BIOL2302 Chapter 7

Chapter 7: Digestive System

Overview

The digestive system, or gastrointestinal tract (GI tract), is approximately 30 feet long, extending from the mouth to the anus, and is responsible for nutrient supply through ingestion, motility, digestion, absorption, and elimination.

Digestive System Anatomy

The GI tract includes the oral cavity, tongue, teeth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon), rectum, and anus. Accessory organs, which aid digestion but are not part of the GI tract, include the salivary glands, liver, gallbladder, and pancreas.

Physiological Processes

1. Ingestion: Masticating food in the mouth and swallowing.

2. Motility: Propulsion through peristalsis and mixing via segmental contractions.

3. Secretion: Release of digestive enzymes, bile, and hydrochloric acid.

4. Digestion: Hydrolytic processes breaking down complex molecules.

5. Absorption: Movement of digested products into the bloodstream or lymphatic vessels.

6. Elimination: Removal of indigestible substances as waste.

Structure of the Wall of GI Tract

The GI tract wall consists of three layers:

- Mucosa: Lining with epithelium, lamina propria, and muscularis mucosa.

- Submucosa: Dense connective tissue with nerves.

- Muscularis externa: Smooth muscle layers for movement.

GI Tract Nervous System

Includes the submucosal plexus (regulating secretions) and myenteric plexus (controlling muscle contractions).

GI Tract Sphincters

Sphincters regulate food passage, including the upper and lower esophageal sphincters, pyloric sphincter, sphincter of Oddi, ileocecal valve, and anal sphincters.

The Esophagus

A muscular tube transporting food from the pharynx to the stomach via peristalsis, with the lower esophageal sphincter preventing acid reflux.

The Stomach

Located in the abdominal cavity, it mixes food with gastric secretions to form chyme. The gastric mucosa contains gastric pits and glands that secrete mucus, hydrochloric acid, and pepsinogen.

The Small Intestines

Divided into the duodenum, jejunum, and ileum, where 90% of digestion and absorption occurs. The duodenum receives chyme, bile, and pancreatic enzymes.

The Large Intestine

About 3 to 4 feet long, responsible for water absorption and feces compaction, consisting of the cecum, colon, rectum, and anus.

The Accessory Organs

Include salivary glands (producing saliva), liver (producing bile), gallbladder (storing bile), and pancreas (secreting digestive enzymes and buffers).

Key Terms

Digestive system, gastrointestinal system, digestion, absorption, peritoneum, mucosa, submucosa, muscularis externa, esophagus, stomach, small intestine, large intestine, salivary glands, liver, gallbladder, pancreas.