Chapter 3

Carbohydrates Mouth Digestion of carbohydrate begins in the mouth, with the secretion of the enzyme salivary amylase from the serous cells of the salivary gland. This enzyme breaks starch and glycogen into disaccharides. The mucous cells of the salivary gland secrete a mucus, which causes the food to stick together, and acts as a lubricant to aid in swallowing. The salivary glands are grouped into three categories: the parotid gland, submandibular glands, and sublingual, all located near the mouth.

Lipids Mouth Mechanical digestion occurs in the mouth by the mixing and churning of food and the release of mucus to lubricate the food. The food containing fats essentially follow the same pathway as carbohydrate foods, with chemical digestion primarily beginning in the stomach. No chemical digestion of lipids occurs in the mouth.

Protein Mouth As food containing protein enters the mouth, mucus is secreted to bind the particles together. No chemical digestion of protein occurs in the mouth.

Esophagus The food (bolus) is forced into the pharynx by the tongue. As the food is swallowed, it moves into the esophagus (a straight collapsible tube), which essentially provides a passageway from the pharynx to the stomach. The mucus glands of the esophagus secrete mucus to aid in moistening and lubricating the bolus. The peristaltic reactions of the muscles push the bolus through the esophagus.

Cardiac Sphincter The bolus passes through the cardiac sphincter, into the first section of the stomach.

Regions of the Stomach The stomach is divided into several regions: the cardiac region, body region, fundic region, and pyloric region.

Carbohydrates Stomach The stomach works to mix and churn the food, which aids in further digestion of carbohydrates. At this point, the bolus is converted into a semifluid paste of bolus and gastric juices called chyme. No chemical digestion occurs in the stomach.

Lipids Stomach The surface of the inner lining of the stomach contains openings called gastric glands, which are made up of three types of cells: mucous cells, chief cells, and parietal cells. A hormone called gastrin stimulates the gastric glands to secrete their fluids.

Lipids Stomach Gastric enzymes are secreted by the chief cells, while hydrochloric acid is secreted by the parietal cells. The combination of mucus, hydrochloric acid and enzymes is referred to as gastric juices. The gastric juices contain small amounts of gastric lipase, which begin the breaking down of specific lipids. A hormone called somatostatin inhibits the release of acid. As the stomach churns, it mixes the bolus and converts it into a semifluid referred to as chyme.

Protein Stomach The chief cells located in the stomach secrete gastric juices containing an enzyme precursor called pepsinogen. As it comes in contact with hydrochloric acid, it is converted into an active form referred to as pepsin.

Pyloric Sphincter The chyme then travels through the pyloric sphincter into the first section of the small intestines.

Sections of Small Intestines The small intestines is divided into three sections: duodenum, jejunum, ileum.

Carbohydrates Small Intestines As the chyme moves into the duodenum, an enzyme called pancreatic amylase is released through the pancreatic duct. This enzyme splits molecules of starch and glycogen into disaccharides. Enzymes released by the walls of the small intestines sucrase, maltase, and lactase, break down the disaccharides into monosaccharide.

Lipids Small Intestines As the chyme passes into the duodenum, cholecystokinin stimulates the gallbladder to release a substance called bile through the common bile duct. The function of bile is to emulsify fats, or break them into smaller droplets. This allows the small droplets of fat to be digested more effectively. Pancreatic Amylase is released by the Pancreas to continue to work on the Lipids. The mucosal cells of the SI release an enzyme called intestinal lipase, which splits fats into fatty acids and glycerol.

Protein Small Intestines When protein reaches the duodenum, the pancreas releases three protein splitting enzymes in inactive forms called trypsinogen, chymotrypsinogen, and procarboxypeptidase. Once trypsinogen comes in contact with an enzyme called enterokinase, which is secreted by the mucosal cells of the small intestines, then trypsin is activated. The presence of trypsin then activates the inactive procarboxypeptidase and chymotrypsinogen, and they are converted into chymotrypsin and carboxypeptidase.

Illeocecal Valve The remainder (waste, fiber and water) moves through the Illeocecal valve into the colon.

Colon Sections: Cecum, Ascending Colon, Transverse, Descending, Sigmoid

Rectum Anus Waste is then removed from the body from this passage.