 This video will cover the following objective from the digestive system. List and describe the functions of the digestive system. List the organs of the elementary canal, also known as the gastrointestinal tract or GI tract. The major functions of the digestive system include ingestion of food, taking food in, which brings food into the mouth. And then after bringing food into the mouth, we can start the process of digestion, which is breaking down food into smaller molecules or smaller particles. The process of mechanical digestion is physically breaking large pieces into smaller pieces, such as mastication, where we chew up the food in the mouth, the teeth are chopping the food and grinding the food into smaller pieces. That's mechanical digestion. In contrast, chemical digestion is when there are chemical reactions breaking down the bonds between atoms within large molecules to produce smaller molecules. So the chemical nature of the molecules in food is changed during chemical digestion and this process involves digestive enzymes, which are proteins that can catalyze these chemical reactions. And so both the processes of mechanical and chemical digestion begin immediately after ingestion as the saliva contains some digestive enzymes and during mastication the food is mechanically digested as it's chewed up and then mixed with the saliva. Another major function of the digestive system is propulsion. And so swallowing, also known as deglutition, is the process where the food that's been chewed up in mastication and mixed with saliva to form a rounded mass known as a bolus, is then moved from the oral cavity down through the pharynx, into the esophagus, and then down through the esophagus into the stomach. And then propulsion will continue to move the contents from the stomach down through the small intestine and large intestine and the primary mechanism that will be responsible for propulsion through the intestines is known as peristalsis. Peristalsis is an involuntary wave of contraction and relaxation that will spread down the intestine in order to propel the contents through from the stomach down through the small intestine and large intestine. And as the contents are traveling through the small intestine there will be absorption of nutrients and water, water soluble nutrients like glucose and amino acids and minerals and many water soluble vitamins will all be taken up by the blood and carried into the small, the capillaries in the small intestine transported into the blood. But lipid soluble nutrients instead will be absorbed into the lacteals, which are lymphatic vessels, and then transported through the lymphatic vessels to drain into the thoracic duct into the left subclavian vein to return ultimately that lipid soluble nutrients will be drained into the blood with the lymph, but they don't enter into the blood at the capillaries within the small intestine because the lipid droplets are too large to pass into the blood at the capillaries and they first have to enter the lacteals and travel through the lymphatic pathway. And so after nutrient absorption the the remaining indigestible material known as the feces is removed in another process of propulsion so the feces are removed from the body in the process of defecation. Deglutition, commonly referred to as swallowing, is the process of moving a bolus from the oral cavity into the stomach. There's three phases of deglutition. It's started off with the voluntary phase of deglutition where the tongue pushes the bolus into the pharynx and then there's the pharyngeal phase of deglutition which is coordinated by the deglutition center in the medulla oblongata regulating the skeletal muscles of the pharynx in order to propel the bolus into the esophagus which will transition into the esophageal phase of deglutition where the muscularis will propel the bolus from the proximal end where there's skeletal muscle initially down into the smooth muscle that's found in the distal two-thirds of the esophagus. Eventually the bolus will then be propelled through the esophagus into the stomach. And so the smooth muscle in the walls of the esophagus will propel the bolus down through using the mechanism of peristalsis. Peristalsis is a wave of contraction and relaxation of the smooth muscle in the wall of the gastrointestinal tract or elementary canal. So peristalsis propels a bolus through the esophagus during deglutition. Then peristalsis will propel the contents through the stomach into the small intestine and down through the small intestine into the large intestine and then through the large intestine and out during defecation. The elementary canal is a tube from the proximal opening of the mouth, the oral cavity, all the way down through to the distal opening of the anus the contents inside of the elementary canal are outside of the body technically and the nutrients get absorbed into the blood to be distributed throughout the body. The elementary canal is also commonly referred to as the gastrointestinal tract or just the GI tract. So the GI tract extends all the way from the mouth to the anus and is divided into several regions while the the mouth is important for initiating the digestion process. When food is brought into the mouth, that's known as ingestion and then as you start to chew food up, that's known as mastication. As food is being chewed up in mastication, it's being mixed with saliva that's produced from the salivary glands. That saliva contains some digestive enzymes in particular an enzyme known as salivary amylase. That's important for initiating the chemical digestion of carbohydrates. So mechanical digestion is chopping up large pieces of food into smaller particles and the chemical digestion is breaking the chemical bonds in large molecules to release smaller nutrient molecules. Digestive enzymes catalyze the chemical reactions for chemical digestion. The mouth is also important for initiating the swallowing process. Swallowing is also known as deglutition and the voluntary phase of deglutition is initiated in the mouth as the tongue pushes the food back into the throat. As the food is mechanically digested during mastication and mixed with saliva, it forms into a rounded mass known as a bolus. And the tongue pushes the bolus into the back of the mouth and then into the throat. The throat is also known as the pharynx and the pharynx takes over swallowing with the pharyngeal phase of deglutition, which propels the bolus down into the esophagus and then the esophagus takes over with the esophageal phase of deglutition that propels the bolus down into the stomach. Within the stomach, the bolus mixes with gastric juice to form a semi-liquid substance known as chyne. Gastric juice has a very low pH, it's very acidic and this helps to protect against infection. It has an antimicrobial activity of helping to kill bacteria and protect against infection. The acid also helps to denature proteins as well as activate an enzyme known as pepsin. And pepsin is a protease that initiates the chemical digestion of proteins, breaking long polypeptides into shorter polypeptides. The stomach has a muscular wall that can contract in a mechanical pattern known as churning, and the churning activity of the stomach breaks down the bolus and mixes it with the gastric juices. Then when the contents are propelled out of the stomach, they enter into the small intestine. The small intestine is the location where the majority of chemical digestion occurs as well as most nutrient absorption. The first segment of the small intestine is known as the duodenum, and the duodenum is the location where the majority of chemical digestion occurs. The chyne that exits the stomach and enters into the duodenum mixes with secretions from the liver and the pancreas and the gallbladder. The pancreatic secretions contain digestive enzymes that are important for chemical digestion. Pancreatic secretions also include bicarbonate, which is a base that functions to neutralize the acidity of the gastric juice coming from the stomach. The liver secretes something known as bile, and bile is important for helping to emulsify lipids, helping to make lipids suspended in an aqueous solution in the watery solution where the digestive enzymes are suspended in order to help facilitate the chemical digestion of lipids and subsequently the absorption of those lipid soluble nutrients. The duodenum is the long middle segment of the small intestine, and it is the location where the majority of nutrient absorption occurs. The duodenum has a highly folded surface in order to increase its surface area. There are large folds called the pleica circularis, with smaller folds called the villi on top of the pleica, and then microscopic folds of the plasma membrane of the cells that are forming the inner lining are called microvilli. So this system of folds on top of folds on top of folds creates an extremely large surface area in the duodenum for nutrient absorption. Then the ileum is the longest and the most distal segment of the small intestine. It's responsible for the final steps of nutrient absorption, and it drains into the large intestine. The ileum contains numerous clusters of lymphoid tissue known as piers patches that are important for regulating the microbiology, regulating the bacterial communities that are living inside of the small intestine and helping to prevent bacteria from migrating from the large intestine to the small intestine. The large intestine functions to absorb most of the remaining water from the contents that it receives from the small intestine, and then the large intestine will absorb some nutrients, especially electrolytes like sodium and potassium, and it will form feces with the indigestible material. So then the feces are removed from the large intestine in the process of defecation. The accessory organs of the digestive system include the salivary glands, the liver, the pancreas, and the gallbladder. The salivary glands produce saliva, a watery secretion that helps to lubricate the oral cavity, lubricating the bolus during deglutition, and also contains enzymes. The major enzyme in saliva is salivary amylase that initiates the chemical digestion of carbohydrates. The pancreas also produces a large amount of digestive enzymes that are important for chemical digestion. The pancreas also produces bicarbonate that functions to neutralize the acidity of chym coming from the stomach. Pancreatic juice is secreted into the duodenum, and so the pancreatic juice will mix with chym coming from the stomach into the duodenum, neutralizing the acidity and enabling chemical digestion. The liver produces bile, and bile is important for emulsifying lipids, which facilitates the chemical digestion and absorption of lipid soluble nutrients. Bile enables emulsification, that is, it enables lipids to become suspended in small droplets, which are mixed throughout a watery solution. This enables the digestive enzymes to access lipid soluble nutrients. For example, the digestive enzyme lipase produced from the pancreas can catalyze hydrolysis of triglycerides releasing fatty acids that can then be absorbed across the wall of the jejunum into the lymphatic lacteals. The function of the gallbladder is to store and concentrate bile, so when the liver secretes bile into bile ducts, those bile ducts will carry the bile into the gallbladder, and the gallbladder will store the bile until it needs to be released. Then, when there's a large amount of food entering into the duodenum, the liver and gallbladder will secrete bile into the duodenum, where it can then enable the emulsification of lipid soluble nutrients, facilitating digestion and absorption of lipid soluble nutrients.