Chapter 23 Digestive System

Overview of the Digestive System Functions of the Digestive System Ingestion (eating) Digestion Absorption Elimination The Wall of the Digestive Tract and Membranes Four layers: mucosal, submucosa, muscle, serosa Peritoneal membranes: mesentery, mesocolon, greater and lesser omentum

Structures and Organs of the Digestive System Mouth Teeth and tongue Salivary glands: parotid, submandibular, and sublingual Pharynx (Throat)

Structures and Organs of the Digestive System - cont’d Esophagus The esophagus is a long tube that connects the pharynx to the stomach. There are two sphincters. Stomach The three parts of the stomach are the fundus, body, and pylorus. The stomach functions in digestion; its most important role is regulate the rate at which chyme is delivered to the small intestine.

Structures and Organs of the Digestive System - cont’d Small Intestine The three parts of the small intestine are the duodenum, jejunum, and ileum. Most of the digestion and absorption occurs within the duodenum. The end-products of digestion are absorbed into villi.

Structures and Organs of the Digestive System - cont’d Large Intestine The large intestine consists of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus. The large intestine functions in absorption of water and electrolytes.

Accessory Digestive Organs Liver The liver has many functions; its most important digestive function is the secretion of bile. The liver receives blood from the portal vein; portal blood is rich in digestive end products.

Accessory Digestive Organs - cont’d Biliary Tree The biliary tree is composed of the bile ducts that connect the liver, gallbladder, and duodenum. The common bile duct empties into the duodenum.

Accessory Digestive Organs - cont’d Gallbladder The gallbladder functions to store and concentrate bile. The gallbladder contracts and releases bile in response to the hormone cholecystokinin (CCK).

Accessory Digestive Organs - cont’d Pancreas The pancreas secretes the most important digestive enzymes. The pancreatic enzymes empty into the duodenum.

Digestion and Absorption Carbohydrate Digestion To be absorbed, carbohydrates must be broken down into glucose. Carbohydrates are digested by enzymes called amylases and disaccharidases.

Digestion and Absorption - cont’d Protein Digestion To be absorbed, proteins must be broken down into amino acids. Proteins are broken down by proteolytic enzymes or proteases.

Digestion and Absorption - cont’d Fat Digestion To be absorbed, fats must be broken down into fatty acids and glycerol. Fats are digested by enzymes called lipases. Fats are first emulsified by bile.

Nutrition and Body Energy Carbohydrates Carbohydrates are either simple or complex. Glucose, the simplest carbohydrate, is the major fuel used by the body for energy.

Nutrition and Body Energy - cont’d Protein The body needs essential amino acids, which it cannot synthesize, and nonessential amino acids, which it can synthesize. Dietary proteins are complete or incomplete.

Nutrition and Body Energy - cont’d Fats (Lipids) Most dietary lipids are triglycerides. Fats are either saturated fats (like butter) or unsaturated fats (like oils).

Nutrition and Body Energy - cont’d Vitamins Vitamins are small organic molecules that help regulate cell metabolism. Dietary vitamin deficiencies give rise to many diseases (see Table 23-2). Vitamins are either water soluble (vitamins B and C) or fat soluble (vitamins A, D, E, and K).

Nutrition and Body Energy - cont’d Minerals Minerals are inorganic substances necessary for normal body function. Mineral deficiencies can cause serious health problems.

Nutrition and Body Energy - cont’d The body needs energy to do its work and maintain body temperature. The basal metabolic rate (BMR) is the amount of energy the body requires per unit time to perform essential activities at rest. Metabolism is determined by many factors, including age; gender; surface area; emotional state; overall health status; and hormones (especially thyroxine).