The Digestion

The Digestion.

    The digestion is the process by which food is cut in simple molecules that can be absorbed by the body. These molecules are the nutrients, they are mainly the proteins (protids), the fat (lipids) and the sugars (carbohydrates). Other molecules like the minerals, the vitamins and the water, are absorbed directly without modification.

    Along the digestive tract, from mouth to the end of the small intestine (ileum), there are the mucus that lubricates the intestines and the enzymes that cleave the complex molecules into simple molecules. Mucus secretion is mainly due to local mechanical stimulation. It is secreted directly in the lumen of the gastrointestinal tracr by billion of goblet cells located mostly on the surface of the intestinal crypts.

    The enzymes are secreted by glands (clusters of cells that secrete in a channel, which also serves as a reservoir). These enzymes are secreted mainly by elongated tubular glands found primarily in the stomach. Other glands, a little more specialized, such as salivary glands and pancreas are of acinar type and are located outside the digestive tract per se. It is a network of small canals that brings the enzymes to the site of secretion.

    After the nutrients have been cut into small molecules, the latter are absorbed into the blood stream which goes to the liver before going to the rest of the body.

    The nutrients enter the liver via the portal vein. The role of the liver is to filter these small molecules and destroy the undesirable ones. This constitute the first-pass metabolism and undesirable molecules will end up in the bile and secreated back in the gut.