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.
If colonic muscles propel the excretory mass through the colon too quickly, it remains semiliquid. The result is diarrhea. Insufficient activity of the colonic musculature, on the other hand, produces constipation. The stool is held in the rectum until excreted through the anus.