How Digestion Occurs in Humans

The digestive system of man consist of a long coiled tube that extend from the mouth to the anus. The main parts in the direction of passage of food, are the oral cavity or buccal cavity, oesophagus, stomach, small intestine( duodenum, jejunum, and ileum ), large intestine( ascending colon, transverse colon, descending colon, rectum, caecum ). Associated with the various regions are the glands, especially salivary gland, liver, and pancreas. There are three sites of digestion in the digestive system of man, oral cavity, stomach, and small intestine.

• Digestion in oral cavity

There are several functions of oral cavity, the most obvious being the, selection of food, grinding or mastication, lubrication and digestion. Lubrication and digestion are the main functions of the oral cavity accomplished by saliva. Saliva is secreted by three pairs of salivary glands namely bilingual glands situated below the tongue, sub maxillary glands and parroted glands. Saliva produce by these glands contains three important ingredients water and mucus, sodium bicarbonate and other salts, carbohydrate digesting enzymes, amylase or ptylin.

• Digestion in stomach

 At the junction between oesophagus and the stomach there is a special ring of muscles called cardiac sphincter. The stomach is situated below the diaphragm on the left side of abdominal cavity. It is an elastic bag that stores food from meals for some time, making discontinuous feeding possible. The mucosa of the stomach possesses numerous tubular gastric glands, which are composed of three types of cells. Mucous cells, that secretes mucus, parietal cells secrete hydrochloric acid and zymogen cells, which secrete pepsinogen. The secretion of all these cells is collectively called gastric juice. The secretion of gastric juice is regulated by smell, sight and quality of food.

• Digestion in small intestine

Small intestine in man consist of duodenum, jejunum and ileum. Duodenum is about 20-25 cm long which leads into jejunum and then into ileum. When chyme passes from stomach into duodenum, its acidity stimulates the release of secretions from pancreas, liver and duodenal cells.

• Pancreas 

Is a large gland which act as both exocrine and endocrine, the exocrine tissue secretes a juice that flows through pancreatic duct into duodenum . Included in this juice are enzymes that digest all principle components of food. It contains carbohydrate digesting enzyme amylase, fat digesting enzyme lipase and protein digesting enzyme enzyme trypsin, which is secreted in inactive form trypsinogen, which is than activated by enterokinase.

• Jejunum

It is the second portion of the small intestine extending from the duodenum to the ileum. The food which escapes undigested from the duodenum, is completely digested in the jejunum and ileum by a group of enzymes in the intestinal juice.

• Absorption of food

Nearly all absorption of the products of digestion takes place in the ileum.  The internal surface of ileum has many folds, which exhibit velvety appearance due to the presence of finger-like outgrowths called villi. Each villi is richly supplied with blood capillaries and a vessel called lacteal of lymphatic system with a covering of epithelial cells. Simple sugars and amino acids are absorbed by diffusion or active transport into the blood capillaries through the micro villi. A large proportion of fatty acids and glycerol enter the epithelial cells of villi, where they recombines into fats.  These fats than enters lacteal. Thereafter the intestinal contents are than pushed into large intestine.

• Large intestine 

Large intestine consist of ascending colon, transverse colon, descending colon, caecum and rectum.  Here water and salts are absorbed into blood, while undigested material is rejected as a feces, the fecal  matter contains large number of bacteria, plant fibres, mucosa cells, cholesterol, bile pigments and water.