Digestive System: Structure and Function (Ch. 22 Anatomy & Physiology)

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 continuous tube called the GI tract and several accessory organs.

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

• Functional segments: Mouth, esophagus, stomach, intestines (small & large).

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

• Processing of food: Occurs during transit through the GI tract.

Layers of the GI Tract

Histological Organization

The wall of the GI tract is organized into 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 and nervous tissues; contains blood vessels, lymphatics, and nerves.

• 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

Serous Membrane of the Abdominopelvic Cavity

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

• Parietal peritoneum: Adheres to the inner wall of the cavity.

• Visceral peritoneum: Adheres to organs, continuous with serosa.

• Major folds: Mesentery, greater & lesser omentum.

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.

• Contains teeth and salivary glands.

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

• Extends to the pharynx.

Teeth

Teeth are specialized structures for mechanical digestion.

• Regions: Crown, neck, root.

• Dentitions: Two sets during lifetime: primary (deciduous) and secondary (permanent).

• Types: Incisors (cutting), canines (tearing), premolars & molars (crushing & grinding).

• Mastication: Increases surface area of food, enhancing enzyme activity.

Salivary Glands

Salivary glands secrete saliva, which aids in digestion and oral health.

• Three paired glands: Parotid, submandibular, sublingual.

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

• Lubricates and moistens food.

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

Pharynx & Esophagus

The pharynx connects the oral cavity to the esophagus, which transports food to the stomach.

• Pharynx secretes mucus and moves food.

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

• Fourth layer is adventitia.

• Contains upper & lower esophageal sphincters.

Deglutition (Swallowing)

Deglutition is the process of moving food from the mouth to the stomach.

• Facilitated by saliva and mucus secretions.

• Three stages:

  1. Voluntary: Tongue pushes food to oropharynx.

  2. Pharyngeal: Epiglottis blocks airway, food moves to esophagus.

  3. Esophageal: Food moved through esophagus by peristalsis.

22.3 The Stomach

Stomach: External Anatomy

The stomach is a J-shaped organ involved in food storage, mixing, and initial digestion.

• Bounded by esophageal & pyloric sphincters.

• Mixing (churning) and holding area for food; some absorption occurs.

• Four anatomical regions: cardia, fundus, body, pylorus.

• Three layers of smooth muscle.

• When empty, mucosa is folded into rugae.

Stomach: Histology

The stomach wall contains specialized cells and glands for digestion.

• Mucosa: Lined by simple columnar epithelium.

• Mucosal layer folded into gastric pits and gastric glands.

• Gastric glands:

  • Mucous neck cells: Secrete acidic mucus.

  • Parietal cells: Secrete HCl and intrinsic factor (for vitamin B12 absorption).

  • Chief cells: Secrete pepsinogen (converted to pepsin) and gastric lipase.

  • Enteroendocrine cells: Secrete hormones (e.g., G cells secrete gastrin).

Stomach: Mechanical & Chemical Digestion

The stomach performs both mechanical and chemical digestion.

• Mechanical: Peristaltic muscle contractions mix food.

• Chemical: Food molecules broken down into absorbable units by enzymes and acid.

• Pepsin: Digests proteins in the presence of HCl.

• Gastric lipase: Digests some lipids.

• Minimal absorption in stomach (e.g., alcohol, aspirin, some electrolytes).

Phases of Digestion

Digestion is regulated in three overlapping phases, each controlled by neural and hormonal mechanisms.

• Cephalic phase: Initiated by sensory receptors in the head; stimulates gastric secretion and motility.

• Gastric phase: Stimulated by the presence of food in the stomach.

• Intestinal phase: Digestion within the small intestine.

• Both gastric and intestinal phases are under neural and hormonal control.

Table: Layers of the GI Tract

Layer	Main Components	Function
Mucosa	Epithelium, lamina propria, muscularis mucosa	Secretion, absorption, protection
Submucosa	Connective tissue, blood vessels, nerves	Support, transport, innervation
Muscularis	2-3 layers of smooth muscle	Peristalsis, mixing
Serosa/Adventitia	Serous membrane or connective tissue	Protection, structural support

Key Equations

• Peristalsis: Described as rhythmic contractions of smooth muscle, but no specific equation. The process is regulated by neural and hormonal signals.

Example: During the cephalic phase, the sight and smell of food stimulate the vagus nerve, increasing gastric secretions before food enters the stomach.

Additional info: The notes have been expanded to include definitions, examples, and a table for clarity and completeness.