ANAT Week 11.docx

Week 11 **Module: Digestive System 1 - Introduction** ============================================= **Peritoneum** -------------- ***LO4: Describe the peritoneum and its main folds*** In the video on the development of the digestive system, we also saw how the digestive organs are related to a s...

Week 11 **Module: Digestive System 1 - Introduction** ============================================= **Peritoneum** -------------- ***LO4: Describe the peritoneum and its main folds*** In the video on the development of the digestive system, we also saw how the digestive organs are related to a structure called the peritoneum. This is the serous membrane lining the abdominopelvic cavity, which is where most of the digestive organs are located. Recall from the Skin module, where the concept of body membranes was introduced, that a serous membrane has an outer parietal layer lining the internal surface of the body cavity and an inner visceral layer lining the external surface of some of the organs in the cavity, and that in between these two layers is a potential space containing a thin film of fluid called serous fluid that is produced by the serous membrane. Like any serous membrane, the peritoneum has these two layers with a potential space in between them, which in this case is called the peritoneal cavity. Click on the hotspots on the image below to learn more about the layers of the peritoneum. The peritoneal cavity can be divided into two spaces called the greater sac and the lesser sac. As we saw in the video, these two spaces are formed as a result of changes in the dorsal and ventral mesenteries of the primitive gut tube and the development of certain digestive organs within them. Another important concept that was addressed in the video is that of peritoneal folds. As the peritoneum reflects from the walls of the abdominopelvic cavity (parietal peritoneum) onto the surface of the organs within this cavity (visceral peritoneum), it forms folds which support some of the digestive organs. These folds are often collectively referred to as mesenteries and include the mesentery proper, greater omentum, lesser omentum and mesocolon. As we saw in the video, it is the greater and lesser omenta that develop from the dorsal and ventral mesenteries of the primitive gut tube, respectively. Click on the hotspots on the image below to learn more about each of these peritoneal folds. **Intraperitoneal vs Retroperitoneal Digestive Organs** ------------------------------------------------------- ***LO5: Define the terms intraperitoneal and retroperitoneal and name the digestive organs that are intraperitoneal and retroperitoneal*** So far in this module, we have looked at how the digestive organs can be classified based on the part of the digestive system they belong to and the part of the primitive gut tube they are derived from. A third way to classify the digestive organs is based on their relationship to the peritoneum. Other organs in the abdominopelvic cavity can also be classified in this way. Organs that are completely surrounded by visceral peritoneum are referred to as intraperitoneal ('intra' = within), while organs that are only covered by parietal peritoneum on their anterior surfaces are referred to as retroperitoneal ('retro' = behind). It is the intraperitoneal organs that are associated with peritoneal folds like the ones we have just looked at. Click on each of the cards below to find out which digestive organs are classified as intraperitoneal and which are classified as retroperitoneal. In subsequent modules, we will learn more about these organs and re-visit their classifications as either intraperitoneal or retroperitoneal. Intraperitoneal Digestive Organs A useful mnemonic that you can use to remember the retroperitoneal organs is **SAD PUCKER**. The digestive organs in this mnemonic have been bolded and we will come across the others in the modules on other body systems. - - - - - - - - - **Functions of the Digestive System** ------------------------------------- ***LO6: Describe the basic functions of the digestive system*** Now that we have looked at the basics of the organisation, development and location of the digestive system, we are going to look at its basic functions. The digestive system has six main functions, which are ingestion, motility, secretion, digestion, absorption and elimination of wastes. Click on the hotspots below to learn more about these six functions. **Knowledge Check** ------------------- Now let's see what you've learnt! Have a go at classifying the digestive organs below based on the part of the digestive system they belong to, the part of the primitive gut tube they are derived from and their relationship to the peritoneum. **Module: Digestive System 2 - Gastrointestinal Tract** ======================================================= **Large Intestine** ------------------- ***LO7: Describe the location, function, gross anatomy and histology of the stomach, small intestine and large intestine*** The final organ of the GI tract is the large intestine, which is the tube that continues from the small intestine and forms a border around it in the abdominopelvic cavity. It is called the "large" intestine as its diameter is greater than that of the small intestine. Its functions are to absorb most of the water from the remaining digested material that has not been absorbed in the small intestine and compact any undigested material and waste products into faeces, which are then defecated. It spans several abdominopelvic quadrants and has both intraperitoneal and retroperitoneal parts. The large intestine consists of several parts, which are the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. The ascending, transverse, descending and sigmoid colons are the four segments of what is collectively referred to as the colon. Click on the hotspots on the image below to learn more about these parts Now we are going to look at the histology of the large intestine. Like the oesophagus, stomach and small intestine, some of the layers in the wall of the large intestine have variations that are directly related to its functions. Click on the hotspots below to learn more about the composition of the layers in the wall of the large intestine where variations are present. **Blood Supply** ---------------- ***LO8: Describe the blood supply of the stomach, small intestine and large intestine and link this to their primitive gut tube origins*** Now that we have looked at the stomach, small intestine and large intestine individually, we are going to look at their blood supply and link this to their primitive gut tube origins. Recall from the Digestive System 1 module that the stomach and the first part of the duodenum are derived from the foregut, the rest of the duodenum, jejunum, ileum, caecum (and vermiform appendix), ascending colon and proximal 2/3 of the transverse colon are derived from the midgut, and the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and superior part of the anal canal are derived from the hindgut. Recall from the Cardiovascular System 2 module that the digestive organs derived from the foregut are supplied by the celiac trunk, those derived from the midgut are supplied by the superior mesenteric artery and those derived from the hindgut are supplied by the inferior mesenteric artery, which are the three unpaired branches of the abdominal aorta. Therefore, the blood supply of the stomach and the first part of the duodenum is from the celiac trunk, the blood supply of most of the duodenum up to and including the proximal 2/3 of the transverse colon is from the superior mesenteric artery and the blood supply of the distal 1/3 of the transverse colon up to and including the superior part of the anal canal is from the inferior mesenteric artery. The inferior part of the anal canal is supplied by another artery which you do not need to know. **Module: Digestive System 3 - Accessory Digestive Organs** **Learning Outcomes** --------------------- **By the end of this module, you should be able to:** **LO1: Describe the structure of a tooth and the composition of a full set of permanent teeth** **LO2: Describe the location, function and gross anatomy of the tongue** **LO3: Describe the function of saliva and the location and drainage of the three pairs of major salivary glands** **LO4: Revise the division of the abdominopelvic cavity into quadrants and the peritoneal relationships and primitive gut tube origins of the liver, gallbladder and pancreas** **LO5: Describe the location, function, gross anatomy and histology of the liver** **LO6: Describe the dual blood supply of the liver and link its arterial supply to its primitive gut tube origin** **LO7: Describe the location and function of the gallbladder** **LO8: Describe the biliary apparatus and how bile and pancreatic juice are drained into the duodenum** **LO9: Describe the location, function, gross anatomy and histology of the pancreas** **LO10: Describe the blood supply of the gallbladder and pancreas and link this to their primitive gut tube origins** **Parts of the Digestive System** --------------------------------- **In the Digestive System 1 module, the two main parts of the digestive system were introduced -- the gastrointestinal or GI tract and the accessory digestive organs. In the Digestive System 2 module, we focused on the GI tract. In this module, we are going to focus on the accessory digestive organs. Recall from the Digestive System 1 module that the accessory digestive organs are found along the path of the GI tract and are connected to it, allowing them to assist in the digestion of ingested material. They include the teeth, tongue, salivary glands, liver, gallbladder and pancreas. In this module, we are going to look at each of these organs individually.** **Teeth** --------- ***LO1: Describe the structure of a tooth and the composition of a full set of permanent teeth*** **We are going to start with the teeth, which are accessory digestive organs located within the oral cavity. The teeth are collectively referred to as the dentition. They are responsible for beginning the mechanical digestion of ingested material in the oral cavity through the process of mastication, or chewing. A tooth has three main parts, which are the crown, neck and root. The crown is the exposed part of a tooth. Recall from the Articular System 1 module that the root is the part of a tooth that forms fibrous joints called gomphoses with the sockets of the mandible or maxillae. The neck is the narrow part between the crown and the root and is covered by the gums, or gingivae.** **Humans will develop two sets of teeth during their lifetime. The first set contains 20 teeth that are referred to as deciduous teeth, also known as milk or baby teeth. These are then gradually replaced by permanent teeth, with a full set of permanent teeth in an adult containing 32 teeth. There are four different types of permanent teeth, which are incisors, canines, premolars and molars. These are each shaped differently to allow for the mechanical digestion of ingested material in different ways (e.g. cutting, tearing, grinding). In each quadrant of the mouth, a full set of permanent teeth contains 2 incisors, 1 canine, 2 premolars and 3 molars, with the third molars being the wisdom teeth. This adds to 8 teeth per quadrant of the mouth and therefore 32 teeth in total.** **Tongue** ---------- ***LO2: Describe the location, function and gross anatomy of the tongue*** **Now we are going to look at the tongue, which is another accessory digestive organ located within the oral cavity. The tongue is primarily formed by skeletal muscles, which can be divided into extrinsic and intrinsic muscles. The extrinsic muscles move the tongue, while the intrinsic muscles change its shape. This allows the tongue to mix ingested material with saliva in the oral cavity and compress it against the palate to form a bolus. It then pushes the bolus towards the oropharynx to be swallowed. As well as digestion, the tongue also plays a role in sound production.** **The tongue has two parts, which are an oral part and a pharyngeal part. The oral part of the tongue makes up the anterior 2/3 and is the part that sits on the floor of the oral cavity within the oral cavity proper. It has many small projections called papillae (singular = papilla; 'papilla' = nipple-like projection) on its superior or dorsal surface that aid in gripping ingested material and some also contain taste buds and touch receptors. It is anchored inferiorly to the floor of the oral cavity via a fold of the mucosa lining the oral cavity called the lingual frenulum ('lingual' = tongue), which limits its movement. The pharyngeal part of the tongue makes up the posterior 1/3 and extends into the oropharynx, forming its anterior wall. It has many lymphatic nodules which form an aggregation called the lingual tonsil. Like the palatine tonsils, the lingual tonsil acts as a first line of defence against any foreign substances that may be inhaled or ingested.** **Salivary Glands** ------------------- ***LO3: Describe the function of saliva and the location and drainage of the three pairs of major salivary glands*** **Now we are going to look at the salivary glands, which are accessory digestive organs located within and around the oral cavity that drain their secretions into it. The salivary glands produce and secrete saliva, which assists with the initial stages of digestion by moistening ingested material and mixing with it to form a bolus, as well as beginning the process of chemical digestion through the production of enzymes. Saliva also dissolves the molecules in ingested material, allowing for the stimulation of taste buds on the tongue. In addition, saliva lubricates the oral cavity and has antibacterial properties that help to maintain oral hygiene.** **The salivary glands can be classified as either major or minor salivary glands, with most saliva being produced by the major salivary glands. There are three pairs of major salivary glands, which are the parotid, submandibular and sublingual glands.** **Click on the hotspots on the image below to learn more about these three pairs of major salivary glands.** **Abdominopelvic Quadrants, Peritoneal Relationship and Primitive Gut Tube Origin** ----------------------------------------------------------------------------------- ***LO4: Revise the division of the abdominopelvic cavity into quadrants and the peritoneal relationships and primitve gut tube origins of the liver, gallbladder and pancreas*** **The remaining accessory digestive organs are the liver, gallbladder and pancreas, all of which are located in the abdominopelvic cavity. Therefore, similar to what we did in the Digestive System 2 module with the GI tract organs, we are going to place each of the remaining accessory digestive organs in the abdominopelvic quadrant(s) in which they are located, as well as re-visit their classification as either intraperitoneal or retroperitoneal and the part of the primitive gut tube they are derived from, looking at how the latter is associated with their blood supply.** **Liver** --------- ***LO5: Describe the location, function, gross anatomy and histology of the liver*** **We are going to start with the liver, which is the largest gland in the body. The liver is an intraperitoneal organ located primarily in the right upper quadrant of the abdominopelvic cavity, extending into the left upper quadrant, sitting just underneath the diaphragm. Only a small area of the liver called the bare area is not covered by peritoneum. It has several important functions, including the production of a fluid called bile to assist with the chemical digestion of lipids, the detoxification of the blood, the processing, storage and release of nutrients and vitamins from the blood, as well as the synthesis of blood plasma proteins.** **The liver has two surfaces, which are an anterosuperior surface called the diaphragmatic surface and a posteroinferior surface called the visceral surface. The diaphragmatic surface sits in contact with the diaphragm, while the visceral surface bears the impressions of several organs that are in contact with the liver ('viscera' = organs). The liver consists of four lobes, which are the right, left, caudate and quadrate lobes. These lobes are anatomical divisions, but the liver can also be divided functionally into right and left halves (note that these do not directly correspond to the right and left lobes). The right and left lobes are the two major lobes and are separated on the diaphragmatic surface of the liver by a peritoneal fold called the falciform ligament, which anchors the liver to the anterior abdominal wall. The right lobe is then further subdivided into the caudate and quadrate lobes, which can be seen on the visceral surface of the liver. This surface also has a central region called the porta hepatis in between the caudate and quadrate lobes.** **Click on the hotspots on the image below to learn more about these parts of the liver.** **Now we are going to look at the histology of the liver. The liver is surrounded by a dense irregular connective tissue capsule that forms septa, which divide the liver into discrete units called hepatic lobules ('hepatic' = liver). These lobules form the basic structural and functional units of the liver. Each lobule is polyhedral in shape, with a central vein in the middle and several portal triads at the periphery in the corners. Within the lobules are hepatocytes, hepatic sinusoids and bile canaliculi.** **Click on the hotspots on the image below to learn more about these hepatic lobule structures.** **Blood Supply of the Liver** ----------------------------- ***LO6: Describe the dual blood supply of the liver and link its arterial supply to its primitive gut tube origin*** **Now we are going to look at the blood supply of the liver. The liver has a dual blood supply, meaning that it receives blood from two sources. The two blood vessels that carry blood to the liver enter the liver at the porta hepatis and as we have already seen, these two blood vessels are the hepatic artery proper and the portal vein.** **Click on the hotspots on the image below to learn more about these two blood vessels.** **The numbers on the image below show the path that blood takes into, through and out of the liver.** **Click on the hotspots next to these numbers to follow this path of blood** **It is important to note that the flow of blood through the liver is in the opposite direction to the flow of bile through the liver. Blood enters the liver at the porta hepatis and then travels through the hepatic lobules towards the central vein, while bile is produced by the hepatocytes within the hepatic lobules and then travels out of the hepatic lobules and ultimately out of the liver at the porta hepatis. We will look more closely at the flow of bile out of the liver later in this module.** **The diagram below is a simple summary of the blood supply of the liver.** **Gallbladder** --------------- ***LO7: Describe the location and function of the gallbladder*** **Now that we have looked at the liver, we are going to look at the gallbladder, which is a sac-like intraperitoneal organ located in the right upper quadrant of the abdominopelvic cavity. As we have already seen, it sits in a depression, or fossa, next to the quadrate lobe on the visceral surface of the liver. The gallbladder functions to store and concentrate the bile produced in the liver until it is ready to be drained into the GI tract, specifically the duodenum. Once drained into the duodenum, the bile assists with the chemical digestion of lipids by emulsifying large lipid globules, breaking them down into smaller pieces so that enzymes can more easily break them down further. The duct that carries bile to and from the gallbladder is called the cystic duct.** **Biliary Apparatus** --------------------- ***LO8: Describe the biliary apparatus and how bile and pancreatic juice are drained into the duodenum*** **Now that we have looked at the liver and gallbladder, we are going to look at how bile is carried from these two accessory digestive organs to drain into the duodenum. The bile is carried by a network of ducts that is referred to as the biliary apparatus. As we have already seen, bile is produced by the hepatocytes within the hepatic lobules in the liver. It is carried by bile canaliculi located in between the hepatocytes to the bile ductules in the portal triads at the periphery of the hepatic lobules. The bile ductules merge to form bile ducts, which merge to form the left and right hepatic ducts, marking the beginning of the network of ducts referred to as the biliary apparatus.** **The numbers on the image below show how the ducts within the biliary apparatus are formed, the path that bile takes through these ducts and how bile drains together with secretions from the pancreas into the duodenum.** **Click on the hotspots next to these numbers to follow this path of bile and pancreatic secretions.** **Pancreas** ------------ ***LO9: Describe the location, function, gross anatomy and histology of the pancreas*** **The final accessory digestive organ that we are going to look at is the pancreas, which is a mixed gland with both endocrine and exocrine functions. We have already looked at the endocrine function of the pancreas in the Endocrine System module. In this module, we are going to focus on the exocrine function of the pancreas, which is to produce and secrete digestive enzymes to assist with the chemical digestion of ingested material in the GI tract, as well as bicarbonate to neutralise the acidity of chyme from the stomach. The digestive enzymes and bicarbonate are collectively referred to as pancreatic juice. The pancreas is located primarily in the left upper quadrant of the abdominopelvic cavity, posterior to the stomach, although extends into the right upper quadrant where it is related to the duodenum. It is mostly retroperitoneal, although a small region called the tail is intraperitoneal.** **The pancreas has four main regions, which are the head, neck, body and tail.** **Click on the hotspots on the image below to learn more about these four regions.** **Now we are going to look at the histology of the pancreas, specifically the exocrine portion of the pancreas, as we have already looked at the histology of its endocrine portion in the Endocrine System module. The exocrine portion of the pancreas makes up the majority of the pancreatic tissue. Recall from the Epithelium module that exocrine glands contain secretory portions called acini, which are clusters of secretory cells called acinar cells, and conducting portions called ducts, which drain the gland of its secretions. In the pancreas, the acinar cells are modified simple cuboidal epithelial cells that produce the digestive enzymes in the pancreatic juice. The ducts are lined with simple cuboidal epithelial cells that produce the bicarbonate in the pancreatic juice. The pancreatic juice containing digestive enzymes and bicarbonate is carried by these small ducts, which merge to form larger ducts. The largest of these ducts is called the main pancreatic duct, which drains most of the pancreatic juice from the pancreas into the duodenum. As we have already seen, this duct merges with the common bile duct to form the hepatopancreatic ampulla, which drains into the duodenum via the major duodenal papilla. A small amount of pancreatic juice is drained into the duodenum by a smaller duct called the accessory pancreatic duct. Recall from the Digestive System 2 module that superior to the major duodenal papilla in the wall of the duodenum is another smaller opening called the minor duodenal papilla. It is via this opening that the accessory pancreatic duct drains into the duodenum.** **Blood Supply of the Gallbladder and Pancreas** ------------------------------------------------ ***LO10: Describe the blood supply of the gallbladder and pancreas and link this to their primitive gut tube origins*** **Now that we have looked at the gallbladder and pancreas individually, we are going to look at their blood supply and link this to their primitive gut tube origins. Recall from the Digestive System 1 module that the gallbladder and pancreas are derived from the foregut. Therefore, their blood supply is from the celiac trunk.**