This is the process through which the soluble end products of digestion diffuse into the cellular lining of the villi.
Absorption of micro-nutrients such as water soluble vitamins, mineral salts and alcohol are absorbed at the stomach. Alcohol is equally absorbed here without undergoing digestion.
Most absorption of end products of digestion occurs in the ileum.
Molecules of amino acids and glucose pass through the epithelial lining and capillary walls into the blood system by active transport.
The capillaries drain into the hepatic portal vein where the absorbed products are transported to the liver before they are circulated to other body parts.
The fatty acids are absorbed into the lacteals of the villi which drain into the lymphatic vessels. The lymphatic vessels later join the blood circulatory system which transports them to other body parts.
The ileum is adapted to absorption in many ways:
a) It is long to provide a large surface area for absorption
b) It has a narrow lumen so as to bring the digested food into close contact with the walls of the ileum for easier absorption
c) It is highly coiled to slow down movement of food thus allowing more time for digestion and absorption of food.
d) The inner surfaces have numerous villi and microvilli to increase surface area for absorption of end products of digestion.
e) The epithelial lining is one cell thick to reduce the distance through which digested food diffuses.
f) Has a dense network of blood capillaries into which digested food materials diffuse to increase transport and thus maintain a steep concentration gradient.
g) Have lacteal vessels in the villi for absorption of fatty acids and glycerol.