Digestive System: Structure and Function (Anatomy & Physiology Study Notes)

22.1 Overview of the Digestive System

Digestive System Structure and Function

The digestive system is responsible for the breakdown and absorption of nutrients from food. It consists of a long tube (the GI tract) and several accessory organs that aid in digestion.

• GI TRACT: A continuous tube through the body with two openings (mouth and anus).

• Functional segments: Includes the mouth, esophagus, stomach, and intestines (small & large).

• Accessory structures: Salivary glands, tongue, teeth, gall bladder, liver, pancreas.

• Processing of food occurs during transit through the system.

Layers of the GI Tract

The wall of the GI tract is composed of four distinct layers, each with specialized functions:

• Mucosa: Innermost layer; consists of epithelium, lamina propria, and muscularis mucosa. Responsible for secretion and absorption.

• Submucosa: Connective tissue containing blood vessels, nerves, and glands.

• Muscularis: Two to three layers of smooth muscle; responsible for peristalsis and mixing.

• Serosa / Adventitia: Outermost layer; serosa if within the peritoneal cavity, adventitia if outside. Provides structural support and protection.

Peritoneum

The peritoneum is a serous membrane lining the abdominopelvic cavity and covering abdominal organs.

• Consists of two layers:

  • Parietal peritoneum: Lines the inner wall of the cavity.

  • Visceral peritoneum: Covers the organs and is continuous with the serosa.

• Additional major folds include the mesentery, greater and lesser omentum, which support and protect organs.

22.2 The Oral Cavity, Pharynx, and Esophagus

Oral Cavity

The oral cavity (mouth) is the entry point for food and the site of initial digestion.

• Formed by cheeks, lips, tongue, hard & soft palate.

• The space within the cavity is called the vestibule.

• Contains teeth and salivary glands.

• Begins processes of ingestion, secretion, mixing/propulsion, and chemical digestion.

• Extends posteriorly to the pharynx.

Teeth

Teeth are specialized structures for mechanical digestion, increasing the surface area of food for enzyme action.

• Three regions: crown, neck, root.

• Two dentitions during lifetime:

  • Primary (deciduous): baby teeth.

  • Secondary (permanent): adult teeth.

• Four types of teeth:

  • Incisors: cutting.

  • Canines: tearing.

  • Premolars & molars: crushing & grinding.

• Mastication increases the surface area of food, enhancing the rate of enzyme activity.

Salivary Glands

Salivary glands secrete saliva, which initiates chemical digestion and lubricates food.

• Three paired glands: parotid, submandibular, sublingual.

• Saliva is 99.5% water, 0.5% solutes (enzymes, salts, organics, dissolved gases).

• Functions:

  • Lubricates and moistens food.

  • Enzymes (e.g., amylase) begin chemical breakdown of carbohydrates.

Pharynx & Esophagus

The pharynx (throat) and esophagus transport food from the mouth to the stomach.

• Pharynx connects oral cavity to esophagus.

• Secretes mucus and transports food to the stomach.

• Esophagus is divided into three regions: upper, middle, lower.

• The fourth layer is adventitia (outer connective tissue).

• Contains upper and lower esophageal sphincters to regulate passage of food.

Deglutition (Swallowing)

Deglutition is the act of swallowing, moving the bolus from the mouth to the stomach, facilitated by saliva and mucus.

• Three stages:

  1. Voluntary: Tongue pushes food to oropharynx.

  2. Pharyngeal: Epiglottis flips downward, food moves to esophagus.

  3. Esophageal: Food moves through esophagus via peristalsis.

22.3 The Stomach

Layers of the GI Tract (Stomach)

The stomach wall contains the same four layers as the rest of the GI tract, with specialized features for digestion.

• Mucosa: Epithelium, lamina propria, muscularis mucosa; contains gastric glands.

• Submucosa: Connective and nervous tissues.

• Muscularis: Two to three layers of muscle (including an oblique layer unique to the stomach).

• Serosa / Adventitia: Outermost layer; serosa if within the peritoneal cavity.

Additional info:

• The stomach is divided into four gross anatomical regions: cardia, fundus, body, and pylorus.

• Gastric glands in the mucosa secrete digestive enzymes and acid.

• Mechanical digestion occurs via churning; chemical digestion via enzymes and acid.