 This video will cover the following objective for the digestive system. Describe the gross structure and histological organization of the esophagus, stomach, duodenum, dejunum, ilium, liver, pancreas, and colon. The esophagus is a muscular tube extending from the upper esophageal sphincter which is a circular ring of muscle that guards the entrance into the esophagus from the laryngeal pharynx in the cervical region. The esophagus extends inferiorly down through the mediastinum of the thoracic cavity and then into the abdominal cavity through the diaphragm at the esophageal hiatus and opening in the diaphragm where the esophagus enters into the abdominal cavity to attach to the stomach. Lower esophageal sphincter is another ring of muscle that's guarding the exit of the esophagus and preventing acid reflux which is when the contractions of the stomach force gastric juice to move through the lower esophageal sphincter back into the esophagus and burn the inner lining of the esophagus causing a painful sensation commonly known as heartburn. The esophagus has an outer layer of adventitia. The adventitia consists of mostly fibrous dense irregular connective tissue providing structural support in anchoring the esophagus to the surrounding tissues. Deep under the adventitia is the muscularis externa. The proximal third of the esophagus contains skeletal muscle in the muscularis externa but the distal two-thirds of the esophagus, the majority of the esophagus contains smooth muscle in the muscularis externa. This muscularis externa is responsible for the patterns of contraction that propel the bolus through the esophagus during the esophageal phase of deglutition. Deep to the muscularis externa is the submucosa. The submucosa contains dense irregular connective tissue and that stabilizes the structural framework where large blood vessels and large nerves and lymphatic vessels are traveling through the esophagus. Then deep to the submucosa is the mucosa. The mucosa consists of a layer of loose areolar connective tissue with lots of capillaries that are then supporting the deepest layer of epithelium, a stratified squamous epithelium lines the lumen forming the inner lining of the esophagus. Here at 40x magnification we can see the stratified squamous epithelium forming the inner lining of the mucosa which is then supported by a layer of areolar connective tissue that is then anchored to the dense irregular connective tissue in the submucosa and then surrounding the submucosa is the muscularis externa and then surrounding the muscularis externa is an outer layer and notice adventitia. Here with 100x magnification we can see a little bit better the stratified squamous epithelium that forms that inner layer of the mucosa and then with 400x magnification you can very clearly see the stratified squamous epithelium forming that inner layer of the mucosa and there's a thin border of loose connective tissue that contains capillary supporting the epithelium and then deep to the mucosa layer is some dense irregular connective tissue forming that submucosa. This illustration shows us the general structure of the alimentary canal that is found from the stomach through the small intestine and large intestine there's four major layers the most superficial outer layer is known as the cirrhosa so in contrast to the esophagus where the most superficial layer is the adventitia and the adventitia is a layer of dense irregular connective tissue in the stomach some intestine and large intestine there is a cirrhosa which is a cirrus membrane containing a simple squamous epithelium and a supporting layer of aerial or connective tissue the cirrhosa is also known as the visceral layer of the peritoneal membrane so the peritoneum is the cirrus membrane lining the abdominal cavity and the visceral layer is also known as the cirrhosa and is covering the abdominal organs the mesentery are folds of the peritoneal membrane that connect between the outer parietal layer of the peritoneum and the visceral layer or cirrhosa deep to the cirrhosa is a layer of smooth muscle known as the muscularis externa so sometimes just abbreviated muscularis we like to call it muscularis externa to distinguish it from an inner layer of smooth muscle known as muscularis interna muscularis interna will form the border between the submucosa and mucosal layer but the muscularis externa has two layers of smooth muscle through the small intestine and large intestine an outer longitudinal layer that contracts to decrease the length of the alimentary canal and an inner circular layer that contracts to decrease the diameter of the alimentary canal then there's the my enteric plexus a network of nervous tissue located in the muscularis externa found in between the outer longitudinal and inner circular layers then deep to the muscularis is the submucosal layer so the submucosa is a layer of dense irregular connective tissue it does contain large nerves and blood vessels and the nervous tissue forms the submucosal plexus or the plexus of meisner the network of nervous tissue that's involved in regulating the glands in the submucosa glands that will secrete mucus into the lumen through ducts that extend into the mucosa and so the mucosa is the deepest layer of the alimentary canal the border between the submucosa and the mucosa is the muscularis mucosa or muscularis interna this layer of smooth muscle then forms the boundary with the submucosa and the mucosa just deep to the smooth muscle is a layer of loose areolar connective tissue known as lamina propria where there's lots of capillaries and small lymphatic vessels and then the inner lining of the mucosa is an epithelium a simple columnar epithelium forms the epithelium forming the inner lining in the mucosa lining the lumen the peritoneal membrane is the cirrus membrane lining the abdominal cavity it has two layers the outer parietal peritoneum lines the walls of the abdominal cavity and the deeper layer the visceral layer of the peritoneal membrane lines the surface of the organs inside of the abdominal cavity and when the visceral peritoneum is lining the alimentary canal it's also known as the cirrosa then the extensions or folds of the peritoneum that connect between the parietal and visceral layers are known as the mesentery the stomach has four basic layers to its wall the innermost layer is the mucosa a mucous membrane with a simple columnar epithelium surrounded by a layer of loose areolar connective tissue called the lamina propria then there is a muscularis interna a layer of smooth muscle that forms the border between the mucosa layer and the submucosal layer the submucosal layer is a layer of dense connective tissue just superficial to the mucosa then surrounding the submucosa is the muscularis externa and in the stomach there are three layers of smooth muscle in the muscularis externa the deepest layer is the oblique layer then surrounding the oblique layer is a circular layer and then the most superficial layer of smooth muscle is the longitudinal layer then superficial to the muscularis externa the outer layer of the stomach is the cirrosa also known as the visceral layer of the peritoneal membrane a cirrus membrane that forms the outer lining of the digestive organs in the abdominal cavity so the regions of the stomach are the cardia of the stomach where the esophagus attaches to the stomach the fundus the bulge that is superior to the cardia the body is the majority of the stomach and then the pylorus as the funnel shaped distal end that attaches to the duodenum pylorus sphincter is a circular shaped ring of muscle that regulates the emptying of the stomach the right side of the stomach is a lesser curvature of the stomach and there is a fold of the peritoneal membrane attaching from the lesser curvature towards the liver which is known as the lesser omentum then the left side of the stomach is the greater curvature and the greater omentum is a mesentery a fold of peritoneal membrane that extends from the greater curvature and drapes anteriorly and inferiorly over the intestines so it drapes down over the anterior of the abdominal cavity and then attaches to the transverse colon the four layers of the wall of the stomach are the outer cirrhosa surrounding the muscularis externa then the submucosa and then the mucosa and while the mucosa of the stomach contains a simple columnar epithelium that epithelium forms inward folds and so these deep inward folds of the mucosa are gastric pits the gastric pits function as exocrine glands there are chief cells within the gastric pits that produce an enzyme called pepsin but it's secreted as an inactive precursor pepsinogen that becomes activated by the hydrochloric acid produced by the parietal cells of the gastric pits the enteroendocrine cells are cells that are producing a hormone known as gastrin that's regulating the gastric secretion of this enzyme-rich acidic gastric juice which is important for chemical digestion of protein inside of the stomach and so as the muscularis externa has three layers in the stomach and can produce forceful contractions known as churning that mix the gastric juice with a bolus to form a semi-liquid mixture known as chym then the stomach will empty the chym through the pyloric sphincter into the duodenum the small intestine has three major segments the duodenum is the short c-shaped proximal region that receives chym from the stomach the duodenum is shown in a purple color here in the illustration then the the middle region the jejunum is around two to three meters long shown with a greenish blue color in the illustration here the jejunum is the primary location where nutrient absorption occurs so while the duodenum is the primary location where chemical digestion of nutrients occurs the duodenum receives the secretions from the pancreas and the liver and gallbladder that are helping with chemical digestion once those nutrients have been fully broken down the jejunum has an enormously high surface area with lots of folds of the mucosa large folds called plica circularis with smaller villi and even smaller micro villi on top of those this enormous surface area enables a high rate of membrane transport for nutrient absorption in the jejunum then the ilium is the longest most distal segment of the small intestine the ilium drains into the colon into the large intestine and the ilium while it's responsible for some nutrient absorption it also contains a large number of pyres patches which are clusters of mucosa associated lymphoid tissue that are responsible for regulating the bacterial environment of the intestines so the histology of the small intestine includes the four basic layers that we see in the elementary canal an outer layer of cirrhosa surrounding a muscularis externa surrounding some mucosal layer and an inner mucosal layer the mucosa forms large folds large circular folds known as the plica circularis and then there are smaller folds known as villi so in the illustration in the center here you can see each villus contains a lymphatic lacteal and blood capillaries that are responsible for nutrient absorption while the blood capillaries absorb water soluble nutrients that lacteals absorb the fat soluble nutrients so the simple columnar epithelium forms this inner line of the mucosa and the cells the columnar cells that form that epithelium have even smaller folds on their surface so there's micro villi which are folds of the plasma membrane of the epithelial cells so the small intestine has folds on top of folds on top of folds lots of folds in order to provide an enormous surface area for membrane transport enabling the absorption of nutrients the illustration on the left here is a low magnification image showing the large folds known as plica circularis then the image in the middle for c shows us the smaller folds the villi and then d shows us an electron microscope image very high magnification so we can see the micro villi the folds of the plasma membrane of the individual epithelial cells so here we can see the histology of the duodenum so the most distinctive feature of the duodenum is there are numerous exocrine glands found in the submucosal layer known as bruner's glands so the bruner's glands produce an alkaline rich mucous which helps to protect the epithelium of the mucosa from the acidity of the chym that's entering from the stomach so here's another view showing us the histology of the duodenum where there are numerous bruner's glands found in the submucosal layer we can also see here there are villi in the mucosal layer large folds of the mucosa and then superficial to the submucosal layer we can see the muscularis externa is smooth muscle organized into an inner circular layer and an outer longitudinal layer and then the outermost layer is the cirrosa a simple squamous epithelium and loose arular connective tissue this picture shows us the histology of the duodenum at low magnification 40 times magnification we can see the large circular fold the pleica circularis with smaller long villi on top of the larger fold so these smaller folds the villi are very long in the duodenum to enable lots of nutrient absorption and so the inner most layer the mucosa has these distinctive large circular folds and long villi in the duodenum then the submucosal layer does not contain bruner's glands in the duodenum there's only a few scattered submucosal glands then we see the thick muscularis externa in the duodenum and then an outer layer of the cirrosa this is the histology of the ilium at 100 times magnification we can still see there are some villi in the ilium and then there are large deep infoldings in the mucosa known as crips that connect in the ilium down into large aggregates of lymphoid tissue these mucosa associated lymphoid tissues are known as the pyres patches that are numerous in the ilium and really give a distinctive histology to the ilium these large clusters of lymphatic tissue found in the submucosal layer the accessory organs or accessory structures of the digestive system include the liver gallbladder and pancreas the liver is located in the right upper quadrant of the abdominal cavity just inferior to the diaphragm protected by the ribcage there's four lobes of the liver the right lobe is the largest lobe and then there's a large left lobe separated from the light the right lobe by a falsiform ligament the falsiform ligament will then be a fold of the peritoneal membrane and mesentery that attaches to the anterior wall of the abdominal cavity there's two smaller lobes of the liver on the inferior and posterior surface you can view the caudate lobe and quadrate lobe so the caudate lobe is just to the right side of the inferior vena cava whereas the quadrate lobe is a rectangular shaped lobe that's located just to the left of the gallbladder at the microscopic level the liver is organized into functional units known as lobules so the illustration here is showing us a liver lobule where blood flows in at portal triads a portal triad contains three structures one branch of a hepatic portal vein which is carrying blood in from the digestive system one branch of the hepatic artery proper which carries oxygen rich blood in and then there's a bile duct and the bile duct will carry bile out from the lobule so bile is the major excretory product excreted from the liver the cells that are found inside of the liver the epithelial cells are known as hepatocytes that produce the bile so hepatocytes are processing blood filtering the blood and then excreting bile the hepatocytes are found lining the sinusoids that connect from the portal triads where the blood in the portal venule and portal arterial flow into the sinusoids and then as that blood flows through the sinusoids it's processed by the hepatocytes and then drains into a central vein at the center of the lobule that central vein will then carry blood out of the lobule to the hepatic veins that will then drain the blood out of the liver into the inferior vena cava here we see the histology of the liver at 40 times magnification where you can see the structure known as a lobule with several portal triads surrounding a central vein here's another image of the liver with a slightly different stain there's still the structural functional unit of a lobule with portal triads surrounding a central vein so the blood will flow in at the portal triad through the hepatic portal vein branches and hepatic artery proper branch and then the blood flows through the sinusoids within the lobule and drains out through the central veins where the bile then is excreted by the hepatocytes in the lobule into the bile connectulae that then drain into the bile ducts at the portal triads bile drains from all of the small bile ducts at each of the liver lobules into larger and larger hepatic ducts the left hepatic duct and right hepatic duct joined together forming the common hepatic duct and then the common hepatic duct connects to the cystic duct which carries bile into the gallbladder the function of the gallbladder is to store and concentrate bile and so while the liver produces bile that bile is stored and concentrated in the gallbladder and then when we eat a meal bile is secreted out from the gallbladder through the cystic duct and out from the liver through the common hepatic duct and the cystic duct and common hepatic duct joined together to form the common bile duct and the common bile duct carries the bile down to the duodenum where bile is released through the duodenal papula into the duodenum where it serves the function of emulsifying fat to help with the chemical digestion and the absorption of fat soluble nutrients this illustration shows us the connection of the common bile duct to the duodenum as well as the structure of the pancreas and the location of the pancreas relative to the duodenum and so both the pancreas as well as the liver and gallbladder are secreting exocrine products into the duodenum to assist with chemical digestion the pancreas has a narrow tail located in the left hypochondriac region and an expanded head region found in the epigastric region connected to the duodenum the major pancreatic duct carries the pancreatic secretions down into the major duodenal papula there's also an accessory pancreatic duct that connects to a lesser duodenal papula but the pancreatic juice is secreted into the duodenum there's a variety of enzymes that are found in the pancreatic juice to help with chemical digestion of nutrients but pancreatic juice also contains bicarbonate which is a chemical that functions as a base to neutralize the acidity of the gastric juice that's coming with chym into the duodenum from the stomach the cells that produce the exocrine secretions of the pancreas are organized into functional units called asinine and so the individual excretory cells are called the asinine cells that form the asinine the functional units that produce the pancreatic juice that's secreted into the pancreatic ducts that then carry the secretions into the duodenum at the duodenal papula so this shows us a low magnification view of the histology of the pancreas where the majority of the cells in the pancreas are the asinine cells those asinine cells secrete pancreatic juice into the pancreatic ducts and the pancreatic ducts carry the secretions into the duodenum there are also endocrine cells inside of the pancreas so the lighter staining circular regions known as the islets of Langerhans are the endocrine regions within the pancreas responsible for producing the hormones insulin and glucagon here's a higher magnification view of the histology of the pancreas where you can see the majority of the cells the asinine cells that produce the pancreatic juice and the pancreatic ducts that carry the pancreatic juice out of the pancreas into the duodenum as well as the lighter staining islets of Langerhans that are the endocrine cells the alpha cells that produce glucagon and the beta cells that produce insulin and so here's another hundred x magnification view with a lighter staining pattern showing us the asinine cells that produce the pancreatic juice and the islets of Langerhans that contain the endocrine cells that produce insulin and glucagon the ilium of the small intestine drains into the large intestine or colon at the cecum the iliocecal valve is a ring of smooth muscle that regulates the emptying of the small intestine the vermiform appendix or commonly just referred to as the appendix is a small extension from the cecum that contains numerous mucosa associated lymphoid tissue important for the immune system to protect against infection and regulate the bacterial environment of the intestines the contents then move from the cecum superiorly through the ascending colon along the right side then the colon turns at the right colic flexure also known as the hepatic flexure to form the transverse colon where the contents are traveling from the right to the left side the transverse colon is anchored to the posterior abdominal wall through a mesentery known as the transverse mesocolon where the middle colic artery is traveling through and supplying blood to the transverse colon then the left colic flexure is where the transverse colon ends and the colon turns inferiorly to form the descending colon the contents then move down through the descending colon into the sigmoid colon where the colon forms a S shape that curves into the pelvic cavity at the midline connecting into the rectum and then the rectum then connects into the anal canal where the feces can exit the body during defecation the surface of the colon contains numerous epiploic appendages that are adipose tissue storage as well as a outer ribbon of smooth muscle that's visible on the surface so the longitudinal layer of the muscularis externa forms a long ribbon along the surface of the large intestine known as the teniac coli the function of the teniac coli is to contract to decrease the length of the colon and it causes the colon to form pouch shaped segments known as the hostra so the large intestine contains the same basic four layers that we saw in the small intestine and outer cirrhosa surrounding a muscularis externa surrounding the submucosa which surrounds the mucosa and the border between the submucosa and the mucosa is a muscularis mucosa layer of smooth muscle the mucosa does still form folds in the colon however there are no outward projections there are no villi in the the colon instead the all the folds of the colon are inward folding intestinal glands also known as intestinal crypts where there are numerous goblet cells that secrete mucus to help lubricate and protect the epithelium that's lining the lumen of the colon so here we see the histology of the colon with numerous intestinal crypts or glands that are filled with lots of mucus being produced by the goblet cells that form the the majority of the cells there in the lining of the intestinal crypt and so the epithelium is a simple columnar epithelium similar to it as we saw in the small intestine and in the stomach but the there are no villi and the the inward folded crypts are full of numerous goblet cells giving this distinctive appearance to the colon so here's another image with low magnification where we can see the mucosa of the colon has numerous inward folding crypts with lots of goblet cells but there's no villi and then there's we can see the smooth muscle forming the muscularis interna as a layer of boundary between the submucosa layer of dense irregular connective tissue and the mucosal layer and so this is a little higher magnification 100 times magnified view focused in on the mucosa of the colon where we can see numerous goblet cells lining the intestinal glands the inward folding intestinal crypts that produce the mucous to help protect and lubricate the simple columnar epithelium that lines the lumen of the colon