Absorption of Nutrients

The intestine is responsible for most digestion and absorption of nutrients. Six times longer than the dog's body, it forms loops that are folded up in the abdomen. All of the abdominal viscera are enclosed by the greater omentum, or epiploon, which holds the organs in position.The inside surface of the small intestine is also folded, which increases the surface area available for absorption. The cells that make up the villi (the smallest folds of the intestinal wall, at the cellular level) have different functions. The lower cells primarily secrete mucus, while the upper cells absorb digested nutrients. Dead cells also release other types of enzymes as they break down. The absorption process differs depending on the type of digested matter present.- Absorption of glucides. It is mainly the basic forms of glucides, the "-oses", that are present in the small intestine and are absorbed by the intestinal cells, whereupon they enter the small intestine's numerous blood vessels.- Absorption of lipids. The micelles are absorbed by the intestinal cells, which alter their various components to reconstitute triglycerides. The triglycerides are attached to proteins and other molecules and are taken up by the small intestine's lymphatic vessels.- Absorption of proteins. Amino acids are absorbed by the intestinal cells in a complex process. Other peptides consisting of amino-acid chains of various lengths are also present in the intestinal lumen. The shortest, chains of two or three amino acids, can be absorbed by an active-transport system. They are then hydrolyzed by enzymes within the cells to form the amino acids that pass into the bloodstream.

Absorption of Other Nutrients

Water and mineral salts are also absorbed in the intestine. Water is only partially absorbed in the small intestine by means of a mechanism involving sodium ions and glucose molecules or amino acids. Mineral salts are absorbed in various parts of the intestine, through differing mechanisms. For example, calcium is absorbed in the duodenum by means of a transport protein.

The intestine's blood vessels join to form the portal vein, which leads to the liver, where nutrients are stored.

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