a. The small intestine is a tube about 22 feet long.

The intestine is attached to the margin of a thin band of tissue called the mesentery, which is a portion of the peritoneum, the serous membrane lining the abdominal cavity.

The mesentery supports the intestine, and the vessels that carry blood to and from the intestine lie within this membrane. The other edge of the mesentery is drawn together like a fan; the gathered margin is attached to the posterior wall of the abdomen. This arrangement permits folding and coiling of the intestine, so this long organ can be packed into a small space. The intestine is divided into three continuous parts: duodenum, jejunum, and ileum.

b. Most of the absorption of food takes place in the small intestine. Muscular contraction of the intestinal walls produces the wave-like motion called peristalsis, which propels the contents through the length of the intestines. The walls of the intestines are covered with small, fingerlike projections called villi, which provide a larger surface area for absorption. After food has been digested, it is absorbed into the capillaries of the villi and carried to all parts of the body via the circulatory system.

c. The small intestine receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder (figure 1-2).

(1) Pancreas. The pancreas is a long, tapering organ lying behind the stomach. The head of the gland lies in the curve of the small intestine near the pyloric valve. The body of the pancreas extends to the left toward the spleen. The pancreas secretes a juice that acts on all types of food. Two enzymes in pancreatic juice act on proteins. Other enzymes change starches into sugars. Another enzyme changes fats into their simplest forms. The pancreas has another important function, the production of insulin.

(2) Liver. The liver is the largest organ in the body. It is located in the upper part of the abdomen with its larger (right) lobe to the right of the midline. It is just under the diaphragm and above the lower end of the stomach. The liver has several important functions. One is the secretion of bile, which is stored in the gallbladder and discharged into the small intestine when digestion is in process. The bile contains no enzymes, but it breaks up the fat particles so that enzymes can act faster.

The liver performs other important functions. It is a storehouse for the sugar of the body (glycogen) and for iron and vitamin B. It plays a part in the destruction of bacteria and worn out red blood cells. Many chemicals such as poisons or medicines are detoxified by the liver; others are excreted by the liver through bile ducts. The liver manufactures part of the proteins of blood plasma. The blood flow in the liver is of special importance. All the blood returning from the spleen, stomach, intestines, and pancreas is detoured through the liver by the portal vein in the portal circulation. Blood drains from the liver by hepatic veins that join the inferior vena cava.

(3) Gallbladder. The gallbladder is a dark green sac, shaped like a blackjack and lodged in a hollow on the underside of the liver. Its ducts join with the duct of the liver to conduct bile to the upper end of the small intestine. The main function of the gallbladder is the storage and concentration of the bile when it is not needed for digestion.