The Human Digestion

Digestion is the process in which nutrients such as proteins, fats and carbohydrates are taken in by an organism and broken down to a form that may be readily absorbed from the gut for circulation to and used by various cells. The host is provided with nutrient required for metabolism. Nutrient supply the molecules from which energy, as calories, may be derived for cellular synthesis, cell and organ growth, and tissue replacement. In addition, nutrient provide the components that are needed in these processes but which the host is unable to synthesize, such as vitamins, minerals and certain essential amino acids.

Food enters the digestive tract by way of the mouth, where it undergoes physical change through chewing. Enzymes, such as ptyalin, that initiate sugar digestion are introduced in salivary secretions, which also provide lubrication to facilitate chewing and swallowing.

The food bolus (soft mass) passes through the esophagus and is retained in the stomach. There food is liquefied by a mixture of hydrochloric acid and pepsin, which is secreted by the stomach wall.

A common duct from the pancreas and the gall bladder into the duodenum serves as a conduit to introduce bicarbonate (to neutralize hydrochloric acid), pancreatic enzymes (for degradation of proteins and carbohydrates) and bile salts (for fat absorption).

Peristalsis moves the chyme into the next portion of small intestine, where the bulk of digested carbohydrate, protein, water, water-soluble vitamins and minerals are absorbed. Remaining nutrients are propelled to the last third of the small intestine, the ileum.

Here fat, fat-soluble vitamins and bile salts are absorbed.