Human Digestive System

Ptyalin or amylase present in saliva catalyses the hydrolysis of starch into maltose and dextrin.

The human anus is the external opening of the rectum. Like other animals, its closure is controlled by sphincter muscles. Feces are expelled from the body through the anus during the act of defecation, which is the primary function of the anus. 

Peristalsis is a distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the oesophagus and intestines. It is an automatic and important process that moves food through the digestive system. 

Chyme is a thick semi-fluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion. Other secretions from the pancreas, gallbladder, liver and glands in the intestinal wall add to the total volume of chyme. 

The epithelial cells of the villi transport nutrients from the lumen of the intestine into the capillaries (amino acids and carbohydrates) and lacteals (lipids). 

Bile or gall is a bitter tasting, dark green to yellowish-brown fluid produced by the liver of most vertebrates that aids the process of digestion of lipids in the small intestine. In many species it is stored in the gallbladder, and upon eating is discharged into the duodenum.

Pepsin is the principal proteolytic enzyme of vertebrate gastric juice. Its inactive precursor form called pepsinogen is produced in stomach mucosa.

Biliverdin and bilirubin are the pigments giving the bile its colour. These are produced by the breakdown of the dead and worn out red blood cells. Biliverdin is a green bile pigment formed by the catabolism of haemoglobin and converted to bilirubin in the liver. It may also arise from oxidation of bilirubin.

Steapsin belongs to the class of digestive enzymes called lipases, and is found in the pancreatic juice that catalyzes the hydrolysis of triglycerides (main constituent in vegetable oils and animal fat) to fatty acids and glycerol.

Defecation is the final action of the digestive system, and the means by which organisms eliminate undigested (mostly indigestible) solid, semi-solid or liquid waste material in the form of faeces from the digestive tract via the rectum, anal canal and finally the anus.

It contains sodium bicarbonate, which neutralizes the acidic material from the stomach. Sodium bicarbonate dissociates into Na⁺ and HCO₃^- in liquids. The Na⁺ component is readily absorbed in the small intestine and enhances water absorption. However, ingestion of this solution raises the pH in the stomach making it alkaline instead of acidic.

The saliva contains ptyalin or salivary amylase, which starts degrading the starch where the chain is linear. It is an enzyme that breaks the bond joining two sugars together, and forms fragments of soluble starches. It then breaks these down further, and the final product is a unit of two glucose molecules known as maltose. This substance is then broken down to glucose in the intestine.

Chymosin or rennin is a proteolytic enzyme synthesized by chief cells in the stomach. Its role in digestion is to curdle or coagulate milk in the stomach, a process of considerable importance in the very young animal. If milk were not coagulated it would rapidly flow through the stomach, and miss the opportunity for initial digestion of its proteins.

The vermiform appendix is a small finger-sized structure found at the end of our small caecum, and located near the beginning of the large intestine. 

Most of the nutrients in food are absorbed by the small intestine and carried to other parts of the body. The walls of the intestine contain tiny blood vessels that transport nutrients first to the liver where any toxins are removed before the nutrients are distributed throughout the rest of the body. On an average, the food spends 4-8 hours in the small intestine.