 This video will cover the following objective from digestive physiology, describe the general functions of the digestive organs of the alimentary canal and accessory organs of the digestive system, define and contrast mechanical and chemical digestion, describe the phases of deglutition and define peristalsis. The alimentary 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 alimentary canal are outside of the body technically and the nutrients get absorbed into the blood to be distributed throughout the body. The alimentary 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. Well, 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 and 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 and 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 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 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 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 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 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 and 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 and 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 the major functions of the digestive system include ingestion which is bringing food into the body which occurs in the mouth when you take a bite of food and then mechanical digestion and chemical digestion are two major subdivisions of digestion breaking down food into smaller particles is the process of digestion major function of the digestive system but mechanical digestion is just breaking down large pieces of food into smaller particles of food mastication, chewing is one example of mechanical digestion turning the muscular contractions of the stomach or another example of mechanical digestion in the small intestine there's a pattern of mechanical activity known as segmentation which is a form of mechanical digestion that helps to mix the mucus and enzymes inside of the small intestine with the contents then chemical digestion refers to the breaking of chemical bonds within large molecules to release the smaller molecules that eventually will release small nutrient molecules that can be absorbed so absorption is a major function of the digestive system that refers to the movement of nutrients from the lumen of the digestive tract through the wall of the elementary canal into the blood so we'll see that water soluble nutrients will be directly absorbed into the blood but lipids fat soluble nutrients are absorbed in large droplets which cannot fit into the capillaries in the wall of the intestine they cannot fit into the blood capillaries and will instead be absorbed by lymphatic vessels known as lacteals and then will be carried in the lymphatic system until eventually being drained into the blood at the veins at the large subclavian vein here we see an illustration of the process of segmentation which is a form of mechanical digestion that occurs in the small intestine segmentation involves alternating contractions of the muscular layer muscularis externa is the muscular layer in the wall of the elementary canal and so if regions on either side contract to form a mass then in the center of the mass a contraction can split the mass into two segments and then alternating back to the original location of contraction and now mixes segments from the two masses together to form a new mass as these alternating contractions continue mucus containing enzymes and bicarbonate are mixed in with the contents of the small intestine enzymes that mix in with the contents of the elementary canal function to catalyze the chemical reactions of chemical digestion so one example is shown here is the chemical digestion of protein so as we take a long chain of amino acids of polypeptide and break it down to the individual amino acids the broad category for the type of enzymes that perform that catalyze those chemical reactions they're known as proteases and we'll see there's lots of specific proteases because each peptide bond has a specific shape and some proteases will be able to remove amino acids from the end of a polypeptide and other proteases will be able to catalyze the hydrolysis of a peptide bond in the middle of a polypeptide deglutition refers to swallowing and deglutition has three phases there's a voluntary phase of deglutition in which the tongue moves upward and backward against the roof of the mouth known as the palate and it pushes the bolus back into the back of the oral cavity and eventually into the oral pharynx which is the region of the throat immediately posterior to the oral cavity so in the oral pharynx stretch receptors detect the entrance of the bolus and initiate the pharyngeal phase of deglutition there's a deglutition center in the medulla oblongata that receives the sensory information from the stretch receptors of the pharynx then the deglutition center stimulates a coordinated muscular pattern of contractions that will squeeze the bolus down through the pharynx and into the esophagus the bolus will move through a sphincter at the superior end of the esophagus the upper esophageal sphincter will relax to allow the bolus to be propelled down into the esophagus the muscles that are responsible for the pharyngeal phase of deglutition are skeletal muscles that are being coordinated by somatic motor neurons that extend from the deglutition center of the medulla oblongata as we transition to the esophagus the proximal regions of the esophagus contain skeletal muscle that's also regulated by the deglutition center of the medulla oblongata but as the bolus is propelled down through the esophagus the muscle in the wall of the esophagus transitions from skeletal muscle to smooth muscle and the smooth muscle in the wall of the elementary canal can propel contents through in a mechanism known as peristalsis peristalsis is a wave of muscular contraction that will propel the bolus down through the esophagus and into the stomach the final step of the esophageal phase of deglutition will involve relaxation of the lower esophageal sphincter which is also known as the cardiac sphincter the lower esophageal sphincter or cardiac sphincter will relax opening the passageway for the bolus to move from the esophagus into the stomach then the lower esophageal sphincter will contract in order to prevent the bolus from moving backwards during the muscular contractions of churning to prevent any acid reflux peristalsis is a pattern of muscular contraction of the smooth muscle in the wall of the elementary canal that is a major mechanism of propulsion that the muscular contraction will spread as a wave of contraction where there will be a relaxation spreading just before the contraction spreads behind and this will enable the bolus to be propelled into the relaxed region as the wave of contraction travels behind