Digestive System n n Digestive System organs for

Digestive System n n Digestive System, organs for changing food chemically for absorption by body tissues. Digestion involves breaking organic compounds into simple soluble substances absorbable by tissues.

Digestive System n This process involves catalytic reactions (reactions that break things down) between ingested food and enzymes secreted into the intestinal tract.

Digestive System Other nutrients such as iron and vitamin B 12 are absorbed by specific "carrier proteins" that make them transferable by the intestinal cells. n Digestion includes both mechanical and chemical processes. n

Digestive System: mechanical The mechanical processes include chewing to reduce food to small particles, the churning action of the stomach, and intestinal peristaltic action. n These forces move the food through the digestive tract and mix it with various secretions. n

Digestive System: Chemical n Three chemical reactions take place: n n 1. conversion of carbohydrates into such simple sugars as glucose 2. breaking down of protein into such amino acids which are the building blocks of proteins. 3. conversion of fats into fatty acids and glycerol. These processes are accomplished by specific enzymes.

Digestive System: Saliva n n When food is eaten, the six salivary glands produce secretions that are mixed with the food. The saliva breaks down carbohydrates (with the enzyme amylase) into maltose, dissolves solid food to make it susceptible to the action of later intestinal secretions, stimulates secretion of digestive enzymes, and lubricates the mouth and esophagus for the passage of solids.

Salivary Glands Holt, Modern Biology

Digestive System Holt, Modern Biology

Stomach n n Gastric juice in the stomach contains agents such as hydrochloric acid and some enzymes, including pepsin, rennin, and lipase. The surface of the stomach itself is thought to be protected from acid and pepsin by its mucous coating. n Pepsin breaks proteins into peptones and proteoses. n Rennin separates milk into liquid and solid portions; n lipase acts on fat.

Stomach

Digestive System: Stomach n n Another function of stomach digestion is gradually to release materials into the upper small intestine, where digestion is completed. Some constituents of gastric juice become active only when exposed to the alkalinity of the small intestine; secretion is stimulated by chewing and swallowing and even by emotion precipitated by seeing or thinking of food.

Stomach and Small Intestine n n The presence of food in the stomach also stimulates production of gastric secretions; in turn, these stimulate the production of digestive substances in the small intestine. The most extensive part of digestion occurs in the small intestine; n here most food products are further hydrolyzed (broken down with the addition of water) and absorbed.

Small Intestine

Small Intestine: Digestive Fluids n n Predigested material supplied by the stomach is subjected to the action of three powerful digestive fluids: n pancreatic fluid, n intestinal juice, and n bile. These fluids neutralize the gastric acid, ending the gastric phase of digestion.

Small Intestine: Pancreatic Fluid n n Pancreatic fluid is introduced into the small intestine through several ducts. Secretion of pancreatic juice is stimulated by the ingestion (eating) of proteins and fats.

Pancreatic Fluid n n n Pancreatic fluid contains trypsin and chymotrypsin, enzymes that split complex proteins into simpler components that can be absorbed and used in reconstructing body proteins. Lipase breaks down fat; amylase hydrolyzes polysaccharides.

Pancreas

Digestive System Holt, Modern Biology

Small Intestine: Intestinal Juices n n Intestinal juice is secreted by the small intestine. It contains a number of enzymes n n n peptidase: for proteins maltase, lactase, and sucrase; its function is to complete the process begun by the pancreatic juice. The flow of intestinal juice is stimulated by the mechanical pressure of food partly digested in the intestine.

Bile n n The role of bile in digestion is to aid in absorption of fats by combining with them to form structures called micelles, which are soluble in the blood and more accessible to lipases. Secreted by the liver and stored in the gallbladder, bile flows in response to fat in the stomach and upper intestine.

Liver, Gall Bladder, Pancreas

Digestive System Holt, Modern Biology

Small Intestine: Active Transport n n n Transport 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.

Digestive System n n The 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 and then on to the heart. 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 to the heart.

Small Intestine Holt, Modern Biology

Small Intestine n n 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 m (1500 sq ft) in an adult. Absorption is also aided by the length of the small intestine, 6. 7 to 7. 6 m (22 to 25 ft).

Digestive System: Large Intestine The stomach and the colon or large intestine also have the ability to absorb water, certain salts, alcohol, and some drugs. n Certain whole proteins are also believed to pass through the intestinal barrier. n

Digestive System Holt, Modern Biology

Large Intestine

Excretion and Water Reabsorption n n 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. n 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.

n "Digestive System, " Microsoft® Encarta® Encyclopedia 99. © 1993 -1998 Microsoft Corporation. All rights reserved.

Enzyme Review n Mouth n n Stomach n n Amylase-breaks down starch Pepsin-breaks peptide bonds, making prote smaller Small Intestine n n peptidase-breaks down proteins lactase- lactose sucrase- sucrose maltase- maltose

Enzyme Review n Pancreas n n Trypsin- Breaks down proteins Amylase-breaks down starch Lipase- breaks down fat Liver n Bile- breaks fat down into droplets which are easier for the enzymes to work with