Digestive System Overview

Overview of the Digestive System

• The digestive system consists of organs of the alimentary canal and accessory digestive system organs.
• The alimentary canal includes: mouth, pharynx, esophagus, stomach, small and large intestines.
• Accessory digestive system organs include: teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestive Processes

• The six digestive processes are: ingestion, propulsion, mechanical breakdown, digestion, absorption, and defecation.
• Ingestion is the intake of food.
• Propulsion is the movement of food through the tract.
• Mechanical breakdown is the physical mixing or breaking of food into smaller fragments.
• Digestion is the breakdown of food by enzymatic action.
• Absorption is the transport of products of digestion through the intestinal mucosa into the blood.
• Defecation is the elimination of undigested residues from the body.

The Stomach

• The stomach is J-shaped and lies in the upper left quadrant of the abdomen.
• The stomach has four major regions: cardia, fundus, body, and pyloric part.
• The stomach mucosa is simple columnar epithelium with gastric pits and glands.
• Gastric glands contain secretory cells: chief cells, parietal cells, mucous neck cells, and enteroendocrine cells.
• The stomach muscularis has a third layer of smooth muscle that allows it to churn and mix food.

Gastric Secretion and Regulation

• Parietal cells secrete hydrochloric acid and intrinsic factor.
• Chief cells produce pepsinogen.
• Mucous neck cells produce mucus.
• Enteroendocrine cells secrete hormones.
• The mucosal barrier protects the stomach from self-digestion and HCl.
• Protein digestion is initiated in the stomach by activated pepsin and requires acidic conditions.

Control of Gastric Secretion

• Gastric secretion is controlled by both nervous and hormonal factors.
• The three phases of gastric secretion are cephalic, gastric, and intestinal.
• Most food-related stimuli stimulate gastric secretion.
• Vagovagal reflexes and sympathetic activity also regulate gastric secretion.

The Small Intestine

• The small intestine is the major digestive and absorptive organ.
• It has three subdivisions: duodenum, jejunum, and ileum.
• The small intestine extends from the pyloric sphincter to the ileocecal valve.
• The bile duct and pancreatic duct join to form the hepatopancreatic ampulla and empty into the duodenum.
• Circular folds, villi, and microvilli increase the intestinal surface area for digestion and absorption.
• The duodenal submucosa contains elaborate mucus-secreting duodenal glands.
• The mucosa of the ileum contains Peyer's patches (lymphoid follicles).