ANAT Week 10.docx

Week 10 **Module: Lymphatic System** **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the main components and describe the basic functions of the lymphatic system **LO2**: Describe the flow of lymph through the body **LO3**: Describe t...

Week 10 **Module: Lymphatic System** **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the main components and describe the basic functions of the lymphatic system **LO2**: Describe the flow of lymph through the body **LO3**: Describe the location, structure and function of the different types of lymphatic vessels **LO4**: Describe the location, structure and function of the major lymphoid tissues and organs **Components and Functions** ---------------------------- ***LO1: Name the main components and describe the basic functions of the lymphatic system*** The lymphatic system consists of lymph, lymphatic vessels and lymphoid tissues and organs. The lymphoid tissues and organs include lymphatic nodules, lymph nodes, the thymus and the spleen. The lymphatic system components are widely spread throughout the body. Only some body regions, such as bone marrow, the teeth and the epidermis of the skin, do not appear to have lymphatics. The lymphatic system plays crucial roles in tissue fluid homeostasis to maintain blood volume, immune surveillance, and the transport of dietary lipids from the small intestine into the blood. This body system is critical in a clinical context, particularly given that it is a major route for cancer metastasis and that the inflammation of lymphatic vessels and lymph nodes is an indicator of pathology. Click on the hotspots below to learn more about the functions of the lymphatic system. **Lymph Flow** -------------- ***LO2: Describe the flow of lymph through the body*** Now we are going to look at the flow of lymph through the body. The image below demonstrates the structural relationship between the blood vascular system and the lymphatic vascular system. Unlike the closed loop formed by the blood vascular system, the lymphatic vascular system is a unidirectional transport system designed to return excess interstitial fluid to the venous circulation, which ultimately drains into the right atrium of the heart. This elaborate system of vessels originates within the interstitial spaces as lymphatic plexuses, or networks, made of lymphatic capillaries. The capillaries merge into larger collecting lymphatic vessels, which then merge into larger lymphatic trunks. The trunks then converge to form two large lymphatic ducts, which drain into large veins at the base of the neck. Therefore, this is a one-way system where lymph flows only in one direction: from the periphery towards the heart. Now, let's follow the path that lymph takes through the body step-by-step. Click on the hotspots next to the numbers on the image below to follow this path. Now, let's look at these vascular components of the lymphatic system in more detail. **Lymphatic Vessels** --------------------- ***LO3: Describe the location, structure and function of the different types of lymphatic vessels*** ### **Lymphatic capillaries** Lymphatic capillaries are the smallest lymphatic vessels. They form lymphatic plexuses that are interspersed amongst the blood capillary beds in the tissues of the body, as seen in image 'a' below. Note that lymphatic capillaries are larger in diameter than the blood capillaries. Image 'b' shows the structure of lymphatic capillaries. They are closed-ended tubes that are similar in structure to blood capillaries in that their walls are composed of a single layer of endothelium, although they lack a basement membrane. The endothelial cells are attached to surrounding structures by anchoring filaments that prevent these thin-walled vessels from collapsing. Note how the overlapping endothelial cells in the walls of lymphatic capillaries create flap-like mini valves that are easily opened. These are one-way valves that prevent fluid from leaving lymphatic capillaries once it enters them from the interstitial spaces. In addition, the lymphatic capillary walls don't have tight junctions between individual endothelial cells. Therefore, large molecules such as plasma proteins, cellular debris, microorganisms and even whole cells (e.g. lymphocytes) can pass into these vessels from the surrounding tissues. There are also specialised types of lymphatic capillaries located exclusively in the villi of the small intestine. Note that there are also collecting lymphatic vessels in a structure called the mesentery, which anchors the small intestine to the posterior abdominal wall. The mucosa of the small intestine is specialised for maximising digestion and absorption of nutrients. Therefore, it is folded to form microscopic finger-like projections called villi that increase the surface area. The lymphatic capillaries inside the villi are called **lacteals**. These absorb dietary lipids and lipid-soluble vitamins, as these are too large to be absorbed directly into the blood. Due to its content, the lymph in the lacteals has a milky appearance and is known as **chyle**. We will look at the villi of the small intestine in more detail in another module on the digestive system. ### **Collecting lymphatic vessels** Collecting lymphatic vessels (collectively called **lymphatics**) are formed by the union of lymphatic capillaries, meaning they have a larger diameter than lymphatic capillaries. The pattern of collecting lymphatic vessels is more variable and complex, but generally parallel, to that of blood vessels. Lymphatics spread widely throughout the body and form two sets of vessels: superficial and deep. The superficial lymphatic vessels are numerous and they follow the superficial veins within the subcutaneous tissue. The superficial lymphatic vessels drain into the deep lymphatic vessels ('from superficial to deep'). The deep lymphatic vessels accompany deep arteries and receive lymph from the internal organs. Collecting lymphatic vessels are punctuated at intervals by small lymphoid organs called **lymph nodes**. These structures remove foreign materials such as infectious microorganisms from the lymph filtering through them. Therefore, they play a role in infection and malignancy. The lymphatic vessels that carry unfiltered lymph from the body tissues to the lymph nodes are called **afferent** lymphatic vessels ('afferent' = 'towards'). The lymphatic vessels that carry filtered lymph from lymph nodes to subsequent lymph nodes and lymphatic vessels further upstream are called **efferent** lymphatic vessels ('efferent' = 'away'). The efferent lymphatic vessels exit the lymph node via a region called the hilum. The various efferent lymphatic vessels converge to form larger lymphatic vessels as they course proximally. On its way through the system of collecting lymphatic vessels, lymph is filtered through a series of lymph nodes. The structure of collecting lymphatic vessels is similar to small veins in that their walls are composed of three layers or tunics (tunica intima, tunica media and tunica externa) and they have valves. The bulging valves give collecting lymphatic vessels a beaded appearance. Smooth muscle in the walls of collecting lymphatic vessels creates peristaltic waves, ensuring the unidirectional flow of lymph. This one-way flow of lymph is also supported by the actions of muscles, respiratory pressure changes, and the pressure gradient towards the veins into which the largest lymphatic vessels drain. ### **Lymphatic trunks** The collecting lymphatic vessels eventually drain into the lymphatic trunks. There are several lymphatic trunks that are strategically positioned to receive lymph from specific body regions. The paired trunks are the jugular, subclavian, bronchomediastinal and lumbar trunks. Some textbooks also include the intercostal trunks in this group. There is also one unpaired lymphatic trunk called the intestinal trunk. Note that the names of the trunks reflect the body regions from which they receive lymph. Click on the hotspots on the image below to learn more about these important lymphatic trunks. ### **Lymphatic ducts** The lymphatic trunks eventually converge to form two major lymphatic channels: the right lymphatic duct (on the right side of the body) and the thoracic duct (on the left side of the body). These two ducts are the largest lymphatic vessels of the body. They return lymph to the venous circulation by emptying it into the junction of the internal jugular vein and subclavian vein on their respective sides of the body. The union of the internal jugular vein and subclavian vein to form the brachiocephalic vein on either side of the body is called the **venous angle**. There is a substantial difference in the areas of the body drained by the two lymphatic ducts. The right lymphatic duct drains lymph from a significantly smaller part of the body: the right upper limb and the right side of the head, neck and thorax (the so-called 'right upper quadrant of the body'). Lymph from the rest of the body drains into the thoracic duct, which originates as a sac-like structure called the cisterna chyli. Click on the hotspots on the image to learn more about these structures. **Lymphoid Tissues and Organs** ------------------------------- ***LO4: Describe the location, structure and function of the major lymphoid tissues and organs*** Now we are going to look at the lymphoid tissues and organs of the body, which are the lymphoid nodules and lymphoid structures such as lymph nodes, the thymus and the spleen. As lymphoid tissues and organs are sites where lymphocytes are concentrated, they can be divided into primary and secondary: - - Since we have just discussed the lymphatic vessels and mentioned the lymph nodes located along them, let's start by having a closer look at these structures. ### **Lymph nodes** Lymph nodes are the principle lymphoid organs in the body. These small, oval or bean-shaped structures located along the path of collecting lymphatic vessels, typically occurring in clusters. The function of lymph nodes is to filter lymph and help generate an immune response against antigens. ***Structure of lymph nodes*** Lymph nodes range in size from 1 to 2 cm in length. Their stroma is mostly composed of reticular connective tissue. Each lymph node is surrounded by a connective tissue capsule, which sends trabeculae into the node radiating towards the centre, thus dividing it into regions. The trabeculae provide a path for blood vessels and nerves to enter and exit the lymph node. Each region of the lymph node has an outer cortex and an inner medulla. The cortex mostly contains lymphocytes that act in immune responses. The medulla contains lymphocytes and macrophages that deal with antigens and enhance immune responses. ***Circulation through lymph nodes*** Lymph enters the nodes via multiple afferent lymphatic vessels and exits via fewer efferent lymphatic vessels. Therefore, lymph flow stagnates within the node, allowing time for circulation. Inside the node, lymph flows through a series of spaces called lymphatic sinuses that are present in both the cortex (called cortical sinuses) and the medulla (called medullary sinuses). Filtering lymph through the sinuses allows for exposure of lymphoid cells such as T- and B-lymphocytes and lymphoid macrophages to a wide range of antigens, and thus allow them to carry out their protective functions. ***Groups of lymph nodes*** Lymph nodes are organised in clusters and like the collecting lymphatic vessels they can be found along, they can be divided into superficial and deep groups. Some examples of lymph node groups are shown on the third image below. Note that the names of the groups represent their anatomical locations. Like the drainage of collecting lymphatic vessels, lymph from the superficial lymph nodes drains into the deep lymph nodes ('from superficial to deep'). Sometimes, lymph nodes become inflamed, swollen and painful to touch, indicating inflammation or infection in a particular body region/organ. Lymph nodes are the initial site of metastasis in many cancers, especially when cancer cells enter the lymphatic system. In this case, lymph nodes become swollen but they are usually not painful. Therefore, knowledge of lymphatic drainage of organs and body regions has significant clinical applications. Lymph nodes are one example of lymphoid organs. Now let's look at the thymus, an essential lymphoid organ involved in immunity. ### **Thymus** The thymus is an incredible organ because it reaches its maturity in utero and degenerates as we age. The thymus is **large in infants and young children**. As we age, it becomes smaller and loses its organised structure while being replaced by adipose connective tissue. **In adults**, it is very small and mostly consists of adipose connective tissue, making it practically non-functional. Therefore, the thymus has a variety of shapes and sizes, even in the same individual. **In young children**, the thymus is located in the inferior neck and extends into the anterior part of the mediastinum where it partially overlies the heart. Most of the thymus is found directly posterior to the sternum, covering the origins of the great vessels. Macroscopically, the thymus is composed of two fused lobes that are each surrounded by a connective tissue capsule. During youth, the thymus has an important function as it is the site of maturation of a type of lymphocyte called T-lymphocytes. These are leukocytes, or white blood cells, responsible for fighting infections. Most thymic cells are T-lymphocytes at varying stages of maturation. Mature T-lymphocytes defend the body against antigens in an immune response. In addition to its immune functions, the thymus produces an array of hormones. Some of these, like thymulin and thymosin, regulate immune cell production. The thymus also synthesizes hormones such as insulin and melatonin. Thus, this organ also has endocrine functions. ### **Spleen** The spleen is the largest lymphoid organ in the body. It is soft, highly vascular and dark purple in colour. Although structurally similar to a lymph node, the spleen filters blood rather than lymph. One of its main functions is to bring blood into contact with lymphocytes so that antigens detected in the blood can be destroyed. The spleen provides a site for lymphocyte proliferation and immune functions, as well as storing platelets (which assist with blood clotting) and erythrocytes, or red blood cells. It also destroys aged or defective platelets and erythrocytes, as well as blood-borne pathogens. In the foetus, the spleen acts as a haematopoietic site (i.e. a site of blood cell formation). The size and shape of the spleen is variable (average 3 x 7 x 12 cm). It is a wedge-shaped organ located in a region of the abdominopelvic cavity called the left upper quadrant (we will look at abdominopelvic quadrants more closely in future modules), sitting directly underneath the left dome of the diaphragm. It lies posterolateral to the stomach and lateral to the pancreas, where it is related to a part of the pancreas called the tail (we will look at the pancreas more closely in another module). The large intestine is inferior to the spleen, and medially is the left kidney. The spleen has two surfaces, which are a posterolateral surface called the diaphragmatic surface and an anteromedial surface called the visceral surface. The diaphragmatic surface is in contact with the diaphragm and lies against the left lower ribs. The visceral surface bears the impressions of several abdominal organs that are in immediate contact with the spleen ('viscera' = organs). The visceral surface also has a region called the hilum, where blood vessels and nerves enter and exit the spleen. ### **Lymphoid nodules** Unlike lymphoid organs such as lymph nodes, the thymus and the spleen that are encapsulated, lymphoid nodules are unencapsulated masses of lymphoid tissue. They contain lymphoid cells which attack antigens. While lymphoid nodules are small on their own, in some areas of the body they form large aggregations. Two examples of these aggregations are mucosa-associated lymphoid tissue (MALT) and the tonsils. In the mucous membranes of the respiratory, digestive, urinary and reproductive tracts, the aggregations of lymphatic nodules are collectively referred to as **mucosa-associated lymphoid tissue (MALT)**. This is particularly prominent in the small intestine, mainly in its distal part called the ileum, where the aggregations are called Peyer's patches. A microscopic view of Peyer's patches is shown in image 'a' below. Another site of lymphoid nodule aggregation is the wall of the appendix, a small tubular pouch connected to the proximal part of the large intestine called the caecum. Appendicitis is inflammation of the appendix, which most often occurs in people between the ages of 10 and 30. **Tonsils** are often considered the simplest lymphoid organs. These aggregations of lymphoid nodules are found at the entrances of the respiratory and digestive tracts, as seen in image 'b' below. They form a protective ring that prevents antigens from penetrating the mucosal lining of these tracts. They appear as swellings of the mucosa and they are named according to their locations: - - - - Tonsillitis is inflammation of the tonsils and is an indication of an active immune response to infection. We will look at both Peyer's patches and tonsils more closely in future modules. **Module: Respiratory System 1 - Upper Respiratory Tract** ========================================================== **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the structural and functional divisions and the basic functions of the respiratory system **LO2**: Describe the features of respiratory and olfactory epithelium **LO3**: Describe the location, function and gross anatmoy of the nose, nasal cavity and paranasal sinuses **LO4**: Describe the location, function, gross anatomy and histology of the pharynx and larynx **Structural and Functional Divisions** --------------------------------------- ***LO1: Describe the structural and functional divisions and the basic functions of the respiratory system*** ### **Structrural Divisions** The respiratory system is composed of the nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi and the progressively smaller airways within the lungs, as well as the lungs themselves. Collectively, the nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi and progressively smaller airways within the lungs are referred to as the respiratory tract. Structurally, the respiratory tract can be divided into the upper respiratory tract and the lower respiratory tract. Click on each of the cards below to find out which structures are included in these two divisions. **Upper Respiratory Tract** **- Nose** **- Nasal cavity** **- Paranasal sinuses** **- Pharynx** **- Larynx** **Lower Respiratory Tract** **- Trachea** **- Bronchi** **- Progressively smaller airways within the lungs** **In this module, we are going to focus on the upper respiratory tract.** ### **Functional Divisions** **The respiratory system can also be divided functionally into a conductive portion and a respiratory portion.** **Click on each of the cards below to learn more about these two portions and the structures within each.** **Conductive Portion** **The conductive portion of the respiratory system consists of the respiratory passageways which transmit air into and out of the lungs. Thus, it is responsible for conducting air to the sites of gas exchange. It includes the nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi and the progressively smaller airways within the lungs up to and including the terminal bronchioles.** **Respiratory Portion** **The respiratory portion of the respiratory system is where gas exchange occurs between the air in the lungs and the blood. It includes the respiratory bronchioles, alveolar ducts and alveolar sacs, all of which contain alveoli.** **In this module, all the structures we will be looking at are part of the conductive portion of the respiratory system.** **Functions** ------------- ***LO1: Describe the structural and functional divisions and the basic functions of the respiratory system*** **Before we focus on the upper respiratory tract, we are going to look at the functions of the respiratory system as a whole. The primary function of the respiratory system is respiration, which a multi-step process by which oxygen is supplied to the body and carbon dioxide is removed from the body. However, the respiratory system also has several other functions, which are gas conditioning, sound production, olfaction and defence.** **Click on the hotspots below to learn more about these functions.** **Click on the card below to learn a little fun fact about how oxygen levels are measured in the blood.** **Pulse Oximetry** **How does pulse oximetry work?** **Pulse oximetres work by detecting changes in the colour of haemoglobin molecules in red blood cells, depending on how much oxygen is being carried by the cell. The more oxygenated blood takes on a brighter appearance.** **Now that we have introduced the overall structure and function of the respiratory system, you may like to watch the video below to summarise this.** **Epithelium of the Respiratory Tract** --------------------------------------- ***LO2: Describe the features of respiratory and olfactory epithelium*** **As we looked at the functions of the respiratory system, two types of epithelium were mentioned -- respiratory epithelium and olfactory epithelium. These are part of the mucous membrane lining the respiratory tract -- recall from the Skin module, where the concept of body membranes was introduced, that a mucous membrane, or mucosa, is composed of an epithelial layer and an underlying layer of areolar connective tissue called the lamina propria. Most of the conductive portion of the respiratory tract is lined by respiratory epithelium. However, the superior part of the nasal cavity is lined by olfactory epithelium.** **Click on each of the cards below to learn more about these two types of epithelium.** **Respiratory Epithelium** **Respiratory epithelium is a pseudostratified ciliated columnar epithelium. Recall from the Epithelium module that this type of epithelium contains goblet cells scattered amongst the epithelial cells, which produce mucous that traps any foreign substances that are inhaled. The cilia on the apical surfaces of the epithelial cells help to move the mucous and any trapped substances towards the pharynx, so that it can either be coughed up and spat out or swallowed. Recall from the Cells and Basic Tissues modules that cilia are elongated, hair-like, motile projections on the surface of some cells that function to move substances along a passage.** **Olfactory Epithelium** **Olfactory epithelium is similar to respiratory epithelium, but it is specialised for olfaction, or the sense of smell. It contains three different types of cells, which are olfactory receptor cells, supporting cells and basal cells. The olfactory receptor cells are bipolar neurons that detect odorant molecules. Recall that bipolar neurons have one dendrite and one axon extending from the cell body. Projecting from the dendrites of the olfactory receptor cells are many non-motile cilia called olfactory hairs. These are embedded within a layer of mucous covering the surface of the olfactory epithelium that is produced by glands in the lamina propria underlying the epithelium called olfactory or Bowman's glands. This mucous traps and dissolves odorant molecules, which are then detected by receptors within the olfactory hairs. The axons of the olfactory receptor cells form the olfactory nerves, which pass through the olfactory foramina in the cribriform plate of the ethmoid bone at the roof of the nasal cavity. The supporting cells in olfactory epithelium provide support and nourishment to the olfactory receptor cells and the basal cells in olfactory epithelium are stem cells that regenerate to replace the olfactory receptor cells.** **To consolidate the structure of a mucous membrane, have a go at labelling the image below** **Click on the card below to learn a little fun fact about why your nose runs on a cold day.** **Curious Minds: Runny Noses** **Have you ever noticed when it is a really cold day, your nose starts to run with a clear, watery mucous?** **This occurs due to the inhaled cold air being exposed to the warm areas of the nasal cavity. This causes water condensation that mixes with the mucous. The cilia area responsible for sweeping that mucous away, but when it is really cold the cilica can become chilled and slightly paralysed, so they have a hard time sweeping the mucous back into the nasopharynx, hence why that watery mucous runs out of the nose.** **Nose, Nasal Cavity and Paranasal Sinuses** -------------------------------------------- ***LO3: Describe the location, function and gross anatomy of the nose, nasal cavity and paranasal sinuses*** **Now we are going to look at the structures of the upper respiratory tract individually, starting with the nose, nasal cavity and paranasal sinuses.** ### **Nose** **The nose is the main conducting airway for inhaled air. The structures of the nose can be divided into the external nose and the internal nasal cavity. For the purpose of this module, when we refer to the "nose", we are referring to the external nose. The nose is located in the centre of the face and is the only externally visible part of the respiratory system. It has three main parts, which are the bridge, dorsum and apex. The opening into the nose is formed by paired nostrils, or nares (singular = naris).** **Click on the hotspots on the image below to learn more about the three parts of the nose.** ### **Nasal cavity** **The nasal cavity is located within and posterior to the nose. Its main functions are the conditioning (i.e. warming, humidifying and filtering) of inhaled air and olfaction. It also contributes to sound production by acting as a resonance chamber.** **The nasal cavity is divided by a structure called the nasal septum into right and left nasal cavities. The most anterior region of the nasal cavity that lies immediately internal to the nares is called the nasal vestibule. It is continuous with the nasal cavity proper that opens posteriorly into the nasopharynx (a part of the pharynx, which we will look at later in this module). If considering the nasal cavity as an elongated box, its boundaries can be described as the roof, floor, anterior boundary, posterior boundary, medial wall and two lateral walls. These are described below -- as you read these descriptions, find the relevant structures on the images below.** - - - - - - **Click on the hotspots on the image below to learn more about some of these features of the nasal cavity.** ### **Paranasal sinuses** **The paranasal sinuses are extensions of the nasal cavity into the surrounding bones of the skull ('para' = around) that communicate with the nasal cavity via their openings into the meatuses. Therefore, they are lined by the same type of epithelium as most of the nasal cavity (i.e. respiratory epithelium). Recall from the Skeletal System 1 module that a sinus is a cavity within a bone. Like the nasal cavity, the paranasal sinuses function to warm and humidify inhaled air and act as resonance chambers to contribute to sound production. They also decrease the weight of the skull. There are four pairs of paranasal sinuses that are named according to the bones in which they are found. These are the frontal, maxillary, ethmoidal and sphenoidal sinuses** **Pharynx** ----------- ***LO4: Describe the location, function, gross anatomy and histology of the pharynx and larynx*** **The next structure of the upper respiratory tract is the pharynx. More commonly referred to as the throat, the pharynx is a funnel-shaped, shared passageway for air and ingested material. This tube-like, muscular structure is located posterior to, and opens into, the nasal cavity, oral cavity and larynx. Please note on the image below that the pharynx is located immediately anterior to the cervical spine. It begins posterior to the nasal cavity and ends as it continues with the oesophagus, which is a digestive organ (we will look at this in another module). The lower boundary of the pharynx coincides with the lower border of a laryngeal cartilage called the cricoid cartilage, which marks the end of the larynx (we will look at the larynx later in this module). The pharyngeal muscles are skeletal muscles and are involved in swallowing.** **The pharynx has three parts, which from superior to inferior are the nasopharynx, oropharynx and laryngopharynx. These three parts are named according to the cavity or structure located immediately anterior to them. The oropharynx and laryngopharynx transmit both air and ingested material, while the nasopharynx transmits air only.** **Click on the hotspots on the image below to learn more about the three parts of the pharynx.** **Click on the card below to learn a little fun fact about why your ears pop on an aeroplane.** **Curious Minds: Popping Ears** **Have you ever wondered why your ears pop during rapid gain or loss of altitude, like travelling in a fast lift or on an aeroplane?** **This occurs due to the difference in pressure between the atmosphere and the inner ear. When gaining altitude, the pressure in the inner ear becomes greater than the atmospheric pressure, straining the eardrum. When swallowing or yawning, the Eustachian tube opens up, allowing the pressure in the inner ear to equalise, producing a popping sound as the strain on the eardrum is released.** **Larynx** ---------- ***LO4: Describe the location, function, gross anatomy and histology of the pharynx and larynx*** **The final structure of the upper respiratory tract is the larynx. More commonly referred to as the voice box, the larynx is a small airway suspended from the hyoid bone in the neck, located immediately anterior to the laryngopharynx and superior to the trachea. The opening of the laryngopharynx into the larynx is called the laryngeal inlet. In addition to serving as a passageway for air, the larynx is the main structure responsible for sound production, as it contains the vocal cords. It also prevents ingested materials from entering the trachea and ultimately the airways in the lungs, assists in increasing intra-abdominal pressure during the Valsalva maneuver and plays a role in the sneeze and cough reflexes.** **The larynx is composed of a framework of cartilages that are held together by ligaments and supported by muscles. There are nine cartilages in total, including three single cartilages and three paired cartilages. The single cartilages are the thyroid cartilage, cricoid cartilage and epiglottis and the paired cartilages are the arytenoid, corniculate and cuneiform cartilages. The thyroid, cricoid and arytenoid cartilages are made of hyaline cartilage, while the epiglottis, corniculate and cuneiform cartilages are made of elastic cartilage.** **Click on the hotspots on the image below to learn more about these cartilages.** **As we have already mentioned, the larynx contains the vocal cords. These are more accurately referred to as the vocal folds, as they are formed by ligaments covered in a mucous membrane. There is also another set of folds in the larynx called the vestibular folds.** **Click on the hotspots on the image below to learn more about the vocal and vestibular folds.** ### **Voice and Speech Production** **Now that we have looked at the larynx, we are going to look briefly at voice and speech production. This involves several parts of the body working together, including the larynx.** **Voice is generated by airflow from the lungs. When air from the lungs flows through the air passages, it passes the vocal folds in the larynx at a high speed. When we speak, the air pressure below the larynx increases until it pushes the vocal folds apart. The vocal folds vibrate and these vibrations lead to the sounds we call voice. These sounds are shaped to form what we call speech. To produce speech, the vocal folds must vibrate normally as air flows past them from the lungs and reaches the mouth and nose. Coordinated movements, mainly by the tongue and lips, produce the recognisablesounds called speech.** **Please watch the short video below to learn more about how we produce voice and speech.** **Module: Respiratory System 2 - Lower Respiratory Tract** ========================================================== **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the location, function, gross anatomy and histology of the trachea, bronchial tree and respiratory portion of the respiratory system **LO2**: Describe the structure and function of the pleura **LO3**: Describe the location and gross anatomy of the lungs **LO4**: Describe the blood supply of the lungs **Structural and Functional Divisions** --------------------------------------- In the Respiratory System 1 module, the structural and functional divisions of the respiratory system were introduced and we focused on the upper respiratory tract. In this module, we are going to focus on the lower respiratory tract, which includes structures that are part of both the conductive and respiratory portions of the respiratory system. **Trachea** ----------- ***LO1: Describe the location, function, gross anatomy and histology of the trachea, bronchial tree and respiratory portion of the respiratory system*** The first structure of the lower respiratory tract is the trachea, which is commonly referred to as the windpipe. It is a part of the conductive portion of the respiratory system. The function of the trachea is to conduct air from the larynx into the main bronchi, which enter the lungs. The trachea is a semi-rigid, tubular airway that continues from the larynx. It travels inferiorly through the neck and the posterior part of the mediastinum in the thoracic cavity. Recall from the Cardiovascular System 1 module that the mediastinum is the central space in the thoracic cavity between the two lungs. Along its course, the trachea lies immediately anterior to the oesophagus. At the level of the sternal angle, the trachea bifurcates (or divides into two -- 'bi'= two) into the right and left main or primary bronchi. Recall from the Skeletal System 2 module that the sternal angle corresponds to the manubriosternal joint. This is usually located at the level of the intervertebral disc between the fourth and fifth thoracic vertebrae. The point of bifurcation of the trachea is called the carina, which is a cartilaginous ridge between the two primary bronchi. The mucous membrane of the carina is the most sensitive area of the trachea that is involved in the cough reflex. The trachea is composed of a framework of 15-20 C-shaped cartilages called tracheal cartilages connected by ligaments, which provide some rigidity to its wall to ensure that it remains open. Posteriorly, the gaps in the C-shaped tracheal cartilages are bridged by a fibromuscular membrane containing a muscle called the trachealis muscle. This allows for the expansion of the oesophagus as ingested material passes through it. Contraction of the trachealis muscle also decreases the diameter of the trachea, causing expired air to rush upward from the lungs with greater force. Now we are going to look at the histology of the trachea. The wall of the trachea is composed of four layers -- from deep to superficial these are the mucosa, submucosa, media and adventitia. Click on the hotspots on the image below to learn more about these four layers. **Bronchial Tree** ------------------ ***LO1: Describe the location, function, gross anatomy and histology of the trachea, bronchial tree and respiratory portion of the respiratory system*** As we have already seen, the trachea bifurcates into the right and left main or primary bronchi at the level of the sternal angle. These then enter the lungs and continue to branch, forming the progressively smaller airways within the lungs. Collectively, this system of airways is referred to as the bronchial tree, as its highly branched structure resembles the branching of a tree. Before we look at the bronchial tree as a whole, we are going to focus on the right and left main or primary bronchi, which mark the beginning of the bronchial tree and enter the right and left lungs, respectively. Compared to the left main bronchus, the right main bronchus is wider, shorter and more vertical. This is clinically significant, as it means that any foreign object that is inhaled is more likely to become lodged on the right side. After entering the lungs, the main or primary bronchi divide into lobar or secondary bronchi. The right main bronchus divides into three lobar bronchi and the left main bronchus divides into two lobar bronchi, which as we will see later in this module, corresponds to the number of lobes in each lung. The right main bronchus actually gives off one of its three lobar bronchi before it enters the right lung and then both of these bronchi enter the lung together. The lobar or secondary bronchi then divide into segmental or tertiary bronchi. Each segmental bronchus will supply a part of the lung called a bronchopulmonary segment ('broncho' = bronchus, 'pulmonary' = lungs) and the number of segmental bronchi will correspond to the number of bronchopulmonary segments in each lung. From here, the tertiary or segmental bronchi divide into smaller bronchi, which divide into smaller airways called bronchioles, which continue to divide and become progressively smaller. The final divisions of the bronchioles are called terminal bronchioles and these mark the end of the conductive portion of the respiratory system. As the bronchial tree divides and the airways become progressively smaller, several changes occur in the histological composition of their walls. These changes are directly related to the functions of the airways as they transition from conducting airways towards the sites of gas exchange. - - - **Respiratory Portion of the Respiratory System** ------------------------------------------------- ***LO1: Describe the location, function, gross anatomy and histology of the trachea, bronchial tree and respiratory portion of the respiratory system*** Now that we have finished looking at the conductive portion of the respiratory system, we are going to look at the respiratory portion of the respiratory system, which is the final part of the bronchial tree and the lower respiratory tract. Recall from the Respiratory System 1 module that this is where gas exchange occurs between the air in the lungs and the blood and it includes the respiratory bronchioles, alveolar ducts and alveolar sacs, all of which contain alveoli. The terminal bronchioles, which mark the end of the conductive portion of the respiratory system, divide into the respiratory bronchioles, which mark the beginning of the respiratory portion of the respiratory system. The respiratory bronchioles then continue to divide and become progressively smaller, until they divide into very small airways called alveolar ducts. Each alveolar duct terminates as a dilated structure called an alveolar sac. The respiratory bronchioles, alveolar ducts and alveolar sacs all contain small, sac-like outpocketings called alveoli (singular = alveolus), which are the functional units of the lungs where gas exchange occurs between the air in the lungs and the blood. Each lung contains around 300-400 million alveoli and they are each surrounded by tiny blood vessels called pulmonary capillaries. As the alveoli are involved in gas exchange, they are lined by simple squamous epithelium which facilitates the diffusion of gases across their walls. The simple squamous epithelial cells form the majority of the cells lining the alveoli and are referred to as alveolar type I cells, but there are also other cuboidal-shaped cells in the alveolar walls referred to as alveolar type II cells that produce and secrete a fluid called surfactant, which is important for reducing surface tension and preventing alveolar collapse. In addition, cells called alveolar macrophages are found either within the alveolar walls or free within the alveoli themselves. Recall from the Connective Tissue module that macrophages are phagocytic cells, meaning they engulf anything that is foreign or damaged. In the lungs, the alveolar macrophages engulf any pathogens or particles that have been inhaled and reached the alveoli. **Pleura** ---------- ***LO2: Describe the structure and function of the pleura*** Now that we have looked at the bronchial tree and the respiratory portion of the respiratory system, we are going to look at the lungs, which are the organs that house these structures. However, before we look at the lungs themselves, we are going to look at the serous membrane surrounding the lungs, which is called the pleura. Recall from the Skin module, where the concept of body membranes was introduced, that a serous membrane has an outer parietal layer and an inner visceral layer, with a potential space in between them containing a thin film of fluid called serous fluid that is produced by the serous membrane. Like any serous membrane, the pleura has these two layers with a potential space in between them, which in this case is called the pleural cavity. Click on the hotspots on the image below to learn more about the layers of the pleura. **Lungs** --------- ***LO3: Describe the location and cross anatomy of the lungs*** Now we are going to look at the gross anatomy of the lungs themselves. The lungs are paired, spongy organs located in the cavities on either side of the mediastinum in the thoracic cavity. Each lung has an apex, base, costal surface and medial surface. On the medial surface, there is a central region called the hilum, where structures enter and exit the lung. This is also referred to as the root of the lung. Click on the hotspots on the image below to learn more about these features. Now that we have looked at the general structure of the lungs, we are going to look at the right and left lungs individually, as there are some differences between the two. Both lungs are divided by deep grooves called fissures into sections called lobes. However, the right lung has three lobes and two fissures, while the left lung only has two lobes and one fissure. Click on each of the cards below to learn more about the structure of the right and left lungs. **Right Lung** **The right lung has three lobes, which are the superior lobe, middle lobe and inferior lobe. These are separated by two fissures -- the horizontal fissure separates the superior and middle lobes and the oblique fissure separates the middle and inferior lobes.** **Left Lung** **The left lung has two lobes, which are the superior lobe and inferior lobe. These are separated by a single oblique fissure. As the heart points towards the left, the left lung also has an indentation on its anterior border called the cardiac notch ('cardiac' = heart).** **Blood Supply of the Lungs** ----------------------------- ***LO4: Describe the blood supply of the lungs*** **Now we are going to look at the blood supply of the lungs. Recall from the Cardiovascular System 1 module that the pulmonary trunk, which emerges from the right ventricle of the heart, divides into the right and left pulmonary arteries which bring deoxygenated blood to the right and left lungs, respectively, to be oxygenated. These arteries enter the lungs at the hilum and then continue to branch until they ultimately reach the pulmonary capillaries surrounding the alveoli, which is where gas exchange occurs. These capillaries then feed the newly oxygenated blood into pulmonary venules, which merge to ultimately form the pulmonary veins that exit the lungs at the hilum and return the oxygenated blood to the left atrium of the heart. This pulmonary circulation is integral for gas exchange between the air in the lungs and the blood.** **However, it is important to understand that the pulmonary arteries are not the only arteries bringing blood to the lungs. Recall from the Cardiovascular System 2 module that the thoracic aorta gives rise to many branches, including visceral branches to supply structures within the thoracic cavity such as the bronchi and lungs. The branches that supply blood to the bronchi and lungs are called the bronchial arteries. These arteries bring high-pressure oxygenated blood to not only the structures of the bronchial tree, but also to the supporting structures of the lungs including the pulmonary arteries. Bronchial veins then drain deoxygenated blood from these structures into a venous system in the thoracic cavity, which ultimately drains into the superior vena cava. The blood in the bronchial circulation does not take part in gas exchange.** **Summary** ----------- **Now that we have finished looking at the respiratory system, you may like to watch the two short videos below. The first video looks at the process of gas exchange and highlights the link between the respiratory system and the cardiovascular system. The second video provides a summary of the respiratory system overall (please note that in this video, thedetails regarding blood pH and the regulation of breathing are not examinable).**