BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The Human Digestive Tract

8.3.9. Absorption of Food in the Small Intestine

The absorption of the End products of Digestion is carried out by the villi of the ileum. As shown in Fig. 8.21 D, E, F, The Structure of the villi is ideally suited for this function. Monosaccharides, dipeptides, and Amino Acids enter the Blood capillaries either by diffusion or by Active Transport (Figs. 8.23 and 5.22).

8.6. What is the advantage of active transport in the absorption of monosaccharides, dipeptides, and amino acids?

The blood capillaries emerging from the villi join to form the HEPATIC PORTAL VEIN, through which the absorbed digestion products are transported to the Liver.

Fatty acids and glycerol enter the columnar Cells of the villi, where they are reassembled into Lipids. Proteins present within the epithelial cells coat the lipid molecules with a thin layer, forming lipoprotein globules known as chylomicrons. They leave the epithelial cells by exocytosis and enter the Lymphatic vessels of the villi (Fig. 8.21, I). Their presence gives the Lymph draining from the intestine a milky appearance, which is why it is sometimes referred to as chyle (from the Latin chylus meaning juice). Chylomicrons are transported along with the lymph through the Lymphatic vessels and enter the Blood Vessels near The Heart, passing into the fluid portion of the blood, or plasma. Here, plasma Enzymes hydrolyze the lipids once again back into fatty acids and glycerol, which are assimilated by cells and can be used in this form for Respiration or stored as fat in the liver, Muscles, mesentery, and subcutaneous tissue.

The Small Intestine also absorbs other essential substances: inorganic salts, Vitamins, and Water.

The sphincter Muscle located between the ileum and the cecum periodically contracts and relaxes, thereby allowing small amounts of material to pass from the small intestine into the Large Intestine.