The Digestive System simplified

Peristalsis, which is rhythmic contractions of the muscle layers, pushes the bolus down the esophagus, the bolus passes into the stomach through the lower esophageal sphincter. This sphincter normally remains closed, preventing reflux of the stomach contents. The stomach is divided into anatomical regions. The cardia, fundus, body, and antrum. The muscular outlet of a stomach is called the pylorus. The surface of the lumen is covered in folds called rugae, peristaltic contractions continue in the stomach, churning and mixing the food bolus with hydrochloric acid, and the digestive enzyme pepsin. Neural and hormonal stimulation causes the release of pepsin by the stomach, which starts the chemical digestion of proteins. 

This helps reduce the bolus to a fluid like substance called chyme, when the chyme reaches its proper softness, the pylorus opens and the chyme is slowly released into the first portion of the small intestine called the duodenum peristaltic contractions continue in the duodenum. It is within the duodenum that the chyme mixes with pancreatic juice and bile the process of digestion causes neural and hormonal stimulation of the pancreas to release pancreatic juice. Specialized pancreatic cells produce the pancreatic juice. The juice contains digestive enzymes and bicarbonate. The major digestive enzymes are amylase, which digests carbohydrates, lipase, which digests fats, and protease which digests proteins when pancreatic cells release the juice. It flows through tiny pancreatic ducts, which lead to the main pancreatic and accessory pancreatic ducts. Then the juice is released into the duodenum through the major and minor papillae, were at mixes with the chyme.

In order to protect the pancreas, proteases released in an inactive form, and is activated once it reaches the duodenum. Bicarbonate within the juice acts to neutralize the acidic chyme, which creates a favorable environment for the digestive enzymes to function. Located above and in front of the pancreas, the liver has many roles, including the maintenance of metabolic homeostasis. The liver maintains this because it receives most of the blood flow via the portal venous system from the small and large intestines. This blood contains nutrients absorbed during digestion. The liver processes these nutrients and vitamins and helps detoxify the blood passing through the portal venous system another role of the liver is the production of bile, which is a liquid that contains bile salts and other substances. Mile salts emulsify fat within their small intestine. Liver cells, known as hepatocytes, secrete the constituents of bile into tiny bile canaliculi, which flows into larger hepatic ductules that eventually combine to form the right and left hepatic ducts. 

Bile then flows into the common hepatic duct and cystic duct, where it enters the gallbladder to be concentrated and stored during digestion, the gallbladder contracts and pushes the bile through the cystic duct and common bile duct, where they join the pancreatic duct at the hepa pancreatic ampulla. The bile then empties into the duodenum through the major papilla. The bile mixes with the chyme and helps to digest fat the chyme continues to mix with the digestive enzymes as it moves through the duodenum and into the jejunum. The second portion of the small intestine. Nutrients from the digested food are absorbed into the bloodstream in the jejunum. The bolus moves through the jejunum and reaches the ileum, where there is more nutrient absorption. The unabsorbed portion of the bolus passes out of the ileum and into the large intestine through the ileocecal valve. The first portion of the large intestine is called the cecum. 

The main function of the large intestine is to absorb water and solutes, such as fatty acids, which help concentrate and form the stool. The bolus is pushed by a peristaltic wave up and through the ascending colon, which passes through the first major bend in the colon, the hepatic flexure. The bolus is now pushed through the transverse colon, and then travels through the second major bend, the splenic flexure, then the bolus moves down the descending colon and into the sigmoid colon, before entering the rectum. The formed stool enters and is stored in the upper portion of the rectum. The anus contains anal sphincter muscles that remain tightly closed until neural input causes the rectum to widen. Then the stool descends, the anal sphincter relaxes, and defecation occurs