycerides TEETH  The teeth, or dentes, are accessory digestive organs located in sockets of the alveolar processes of the mandible and maxillae.  The alveolar processes are covered by the gingivae or gums, which extend slightly into each socket  The sockets are lined by the periodontal ligament or membrane which consists of dense fibrous connective tissue that anchors the teeth to the socket walls TEETH  A typical tooth has three major external regions: the crown, root, and neck  The crown is the visible portion above the level of the gums.  Embedded in the socket are one to three roots  The neck is the constricted junction of the crown and root near the gum line. TEETH  Internally, dentin forms the majority of the tooth  Dentin consists of a calcified connective tissue that gives the tooth its basic shape and rigidity  It is harder than bone because of its higher content of calcium salts (70% of dry weight). TEETH  The dentin of the crown is covered by enamel, which consists primarily of calcium phosphate and calcium carbonate.  Enamel is also harder than bone because of its even higher content of calcium salts (about 95% of dry weight)  In fact, enamel is the hardest substance in the body.  serves to protect the tooth from the wear and tear of chewing  It also protects against acids that can easily dissolve dentin TEETH  The dentin of the root is covered by cementum, another bonelike substance, which attaches the root to the periodontal ligament.  The dentin of a tooth encloses a space  The enlarged part of the space, the pulp cavity, lies within the crown and is filled with pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels TEETH  Narrow extensions of the pulp cavity, called root canals, run through the root of the tooth  Each root canal has an opening at its base, the apical foramen, through which blood vessels, lymphatic vessels, and nerves extend  The blood vessels bring nourishment, the lymphatic vessels offer protection, and the nerves provide sensation. TEETH  The branch of dentistry that is concerned with the prevention, diagnosis, and treatment of diseases that affect the pulp, root, periodontal ligament, and alveolar bone is known as endodontics  Orthodontics is a branch of dentistry that is concerned with the prevention and correction of abnormally aligned teeth  Periodontics is a branch of dentistry concerned with the treatment of abnormal conditions of the tissues immediately surrounding the teeth, such as gingivitis (gum disease) TEETH  Humans have two dentitions, or sets of teeth:  Deciduous - also called primary teeth, milk teeth, or baby teeth— begin to erupt at about 6 months of age, and approximately two teeth appear each month thereafter, until all 20 are present (all will be lost generally by age 6-12 and will be replaced)  Permanent - The permanent dentition contains 32 teeth that erupt between age 6 and adulthood TEETH MECHANICAL AND CHEMICAL DIGESTION IN THE MOUTH  Mechanical digestion in the mouth results from chewing, or mastication in which food is manipulated by the tongue, ground by the teeth, and mixed with saliva  As a result, the food is reduced to a soft, flexible, easily swallowed mass called a bolus  Food molecules begin to dissolve in the water in saliva, an important activity because enzymes can react with food molecules in a liquid medium only  Two enzymes, salivary amylase and lingual lipase, contribute to chemical digestion in the mouth MECHANICAL AND CHEMICAL DIGESTION IN THE MOUTH  Salivary amylase, which is secreted by the salivary glands, initiates the breakdown of starch  Dietary carbohydrates are either monosaccharide and disaccharide sugars or complex polysaccharides such as starches  Most of the carbohydrates we eat are starches, but only monosaccharides can be absorbed into the bloodstream  Thus, ingested disaccharides and starches must be broken down into monosaccharides  The function of salivary amylase is to begin starch digestion by breaking down starch into smaller molecules such as the disaccharide maltose, the trisaccharide maltotriose,and short-chain glucose polymers called -dextrins MECHANICAL AND CHEMICAL DIGESTION IN THE MOUTH  Even though food is usually swallowed too quickly for all the starches to be broken down in the mouth, salivary amylase in the swallowed food continues to act on the starches for about another hour, at which time stomach acids inactivate it  Saliva also contains lingual lipase, which is secreted by lingual glands in the tongue.  This enzyme becomes activated in the acidic environment of the stomach and thus starts to work after food is swallowed  It breaks down dietary triglycerides into fatty acids and diglycerides MECHANICAL AND CHEMICAL DIGESTION IN THE MOUTH PHARYNX  When food is first swallowed, it passes from the mouth into the pharynx  The pharynx is composed of skeletal muscle and lined by mucous membrane, and is divided into three parts: the nasopharynx, the oropharynx, and the laryngopharynx  The nasopharynx functions only in respiration, but both the oropharynx and laryngopharynx have digestive as well as respiratory functions  Swallowed food passes from the mouth into the oropharynx and laryngopharynx; the muscular contractions of these areas help propel food into the esophagus and then into the stomach ESOPHAGUS  The esophagus is a collapsible muscular tube, about 25 cm (10 in.) long, that lies posterior to the trachea  The esophagus begins at the inferior end of the laryngopharynx and passes through the mediastinum anterior to the vertebral column  Then it pierces the diaphragm through an opening called the esophageal hiatus, and ends in the superior portion of the stomach  Sometimes, part of the stomach protrudes above the diaphragm through the esophageal hiatus --termed a hiatus hernia HISTOLOGY OF THE ESOPHAGUS  The mucosa of the esophagus consists of nonkeratinized stratified squamous epithelium, lamina propria (areolar connective tissue), and a muscularis muscosae (smooth muscle)  Near the stomach, the mucosa of the esophagus also contains mucous glands.  The stratified squamous epithelium associated with the lips, mouth, tongue, oropharynx, laryngopharynx, and esophagus affords considerable protection against abrasion and wear-and-tear from food particles that are chewed, mixed with secretions, and swallowed  The submucosa contains areolar connective tissue, blood vessels, and mucous glands HISTOLOGY OF THE ESOPHAGUS  The muscularis of the superior third of the esophagus is skeletal muscle, the intermediate third is skeletal and smooth muscle, and the inferior third is smooth muscle  At each end of the esophagus, the muscularis becomes slightly more prominent and forms two sphincters—the upper esophageal sphincter (UES), which consists of skeletal muscle, and the lower esophageal sphincter (LES), which consists of smooth muscle  The upper esophageal sphincter regulates the movement of food from the pharynx into the esophagus; the lower esophageal sphincter regulates the movement of food from the esophagus into the stomach. HISTOLOGY OF THE ESOPHAGUS  The superficial layer of the esophagus is known as the adventitia, rather than the serosa as in the stomach and intestines  The adventitia attaches the esophagus to surrounding structures. PHYSIOLOGY OF THE ESOPHAGUS  The esophagus secretes mucus and transports food into the stomach. It does not produce digestive enzymes, and it does not carry on absorption. DEGLUTITION  The movement of food from the mouth into the stomach is achieved by the act of swallowing or deglutition  Deglutition is facilitated by the secretion of saliva and mucus and involves the mouth, pharynx, and esophagus.  Swallowing occurs in three stages:  (1) the voluntary stage, in which the bolus is passed into the oropharynx  (2) the pharyngeal stage, the involuntary passage of the bolus through the pharynx into the esophagus  (3) the esophageal stage, the involuntary passage of the bolus through the esophagus into the stomach. DEGLUTITION  Swallowing starts when the bolus is forced to the back of the oral cavity and into the oropharynx by the movement of the tongue upward and backward against the palate  these actions constitute the voluntary stage of swallowing  With the passage of the bolus into the oropharynx, the involuntary pharyngeal stage of swallowing begins  The bolus stimulates receptors in the oropharynx, which send impulses to the deglutition center in the medulla oblongata and lower pons of the brain stem DEGLUTITION  The returning impulses cause the soft palate and uvula to move upward to close off the nasopharynx, which prevents swallowed foods and liquids from entering the nasal cavity  In addition, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the rest of the respiratory tract.  The bolus moves through the oropharynx and the laryngopharynx. Once the upper esophageal sphincter relaxes, the bolus moves into the esophagus DEGLUTITION  The esophageal stage of swallowing begins once the bolus enters the esophagus  During this phase, peristalsis, a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis, pushes the bolus onward  The contractions are repeated in waves that push the food toward the stomach  As the bolus approaches the end of the esophagus, the lower esophageal sphincter relaxes and the bolus moves into the stomach.  Mucus secreted by esophageal glands lubricates the bolus and reduces friction  The passage of solid or semisolid food from the mouth to the stomach takes 4 to 8 seconds; very soft foods and liquids pass through in about 1 second. DEGLUTITION STOMACH  The stomach is a J-shaped enlargement of the GI tract directly inferior to the diaphragm in the epigastric, umbilical, and left hypochondriac regions of the abdomen  The stomach connects the esophagus to the duodenum, the first part of the small intestine STOMACH  Because a meal can be eaten much more quickly than the intestines can digest and absorb it, one of the functions of the stomach is to serve as a mixing chamber and holding reservoir.  At appropriate intervals after food is ingested, the stomach forces a small quantity of material into the first portion of the small intestine  Empty, it is about the size of a large sausage  In the stomach, digestion of starch continues, digestion of proteins and triglycerides begins, the semisolid bolus is converted to a liquid, and certain substances are absorbed. ANATOMY OF THE STOMACH  The stomach has four main regions: the cardia, fundus, body, and pylorus  The cardia surrounds the superior opening of the stomach.  The rounded portion superior to and to the left of the cardia is the fundus  Inferior to the fundus is the large central portion of the stomach called the body  The region of the stomach that connects to the duodenum is the pylorus  ANATOMY OF THE STOMACH  the pylorus has two parts:  Pyloric antrum which connects to the body of the stomach  Pyloric canal, which leads into the duodenum  When the stomach is empty, the mucosa lies in large folds, called rugae  The pylorus communicates with the duodenum of the small intestine via a smooth muscle sphincter called the pyloric sphincter  The concave medial border of the stomach is called the lesser curvature, and the convex lateral border is called the greater curvature HISTOLOGY OF THE STOMACH  The surface of the mucosa is a layer of simple columnar epithelial cells called surface mucous cells  The mucosa contains a lamina propria (areolar connective tissue) and a muscularis mucosae (smooth muscle)  Epithelial cells extend down into the lamina propria, where they form columns of secretory cells called gastric glands.  Several gastric glands open into the bottom of narrow channels called gastric pits  Secretions from several gastric glands flow into each gastric pit and then into the lumen of the stomach. HISTOLOGY OF THE STOMACH  The gastric glands contain three types of exocrine gland cells that secrete their products into the stomach lumen:  mucous neck cells,  chief cells, and  parietal cells  Both surface mucous cells and mucous neck cells secrete mucus  Parietal cells produce intrinsic factor (needed for absorption of vitamin B12) and hydrochloric acid.  The chief cells secrete pepsinogen and gastric lipase.  The secretions of the mucous, parietal, and chief cells form gastric juice, which totals 2000–3000 mL (roughly 2–3 qt.) per day HISTOLOGY OF THE STOMACH HISTOLOGY OF THE STOMACH  In addition, gastric glands include a type of enteroendocrine cell, the G cell, which is located mainly in the pyloric antrum and secretes the hormone gastrin into the bloodstream.  Three additional layers lie deep to the mucosa  The submucosa of the stomach is composed of areolar connective tissue  The muscularis has three layers of smooth muscle (rather than the two found in the esophagus and small and large intestines):  an outer longitudinal layer,  a middle circular layer,  and an inner oblique layer HISTOLOGY OF THE STOMACH  The oblique layer is limited primarily to the body of the stomach  The serosa is composed of simple squamous epithelium (mesothelium) and areolar connective tissue;  the portion of the serosa covering the stomach is part of the visceral peritoneum  At the lesser curvature of the stomach, the visceral peritoneum extends upward to the liver as the lesser omentum  At the greater curvature of the stomach, the visceral peritoneum continues downward as the greater omentum and drapes over the intestines MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  Several minutes after food enters the stomach, gentle, rippling, peristaltic movements called mixing waves pass over the stomach every 15 to 25 seconds  These waves macerate food, mix it with secretions of the gastric glands, and reduce it to a soupy liquid called chyme  Few mixing waves are observed in the fundus, which primarily has a storage function  As digestion proceeds in the stomach, more vigorous mixing waves begin at the body of the stomach and intensify as they reach the pylorus  The pyloric sphincter normally remains almost, but not completely, closed MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  As food reaches the pylorus, each mixing wave periodically forces about 3 mL of chyme into the duodenum through the pyloric sphincter, a phenomenon known as gastric emptying.  Most of the chyme is forced back into the body of the stomach, where mixing continues  The next wave pushes the chyme forward again and forces a little more into the duodenum  These forward and backward movements of the gastric contents are responsible for most mixing in the stomach.  Foods may remain in the fundus for about an hour without becoming mixed with gastric juice  During this time, digestion by salivary amylase continues MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  Soon, however, the churning action mixes chyme with acidic gastric juice, inactivating salivary amylase and activating lingual lipase, which starts to digest triglycerides into fatty acids and diglycerides  Although parietal cells secrete hydrogen ions (H+) and chloride ions (Cl-) separately into the stomach lumen, the net effect is secretion of hydrochloric acid (HCl)  Proton pumps powered by H/K ATPases actively transport H into the lumen whil bringing potassium ions (K) into the cell.  At the same time, Cl and K diffuse out into the lumen through Cl and K channels in the apical membrane. MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  The enzyme carbonic anhydrase, which is especially plentiful in parietal cells, catalyzes the formation of carbonic acid (H2CO3) from water (H2O) and carbon dioxide (CO2).  HCl secretion by parietal cells can be stimulated by several sources:  acetylcholine (ACh) released by parasympathetic neurons,  gastrin secreted by G cells,  and histamine, which is a paracrine substance released by mast cells in the nearby lamina propria.  MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  The strongly acidic fluid of the stomach kills many microbes in food.  HCl partially denatures (unfolds) proteins in food and stimulates the secretion of hormones that promote the flow of bile and pancreatic juice.  Enzymatic digestion of proteins also begins in the stomach.  The only proteolytic (protein-digesting) enzyme in the stomach is pepsin,  Pepsin is most effective in the very acidic environment of the stomach (pH 2); it becomes inactive at a higher pH. QUESTION! What keeps pepsin from digesting the protein in stomach cells along with the food? ANSWER!  First, pepsin is secreted in an inactive form called pepsinogen; in this form, it cannot digest the proteins in the chief cells that produce it.  Pepsinogen is not converted into active pepsin until it comes in contact with hydrochloric acid secreted by parietal cells or active pepsin molecules.  Second, the stomach epithelial cells are protected from gastric juices by a 1–3 mm thick layer of alkaline mucus secreted by surface mucous cells and mucous neck cells. MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  Another enzyme of the stomach is gastric lipase, which splits the short-chain triglycerides in fat molecules (such as those found in milk) into fatty acids and monoglycerides.  This enzyme, which has a limited role in the adult stomach, operates best at a pH of 5–6.  More important than either lingual lipase or gastric lipase is  pancreatic lipase, an enzyme secreted by the pancreas into the small intestine. MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  Only a small amount of nutrients are absorbed in the stomach because its epithelial cells are impermeable to most materials.  However, mucous cells of the stomach absorb some water, ions, and short-chain fatty acids, as well as certain drugs (especially aspirin) and alcohol MECHANICAL AND CHEMICAL DIGESTION IN THE STOMACH  Within 2 to 4 hours after eating a meal, the stomach has emptied its contents into the duodenum.  Foods rich in carbohydrate spend the least time in the stomach; high-protein foods remain somewhat longer, and emptying is slowest after a fat-laden meal containing large amounts of triglycerides. OBJECTIVES  Describe the location, anatomy, histology, and function of the pancreas  Describe the location, anatomy, histology, and functions of the liver and gallbladder  Describe the location, anatomy, histology, and functions of the small intestine  Describe the anatomy, histology, and functions of the large intestine.  Describe the three phases of digestion.  Describe the major hormones that regulate digestive activities.  PANCREAS  a retroperitoneal gland that is about 12– 15 cm (5–6 in.) long and 2.5 cm (1 in.) thick  lies posterior to the greater curvature of the stomach.  The pancreas consists of a head, a body, and a tail and is usually connected to the duodenum by two ducts  The head is the expanded portion of the organ near the curve of the duodenum; superior to and to the left of the head are the central body and the tapering tail. PANCREAS  Pancreatic juices are secreted by exocrine cells into small ducts that ultimately unite to form two larger ducts,  the pancreatic duct and the accessory duct.  The pancreatic duct (duct of Wirsung) is the larger of the two ducts. In most people, the pancreatic duct joins the common bile duct from the liver and gallbladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla (ampulla of Vater).  The ampulla opens on an elevation of the duodenal mucosa known as the major duodenal papilla, which lies about 10 cm (4 in.) inferior to the pyloric sphincter of the stomach.  PANCREAS  The passage of pancreatic juice and bile through the hepatopancreatic ampulla into the small intestine is regulated by a mass of smooth muscle known as the sphincter of the hepatopancreatic ampulla (sphincter of Oddi).  The other major duct of the pancreas, the accessory duct (duct of Santorini), leads from the pancreas and empties into the duodenum about 2.5 cm (1 in.) superior to the hepatopancreatic ampulla. PANCREAS PANCREAS HISTOLOGY OF THE PANCREAS  The pancreas is made up of small clusters of glandular epithelial cells.  About 99% of the clusters, called acini, constitute the exocrine portion of the organ  The cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice.  The remaining 1% of the clusters, called pancreatic islets (islets of Langerhans), form the endocrine portion of the pancreas.  These cells secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide. HISTOLOGY OF THE PANCREAS COMPOSITION AND FUNCTIONS OF PANCREATIC JUICE  Each day the pancreas produces 1200–1500 mL (about 1.2– 1.5 qt) of pancreatic juice, a clear, colorless liquid consisting  mostly of water, some salts, sodium bicarbonate, and several enzymes.  The sodium bicarbonate gives pancreatic juice a slightly alkaline pH (7.1–8.2) that buffers acidic gastric juice in chyme,  stops the action of pepsin from the stomach, and creates the proper pH for the action of digestive enzymes in the small intestine. COMPOSITION AND FUNCTIONS OF PANCREATIC JUICE  The enzymes in pancreatic juice include a starch digesting enzyme called pancreatic amylase; several protein digesting enzymes called  trypsin, chymotrypsin, carboxypeptidase, and elastase;  The principal triglyceride-digesting enzyme in adults, called pancreatic lipase; and nucleic acid– digesting enzymes called ribonuclease and deoxyribonuclease. COMPOSITION AND FUNCTIONS OF PANCREATIC JUICE  The protein-digesting enzymes of the pancreas are produced in an inactive form  Because they are inactive, the enzymes do not digest cells of the pancreas itself.  Trypsin is secreted in an inactive form called trypsinogen  Pancreatic acinar cells also secrete a protein called trypsin inhibitor COMPOSITION AND FUNCTIONS OF PANCREATIC JUICE  Trypsinogen reacts with an activating brush-border enzyme called enterokinase,  which splits off part of the trypsinogen molecule to form trypsin.  In turn, trypsin acts on the inactive precursors (called chymotrypsinogen, procarboxypeptidase, and proelastase)  to produce chymotrypsin, carboxypeptidase, and elastase, respectively. PANCREATITIS  Inflammation of the pancreas, as may occur in association with alcohol abuse or chronic gallstones, PANCREATITIS LIVER AND GALLBLADDER  is the heaviest gland of the body, weighing about 1.4 kg (about 3 lb) in an average adult.  it is second only to the skin in size.  The liver is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity  The gallbladder (gall- bile) is a pear-shaped sac that is located in a depression of the posterior surface of the liver.  It is 7–10 cm (3–4 in.) long and typically hangs from the anterior inferior margin of the liver LIVER AND GALLBLADDER LIVER  The liver is almost completely covered by visceral peritoneum and is completely covered by a dense irregular connective tissue layer that lies deep to the peritoneum.  The liver is divided into two principal lobes—a large right lobe and a smaller left lobe— by the falciform ligament, a fold of the mesentery  Although the right lobe is considered by many anatomists to include an inferior quadrate lobe and a posterior caudate lobe, the quadrate and caudate lobes more appropriately belong to the left lobe. LIVER  The falciform ligament extends from the undersurface of the diaphragm between the two principal lobes  It helpsto suspend the liver in the abdominal cavity.  In the free border of the falciform ligament is the ligamentum teres (round ligament), a remnant of the umbilical vein of the fetus; this fibrous cord extends from the liver to the umbilicus. LIVER AND GALLBLADDER GALLBLADDER  The parts of the gallbladder include the broad fundus, which projects inferiorly beyond the inferior border of the liver; the body, the central portion; and the neck, the tapered portion. The body and neck project superiorly. HISTOLOGY OF THE LIVER AND GALLBLADDER  Hepatocytes  are the major functional cells of the liver and perform a wide array of metabolic, secretory, and endocrine functions.  These are specialized epithelial cells with 5 to 12 sides that make up about 80% of the volume of the liver.  Hepatocytes form complex three-dimensional arrangements called hepatic laminae.  Grooves in the cell membranes between neighboring hepatocytes provide spaces for canaliculi (described next) into which the hepatocytes secrete bile.  Bile, a yellow, brownish, or olive-green liquid secreted by hepatocytes, serves as both an excretory product and a digestive secretion. HEPATOCYTES HISTOLOGY OF THE LIVER AND GALLBLADDER  Bile canaliculi  These are small ducts between hepatocytes that collect bile produced by the hepatocytes.  From bile canaliculi, bile passes into bile ductules and then bile ducts. The bile ducts merge and eventually form the larger right and left hepatic ducts, which unite and exit the liver as the common hepatic duct  The common hepatic duct joins the cystic duct (cystic bladder) from the gallbladder to form the common bile duct.  From here, bile enters the small intestine to participate in digestion. HISTOLOGY OF THE LIVER AND GALLBLADDER HISTOLOGY OF THE LIVER AND GALLBLADDER  Hepatic sinusoids.  These are highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein.  Present in the hepatic sinusoids are fixed phagocytes called stellate reticuloendothelial (Kupffer) cells, which destroy worn-out white and red blood cells, bacteria, and other foreign matter  Together, a bile duct, branch of the hepatic artery, and branch of the hepatic vein are referred to as a portal triad. PORTAL TRIAD HISTOLOGY OF THE LIVER AND GALLBLADDER  The hepatocytes, bile duct system, and hepatic sinusoids can be organized into anatomical and functional units in three different ways:  Hepatic lobule  Portal Lobule  Hepatic Acinus HISTOLOGY OF THE LIVER AND GALLBLADDER HEPATIC LOBULE  For years, anatomists described the hepatic lobule as the functional unit of the liver.  According to this model, each hepatic lobule is shaped like a hexagon (six-sided structure).  left at its center is the central vein, and radiating out from it are rows of hepatocytes and hepatic sinusoids.  Located at three corners of the hexagon is a portal triad. PORTAL LOBULE  This model emphasized the exocrine function of the liver, that is, bile secretion.  Accordingly, the bile duct of a portal triad is taken as the center of the portal lobule.  The portal lobule is triangular in shape and is defined by three imaginary straight lines that connect three central veins that are closest to the portal triad HEPATIC ACINUS  In recent years, the preferred structural and functional unit of the liver is the hepatic acinus.  Each hepatic acinus is an approximately oval mass that includes portions of two neighboring hepatic lobules.  Hepatocytes in the hepatic acinus are arranged in three zones around the short axis, with no sharp boundaries between them HEPATIC ZONES  Cells in zone 1 are closest to the branches of the portal triad and the first to receive incoming oxygen, nutrients, and toxins from incoming blood.  These cells are the first ones to take up glucose and store it as glycogen after a meal and break down glycogen to glucose during fasting.  They are also the first to show morphological changes following bile duct obstruction or exposure to toxic substances.  Zone 1 cells are the last ones to die if circulation is impaired and the first ones to regenerate.  Cells in zone 3 are farthest from branches of the portal triad and are the last to show the effects of bile obstruction or exposure to toxins, the first ones to show the effects of impaired circulation, and the last ones to regenerate.  Zone 3 cells also are the first to show evidence of fat accumulation.  Cells in zone 2 have structural and functional characteristics intermediate between the cells in zones 1 and 3. HISTOLOGY OF THE GALLBLADDER  The mucosa of the gallbladder consists of simple columnar epithelium arranged in rugae resembling those of the stomach.  The wall of the gallbladder lacks a submucosa.  The middle, muscular coat of the wall consists of smooth muscle fibers.  Contraction of the smooth muscle fibers ejects the contents of the gallbladder into the cystic duct.  The gallbladder’s outer coat is the visceral peritoneum.  The functions of the gallbladder are to store and concentrate the bile produced by the liver (up to tenfold) until it is needed in the small intestine. HISTOLOGY OF THE GALLBLADDER BLOOD SUPPLY OF THE LIVER  The liver receives blood from two sources:  From the hepatic artery it obtains oxygenated blood,  and from the hepatic portal vein it receives deoxygenated blood containing newly absorbed nutrients, drugs, and possibly microbes and toxins from the gastrointestinal tract  Branches of both the hepatic artery and the hepatic portal vein carry blood into liver sinusoids, where oxygen, most of the nutrients, and certain toxic substances are taken up by the hepatocytes.  Because blood from the gastrointestinal tract passes through the liver as part of the hepatic portal circulation, the liver is often a site for metastasis of cancer that originates in the GI tract. BLOOD SUPPLY OF THE LIVER ROLE AND COMPOSITION OF BILE  Each day, hepatocytes secrete 800–1000 mL (about 1 qt) of bile,  a yellow, brownish, or olive-green liquid. It has a pH of 7.6–8.6 and consists mostly of water, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions.  The principal bile pigment is bilirubin.  The phagocytosis of aged red blood cells liberates iron, globin, and bilirubin (derived from heme)  The iron and globin are recycled; the bilirubin is secreted into the bile and is eventually broken down in the intestine.  One of its breakdown products—stercobilin—gives feces their normal brown color.  Bile is partially an excretory product and partially a digestive secretion. ROLE AND COMPOSITION OF BILE  Bile salts, which are sodium salts and potassium salts of bile acids (mostly chenodeoxycholic acid and cholic acid),  Play a role in emulsification, the breakdown of large lipid globules into a suspension of small lipid globules.  Bile salts also aid in the absorption of lipids following their digestion.  Although hepatocytes continually release bile, they increase production and secretion when the portal blood contains more bile acids; thus, as digestion and absorption continue in the small intestine, bile release increases. GALLSTONES  If bile contains either insufficient bile salts or lecithin or excessive cholesterol,  The cholesterol may crystallize to form gallstones.  As they grow in size and number, gallstones may cause minimal, intermittent, or complete obstruction to the flow of bile from the gallbladder into the duodenum. FUNCTIONS OF THE LIVER  Carbohydrate metabolism.  The liver is especially important in maintaining a normal blood glucose level.  When blood glucose is low, the liver can break down glycogen to glucose and release the glucose into the bloodstream.  The liver can also convert certain amino acids and lactic acid to glucose, and it can convert other sugars, such as fructose and galactose, into glucose.  When blood glucose is high, as occurs just after eating a meal, the liver converts glucose to glycogen and triglycerides for storage. FUNCTIONS OF THE LIVER  Lipid metabolism.  Hepatocytes store some triglycerides; break down fatty acids to generate ATP; synthesize lipoproteins, which transport fatty acids, triglycerides, and cholesterol to and from body cells; synthesize cholesterol; and use cholesterol to make bile salts. FUNCTIONS OF THE LIVER  Protein metabolism.  Hepatocytes deaminate (remove the amino group, NH2, from) amino acids so that the amino acids can be used for ATP production or converted to carbohydrates or fats.  The resulting toxic ammonia (NH3) is then converted into the much less toxic urea, which is excreted in urine.  Hepatocytes also synthesize most plasma proteins, such as alpha and beta globulins, albumin, prothrombin, and fibrinogen. FUNCTIONS OF THE LIVER  Processing of drugs and hormones.  The liver can detoxify substances such as alcohol and excrete drugs such as penicillin, erythromycin, and sulfonamides into bile.  It can also chemically alter or excrete thyroid hormones and steroid hormones such as estrogens and aldosterone. FUNCTIONS OF THE LIVER  Excretion of bilirubin.  As previously noted, bilirubin, derived from the heme of aged red blood cells, is absorbed by the liver from the blood and secreted into bile.  Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces. FUNCTIONS OF THE LIVER  Synthesis of bile salts.  Bile salts are used in the small intestine for the emulsification and absorption of lipids.  Storage.  In addition to glycogen, the liver is a prime storage site for certain vitamins (A, B12, D, E, and K) and minerals (iron and copper), which are released from the liver when needed elsewhere in the body. FUNCTIONS OF THE LIVER  Phagocytosis.  The stellate reticuloendothelial (Kupffer) cells of the liver phagocytize aged red blood cells, white blood cells, and some bacteria.  Activation of vitamin D. The skin, liver, and kidneys participate in synthesizing the active form of vitamin D. SMALL INTESTINE  begins at the pyloric sphincter of the stomach, coils through the central and inferior part of the abdominal cavity, and eventually opens into the large intestine  Averages 2.5 cm (1 in.) in diameter; its length is about 3 m (10 ft) in a living person and about 6.5 m (21 ft) in a cadaver due to the loss of smooth muscle tone after death  ANATOMY OF THE SMALL INTESTINE  The small intestine is divided into three regions:  The duodenum -the shortest region, is retroperitoneal  It starts at the pyloric sphincter of the stomach and extends about 25 cm (10 in.) until it merges with the jejunum.  Duodenum means “12”; it is so named because it is about as long as the width of 12 fingers  The jejunum is about 1 m (3 ft) long and extends to the ileum.  Jejunum means “empty,” which is how it is found at death.  The final and longest region of the small intestine, the ileum  measures about 2 m (6 ft) and joins the large intestine at a smooth muscle sphincter called the ileocecal sphincter HISTOLOGY OF THE SMALL INTESTINE  The wall of the small intestine is composed of the same four layers that make up most of the GI tract: mucosa, submucosa, muscularis, and serosa  The mucosa is composed of a layer of epithelium, lamina propria, and muscularis mucosae. HISTOLOGY OF THE SMALL INTESTINE  The epithelial layer of the small intestinal mucosa consists of simple columnar epithelium that contains many types of cells:  Absorptive cells of the epithelium digest and absorb nutrients in small intestinal chyme  Also present in the epithelium are goblet cells, which secrete mucus. Cells lining the crevices form the intestinal glands (crypts of Lieberkühn) and secrete intestinal juice  Paneth cells secrete lysozyme, a bactericidal enzyme, and are capable of phagocytosis. Paneth cells may have a role in regulating the microbial population in the small intestine  Three types of enteroendocrine cells are found in the intestinal glands of the small intestine: S cells, CCK cells, and K cells, which secrete the hormones secretin, cholecystokinin (CCK) and glucose-dependent insulinotropic peptide or GIP, respectively. HISTOLOGY OF THE SMALL INTESTINE  The lamina propria of the small intestinal mucosa contains areolar connective tissue and has an abundance of mucosa-associated lymphoid tissue (MALT).  Solitary lymphatic nodules are most numerous in the distal part of the ileum  Groups of lymphatic nodules referred to as aggregated lymphatic follicles (Peyer’s patches) are also present in the ileum  The muscularis mucosae of the small intestinal mucosa consists of smooth muscle HISTOLOGY OF THE SMALL INTESTINE  The submucosa of the duodenum contains duodenal (Brunner’s) glands, which secrete an alkaline mucus that helps neutralize gastric acid in the chyme  Sometimes the lymphatic tissue of the lamina propria extends through the muscularis mucosae into the submucosa HISTOLOGY OF THE SMALL INTESTINE  The muscularis of the small intestine consists of two layers of smooth muscle:  The outer, thinner layer contains longitudinal fibers;  the inner, thicker layer contains circular fibers.  Except for a major portion of the duodenum, the serosa (or visceral peritoneum) completely surrounds the small intestine. HISTOLOGY OF THE SMALL INTESTINE  Special structural features of the small intestine facilitate the process of digestion and absorption:  Circular folds or plicae circulares are folds of the mucosa and submucosa  These permanent ridges, which are about 10 mm(0.4 in.) long, begin near the proximal portion of the duodenumand end at about the midportion of the ileum  Circular folds enhance absorption by increasing surface area and causing the chyme to spiral, rather than move in a straight line, as it passes through the small intestine. HISTOLOGY OF THE SMALL INTESTINE  Also present in the small intestine are villi ( tufts of hair), which are fingerlike projections of the mucosa that are 0.5–1 mm long  The large number of villi (20–40 per square millimeter) vastly increases the surface area of the epithelium available for absorption and digestion and gives the intestinal mucosa a velvety appearance  Each villus is covered by epithelium and has a core of lamina propria  Embedded in the connective tissue of the lamina propria are an arteriole, a venule, a blood capillary network, and a lacteal, which is a lymphatic capillary  Nutrients absorbed by the epithelial cells covering the villus pass through the wall of a capillary or a lacteal to enter blood or lymph, respectively. HISTOLOGY OF THE SMALL INTESTINE  HISTOLOGY OF THE SMALL INTESTINE  Besides circular folds and villi, the small intestine also has microvilli which are projections of the apical (free) membrane of the absorptive cells  Each microvillus is a 1µm-long cylindrical, membrane-covered projection that contains a bundle of 20–30 actin filaments.  When viewed through a light microscope, the microvilli are too small to be seen individually; instead they form a fuzzy line, called the brush border, extending into the lumen of the small intestine  There are an estimated 200 million microvilli per square millimeter of small intestine ROLE OF INTESTINAL JUICE AND BRUSH-BORDER ENZYMES  About 1–2 liters (1–2 qt) of intestinal juice, a clear yellow fluid, are secreted each day. Intestinal juice contains water and mucus and is slightly alkaline (pH 7.6).  Together, pancreatic and intestinal juices provide a liquid medium that aids the absorption of substances from chyme in the small intestine  The absorptive cells of the small intestine synthesize several digestive enzymes, called brush-border enzymes, and insert them in the plasma membrane of the microvilli  Among the brush-border enzymes are four carbohydrate-digesting enzymes called -dextrinase, maltase, sucrase, and lactase; protein- digesting enzymes called peptidases (aminopeptidase and dipeptidase); and two types of nucleotide-digesting enzymes, nucleosidases and phosphatases MECHANICAL DIGESTION IN THE SMALL INTESTINE  The two types of movements of the small intestine—segmentations and a type of peristalsis called migrating motility complexes—are governed mainly by the myenteric plexus  Segmentations are localized, mixing contractions that occur in portions of intestine distended by a large volume of chyme  Segmentations mix chyme with the digestive juices and bring the particles of food into contact with the mucosa for absorption; they do not push the intestinal contents along the tract.  Segmentations occur most rapidly in the duodenum, about 12 times per minute, and progressively slow to about 8 times per minute in the ileum. This movement is similar to alternately squeezing the middle and then the ends of a capped tube of toothpaste. MECHANICAL DIGESTION IN THE SMALL INTESTINE  After most of a meal has been absorbed, which lessens distension of the wall of the small intestine, segmentation stops and peristalsis begins  The type of peristalsis that occurs in the small intestine, termed a migrating motility complex (MMC), begins in the lower portion of the stomach and pushes chyme forward along a short stretch of small intestine before dying out.  The MMC slowly migrates down the small intestine, reaching the end of the ileum in 90–120 minutes.  Then another MMC begins in the stomach. Altogether, chyme remains in the small intestine for 3–5 hours. CHEMICAL DIGESTION IN THE SMALL INTESTINE  In the mouth, salivary amylase converts starch (a polysaccharide) to maltose (a disaccharide), maltotriose (a trisaccharide), and - dextrins (short-chain, branched fragments of starch with 5–10 glucose units).  In the stomach, pepsin converts proteins to peptides (small fragments of proteins), and lingual and gastric lipases convert some triglycerides into fatty acids, diglycerides, and monoglycerides.  The completion of the digestion of carbohydrates, proteins, and lipids is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine. DIGESTION OF CARBOHYDRATES  Even though the action of salivary amylase may continue in the stomach for a while, the acidic pH of the stomach destroys salivary amylase and ends its activity  Thus, only a few starches are broken down by the time chyme leaves the stomach.  Those starches not already broken down into maltose, maltotriose, and a-dextrins are cleaved by pancreatic amylase, an enzyme in pancreatic juice that acts in the small intestine.  Although pancreatic amylase acts on both glycogen and starches, it has no effect on another polysaccharide called cellulose, an indigestible plant fiber that is commonly referred to as “roughage” as it moves through the digestive system.  DIGESTION OF CARBOHYDRATES  After amylase (either salivary or pancreatic) has split starch into smaller fragments, a brush-border enzyme called -dextrinase acts on the resulting -dextrins, clipping off one glucose unit at a time  Ingested molecules of sucrose, lactose, and maltose—three disaccharides— are not acted on until they reach the small intestine.  Three brush-border enzymes digest the disaccharides into Monosaccharides:  Sucrase breaks sucrose into a molecule of glucose and a molecule of fructose  lactase digests lactose into a molecule of glucose and a molecule of galactose  Maltase splits maltose and maltotriose into two or three molecules of glucose  Digestion of carbohydrates ends with the production of monosaccharides, which the digestive system is able to absorb.  DIGESTION OF PROTEINS  Recall that protein digestion starts in the stomach, where proteins are fragmented into peptides by the action of pepsin.  Enzymes in pancreatic juice—trypsin, chymotrypsin, carboxypeptidase, and elastase—continue to break down proteins into peptides  Trypsin, chymotrypsin, and elastase all cleave the peptide bond between a specific amino acid and its neighbor  Carboxypeptidase splits off the amino acid at the carboxyl end of a peptide  Protein digestion is completed by two peptidases in the brush border: aminopeptidase and dipeptidase  Aminopeptidase cleaves off the amino acid at the amino end of a peptide  Dipeptidase splits dipeptides (two amino acids joined by a peptide bond) into single amino acids DIGESTION OF LIPIDS  The most abundant lipids in the diet are triglycerides, which consist of a molecule of glycerol bonded to three fatty acid molecules  Enzymes that split triglycerides and phospholipids are called lipases  there are three types of lipases that can participate in lipid digestion: lingual lipase, gastric lipase, and pancreatic lipase  Triglycerides are broken down by pancreatic lipase into fatty acids and monoglycerides. DIGESTION OF LIPIDS  Before a large lipid globule containing triglycerides can be digested in the small intestine, it must first undergo emulsification - a process in which the large lipid globule is broken down into several small lipid globules  Bile salts are amphipathic, which means that each bile salt has a hydrophobic (nonpolar) region and a hydrophilic (polar) region.  The amphipathic nature of bile salts allows them to emulsify a large lipid globule  The hydrophobic regions of bile salts interact with the large lipid globule, while the hydrophilic regions of bile salts interact with the watery intestinal chyme DIGESTION OF NUCLEIC ACIDS  Pancreatic juice contains two nucleases: ribonuclease, which digests RNA, and deoxyribonuclease, which digests DNA  The nucleotides that result from the action of the two nucleases are further digested by brush-border enzymes called nucleosidases and phosphatases into pentoses, phosphates, and nitrogenous bases. These products are absorbed via active transport LARGE INTESTINE  The large intestine is the terminal portion of the GI tract  The overall functions of the large intestine are the completion of absorption, the production of certain vitamins, the formation of feces, and the expulsion of feces from the body.  The large intestine, which is about 1.5 m (5 ft) long and 6.5 cm (2.5 in.) in diameter, extends from the ileum to the anus  It is attached to the posterior abdominal wall by its mesocolon, which is a double layer of peritoneum  Structurally, the four major regions of the large intestine are the cecum, colon, rectum, and anal canal ANATOMY OF THE LARGE INTESTINE  The opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine  Hanging inferior to the ileocecal valve is the cecum, a small pouch about 6 cm (2.4 in.) long  Attached to the cecum is a twisted, coiled tube, measuring about 8 cm (3 in.) in length, called the appendix or vermiform appendix  The mesentery of the appendix, called the mesoappendix, attaches the appendix to the inferior part of the mesentery of the ileum. ANATOMY OF THE LARGE INTESTINE  The open end of the cecum merges with a long tube called the colon, which is divided into ascending, transverse, descending, and sigmoid portions.  the ascending colon ascends on the right side of the abdomen, reaches the inferior surface of the liver, and turns abruptly to the left to form the right colic (hepatic) flexure.  The colon continues across the abdomen to the left side as the transverse colon  It curves beneath the inferior end of the spleen on the left side as the left colic (splenic) flexure and passes inferiorly to the level of the iliac crest as the descending colon ANATOMY OF THE LARGE INTESTINE  The sigmoid colon begins near the left iliac crest, projects medially to the midline, and terminates as the rectum at about the level of the third sacral vertebra  The rectum, the last 20 cm (8 in.) of the GI tract, lies anterior to the sacrum and coccyx  The terminal 2–3 cm (1 in.) of the rectum is called the anal canal  The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns that contain a network of arteries and veins  The opening of the anal canal to the exterior, called the anus, is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary) HISTOLOGY OF THE LARGE INTESTINE  The wall of the large intestine contains the typical four layers found in the rest of the GI tract: mucosa, submucosa, muscularis, and serosa  The mucosa consists of simple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle)  The epithelium contains mostly absorptive and goblet cells  The absorptive cells function primarily in water absorption; the goblet cells secrete mucus that lubricates the passage of the colonic contents.  Both absorptive and goblet cells are located in long, straight, tubular intestinal glands (crypts of Lieberkühn) that extend the full thickness of the mucosa HISTOLOGY OF THE LARGE INTESTINE  The submucosa of the large intestine consists of areolar connective tissue.  The muscularis consists of an external layer of longitudinal smooth muscle and an internal layer of circular smooth muscle.  Unlike other parts of the GI tract, portions of the longitudinal muscles are thickened, forming three conspicuous bands called the teniae coli  Tonic contractions of the bands gather the colon into a series of pouches called haustra  Small pouches of visceral peritoneum filled with fat are attached to teniae coli and are called omental (fatty) appendices. HISTOLOGY OF THE LARGE INTESTINE MECHANICAL DIGESTION IN THE LARGE INTESTINE  The passage of chyme from the ileum into the cecum is regulated by the action of the ileocecal sphincter.  Normally, the valve remains partially closed so that the passage of chyme into the cecum usually occurs slowly.  Immediately after a meal, a gastroileal reflex intensifies peristalsis in the ileum and forces any chyme into the cecum.  The hormone gastrin also relaxes the sphincter.  Whenever the cecum is distended, the degree of contraction of the ileocecal sphincter intensifies. MECHANICAL DIGESTION IN THE LARGE INTESTINE  Movements of the colon begin when substances pass the ileocecal sphincter.  The time required for a meal to pass into the colon is determined by gastric emptying time.  As food passes through the ileocecal sphincter, it fills the cecum and accumulates in the ascending colon.  One movement characteristic of the large intestine is haustral churning.  In this process, the haustra remain relaxed and become distended while they fill up. When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum. MECHANICAL DIGESTION IN THE LARGE INTESTINE  Peristalsis also occurs, although at a slower rate (3–12 contractions per minute) than in more proximal portions of the tract.  A final type of movement is mass peristalsis, a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives the contents of the colon into the rectum. Because food in the stomach initiates this gastrocolic reflex in the colon, mass peristalsis usually takes place three or four times a day, during or immediately after a meal. CHEMICAL DIGESTION IN THE LARGE INTESTINE  The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen.  Mucus is secreted by the glands of the large intestine, but no enzymes are secreted.  Chyme is prepared for elimination by the action of bacteria, which ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases.  These gases contribute to flatus (gas) in the colon, termed flatulence when it is excessive. CHEMICAL DIGESTION IN THE LARGE INTESTINE  Bacteria also convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, hydrogen sulfide, and fatty acids.  Some of the indole and skatole is eliminated in the feces and contributes to their odor; the rest is absorbed and transported to the liver, where these compounds are converted to less toxic compounds and excreted in the urine.  Bacteria also decompose bilirubin to simpler pigments, including stercobilin, which gives feces their brown color.  Bacterial products that are absorbed in the colon include several vitamins needed for normal metabolism, among them some B vitamins and vitamin K. ABSORPTION AND FECES FORMATION IN THE LARGE INTESTINE  By the time chyme has remained in the large intestine 3–10 hours, it has become solid or semisolid because of water absorption and is now called feces.  Chemically, feces consist of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the gastrointestinal tract, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.  Although 90% of all water absorption occurs in the small intestine, the large intestine absorbs enough to make it an important organ in maintaining the body’s water balance.  Of the 0.5–1.0 liter of water that enters the large intestine, all but about 100– 200 mL is normally absorbed via osmosis.  The large intestine also absorbs ions, including sodium and chloride, and some vitamins. THE DEFECATION REFLEX  Mass peristaltic movements push fecal material from the sigmoid colon into the rectum.  The resulting distension of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that empties the rectum.  The defecation reflex occurs as follows:  In response to distension of the rectal wall, the receptors send sensory nerve impulses to the sacral spinal cord.  Motor impulses from the cord travel along parasympathetic nerves back to the descending colon, sigmoid colon, rectum, and anus.  The resulting contraction of the longitudinal rectal muscles shortens the rectum, thereby increasing the pressure within it. THE DEFECATION REFLEX  This pressure, along with voluntary contractions of the diaphragm and abdominal muscles, plus parasympathetic stimulation, opens the internal anal sphincter.  The external anal sphincter is voluntarily controlled.  If it is voluntarily relaxed, defecation occurs and the feces are expelled through the anus; if it is voluntarily constricted, defecation can be postponed.  Voluntary contractions of the diaphragm and abdominal muscles aid defecation by increasing the pressure within the abdomen, which pushes the walls of the sigmoid colon and rectum inward. THE DEFECATION REFLEX  In infants, the defecation reflex causes automatic emptying of the rectum because voluntary control of the external anal sphincter has not yet developed.  The amount of bowel movements that a person has over a given period of time depends on various factors such as diet, health, and stress.  The normal range of bowel activity varies from two or three bowel movements per day to three or four bowel movements per week. PHASES OF DIGESTION  Digestive activities occur in three overlapping phases:  the cephalic phase,  the gastric phase,  and the intestinal phase. CEPHALIC PHASE  During the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem.  The brain stem then activates the facial (VII), glossopharyngeal (IX), and vagus (X) nerves.  The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva, while the vagus nerves stimulate the gastric glands to secrete gastric juice.  The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten. GASTRIC PHASE  Once food reaches the stomach, the gastric phase of digestion begins.  Neural and hormonal mechanisms regulate the gastric phase of digestion to promote gastric secretion and gastric motility.  Neural regulation. Food of any kind distends the stomach and stimulates stretch receptors in its walls.  Chemoreceptors in the stomach monitor the pH of the stomach chyme. GASTRIC PHASE  When the stomach walls are distended or pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated, and a neural negative feedback loop is set in motion  From the stretch receptors and chemoreceptors, nerve impulses propagate to the submucosal plexus, where they activate parasympathetic and enteric neurons.  The resulting nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands. GASTRIC PHASE  The peristaltic waves mix the food with gastric juice; when the waves become strong enough, a small quantity of chyme undergoes gastric emptying into the duodenum.  The pH of the stomach chyme decreases (becomes more acidic) and the distension of the stomach walls lessens because chyme has passed into the small intestine, suppressing secretion of gastric juice INTESTINAL PHASE  Phase of digestion begins once food enters the small intestine.  In contrast to reflexes initiated during the cephalic and gastric phases, which stimulate stomach secretory activity and motility, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach.  This prevents the duodenum from being overloaded with more chyme than it can handle. In addition, responses occurring during the intestinal phase promote the continued digestion of foods that have reached the small intestine.  These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms. INTESTINAL PHASE  Neural regulation.  Distension of the duodenum by the presence of chyme causes the enterogastric reflex.  Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach.  As a result, gastric motility is inhibited and there is an increase in the contraction of the pyloric sphincter, which decreases gastric emptying. INTESTINAL PHASE  Hormonal regulation.  The intestinal phase of digestion is mediated by two major hormones secreted by the small intestine: cholecystokinin and secretin.  Cholecystokinin (CCK) is secreted by the CCK cells of the small intestinal crypts of Lieberkühn in response to chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides.  CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes.  It also causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder into the cystic duct and through the common bile duct.  In addition, CCK causes relaxation of the sphincter of the hepatopancreatic ampulla (sphincter of Oddi), which allows pancreatic juice and bile to flow into the duodenum. INTESTINAL PHASE  Produces satiety (a feeling of fullness) by acting on the hypothalamus in the brain, promotes normal growth and maintenance of the pancreas, and enhances the effects of secretin.  Acidic chyme entering the duodenum stimulates the release of secretin from the S cells of the small intestinal crypts of Lieberkühn.  Secretin stimulates the flow of pancreatic juice that is rich in bicarbonate (HCO3) ions to buffer the acidic chyme that enters the duodenum from the small intestine. OTHER HORMONES OF THE DIGESTIVE SYSTEM  They include motilin, substance P, and bombesin, which stimulate motility of the intestines;  Vasoactive intestinal polypeptide (VIP), which stimulates secretion of ions and water by the intestines and inhibits gastric acid secretion;  Gastrin-releasing peptide, which stimulates release of gastrin;  Somatostatin, which inhibits gastrin release. PHASES OF DIGESTION  END Thank you for your time!