The monosaccharides, amino acids and other products of digestion must be passed on to other tissues to be useful for the organism. The process by which the digested material from the alimentary canal enters the blood stream is known as absorption. In intracellular digestion the same cells are concerned with digestion and absorption but in higher multicellular animals there are usually separate tissues and areas of gut for enzyme production, digestion and absorption. In this section we will mainly be concerned with absorption of amino acids, sugars and fats released during extracellular digestion in vertebrates. In all vertebrates most of the absorption is localised in the intestine.
As you already know the wall of the vertebrate intestine is folded and ridged to increase the absorptive surface. These ridges or folds are covered by a velvet like pile of minute absorptive villi. These are highly specialised absorptive organs with a core containing a network of capillaries derived from blood vessels in the gut wall. Each villus also contains a central lymph capillary known as lacteal which begins blindly at the tip of the villus and drains into the main lymph channels of the gut wall. Lipids pass mainly into the lacteals while sugars and amino acids are absorbed directly by the blood capillaries. The villi and intestinal folds contain smooth muscles that contract to bring the villi in contact with the food in the intestine; and also maintaining the circulation in lacteals, lymphatics and small blood vessels.
Figure: Lining of mammalian small intestine a) Villus covered with digestive epithelium which consists of absorptive cell and occasional goblet cells. b) An absorptive cell. The apical surface bears a brush border of microvilli.