Small Intestines - Villi
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The small intestines
The small intestine, a twenty-one-foot tube, lies in coils in the abdomen. The first ten to twelve inches are called the duodenum, which is the U-shaped section of the small intestine that joins the stomach and the jejunum. The next eight feet, the jejunum and the remaining twelve feet the ileum, join the duodenum with the large intestine. The muscle layers of the small intestine are arranged longitudinally on the outside and circularly on the inside. Between the muscle layers are a series of nerve fibers which control the muscular movements that mix and move food along. The common bile duct is the opening through which bile and pancreatic juices enter the small intestine. As food reaches the duodenum from the stomach, it is mixed with these juices and further digested. At the base of the duct is a ring of smooth muscle, the sphincter, which regulates the outflow of fluid. Digested food is absorbed into the cells through the fingerlike projections lining the small intestine called the villi. The Crypts of Lieberkuhn, numerous tubular glands which open into the intestine from between the villi, secrete an alkaline fluid rich in digestive enzymes and mucus. The mucus protects the tissues of the small intestine from being digested by it's own enzymes. Brunner's glands pour out an alkaline juice to neutralize the acid chyme reaching the small intestine and to create the necessary conditions for further digestion. Tributaries of the portal vein carry digested materials away from the small intestine to the liver. The major function of the small intestine is to complete the digestion of food passing into it from the stomach and to absorb the final products of digestion.
How the Body Works : The Villi
Each villus contains a network of blood vessels and a small lymphatic vessel, known as the lacteal. Some products of digestion---like amino acids and glucose---pass into the blood vessels and then into the hepatic portal vein, which carries them to the liver. Most fatty acids and glycerol are recombined to become fats in the intestinal lining and are absorbed into the lacteals. The fluid in the lacteals passes into the lymphatic vessels and is emptied into the bloodstream.
2.32 Recall how to carry out a simple experiment to determine the energy content in a food sample.
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