T ransport of the products of digestion through the wall of the small intestine may be either passive or active. Sodium, glucose, and many amino acids are actively transported. The products of digestion are thus assimilated into the body through the intestinal wall, which is able to absorb nutritive substances selectively, rejecting other similar substances. The stomach and the colon or large intestine also have the ability to absorb water, certain salts, alcohol, and some drugs and crystalloids. Certain whole proteins are also believed to pass through the intestinal barrier.
Intestinal absorption has another unique feature.
Many nutrients are more efficiently absorbed when the body need is greater. The absorptive, extensively convoluted surface of the intestine amounts to 140 sq. T he water-soluble substances, including minerals, amino acids, and carbohydrates, are transferred into the venous drainage of the intestine and through the portal blood channels directly to the liver. Many of the fats, however, are resynthesized in the wall of the intestine and are picked up by the lymphatic system , which carries them into the systemic blood flow as it returns through the vena caval system , bypassing an original passage through the liver .Excretion. Undigested material is formed into a solid mass in the colon by reabsorption of water into the body.