Digestive System Structure and Function: ANP Study Notes

Digestive System Overview

Major Organs and Accessory Organs

The digestive system is a long, muscular tube (the digestive tract) with specialized regions and accessory organs that work together to process food, absorb nutrients, and eliminate waste. The tract is lined with ridges and folds to increase surface area for absorption, supporting cell maintenance and growth.

• Major organs: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, anus

• Accessory organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas

The enteric nervous system is a network of sensory neurons and interneurons within the digestive system, controlling most digestive activities. It is influenced by the autonomic nervous system (ANS):

• Parasympathetic stimulation: Increases digestive activity

• Sympathetic stimulation: Decreases digestive activity

Structure of the Digestive Tract

Four Generic Layers (MSMS)

The digestive tract wall is organized into four main layers, each with distinct functions:

1. Mucosa (inner lining): Mucous membrane moistened by glandular secretions; contains the lamina propria (areolar tissue). The specific cell type varies by location.

2. Submucosa: Dense connective tissue with blood vessels, lymphatics, nerve plexus, and exocrine glands. In some regions, it secretes buffers and enzymes.

3. Muscular layer: Smooth muscle in two layers (inner circular, outer longitudinal) for mechanical processing and movement. The stomach has an additional oblique layer.

4. Serosa: Visceral peritoneum covering most of the tract in the abdominal cavity. In the oral cavity, pharynx, esophagus, and rectum, this is replaced by the adventitia (collagen fibers).

Mucosa histology:

• Stratified squamous epithelium at the beginning and end of the tract (for abrasion resistance)

• Simple columnar epithelium with goblet cells in the stomach, small, and large intestines (for secretion and absorption)

• Villi: Small mucosal projections in the small intestine that increase surface area for absorption

Smooth Muscle and Motility

Smooth Muscle in the Digestive Tract

Smooth muscle cells in the digestive tract walls are electrically connected by gap junctions and mechanically by dense bodies, allowing them to contract as a single unit. Stimulation can be neural, hormonal, or chemical, and some regions are rhythmically stimulated by pacesetter cells.

Peristalsis: Propulsion of Food

Peristalsis is a wave-like muscle contraction that propels food (bolus) along the digestive tract. The process involves:

• Circular muscles contracting behind the bolus

• Longitudinal muscles ahead of the bolus contracting

• Sequential contraction of circular muscle layer forces the bolus forward

Segmentation: Mixing of Contents

Segmentation involves cycles of contraction by the inner circular muscle, churning and mixing the contents in the small intestine without forward movement. This process fragments the bolus and mixes it with intestinal secretions, enhancing digestion and absorption.

• No set pattern of contractions; no particular direction of movement

• Occurs mainly in the small intestine and some parts of the large intestine

Regulation of Digestive Activities

Chemical and Physical Factors

Digestive activities are regulated by various factors at specific tract locations:

• pH changes in the lumen

• Physical distortion of the digestive tract wall

• Nutrients or chemical messengers released by the mucosa

Neural (Reflex) Control

• Short reflexes (myenteric reflexes): Triggered by chemoreceptors or stretch receptors in the tract wall; controlled by neurons in the myenteric plexus

• Long reflexes: Involve interneurons and motor neurons of the CNS; control large-scale peristalsis and may involve parasympathetic fibers synapsing in the myenteric plexus

Hormonal Control Mechanisms

Multiple hormones produced by enteroendocrine cells in the digestive tract epithelium regulate digestive function, coordinating secretion and motility.

Oral Cavity, Teeth, and Swallowing

Oral Cavity Structure and Function

The oral cavity is lined by stratified squamous epithelium for abrasion resistance. Keratinized areas (superior tongue, hard palate) resist severe abrasion, while nonkeratinized areas (cheeks, lips, inferior tongue) are more delicate. Thin mucosa under the tongue allows rapid absorption of lipid-soluble drugs (e.g., nitroglycerin). Carbohydrate and lipid digestion begins here, but no nutrient absorption occurs.

Teeth and Supporting Structures

• Gingival sulcus: Epithelial attachment blocks bacteria from deeper tissues

• Periodontal ligament: Connects root dentin to alveolar bone (gomphosis articulation)

• Root canal: Passageway for blood vessels and nerves to the pulp cavity

Clinical note: Infections in the root canal can lead to sepsis.

Pharynx and Esophagus: Swallowing (Deglutition)

Swallowing occurs in three phases:

1. Buccal phase (voluntary): Tongue pushes bolus into oropharynx; soft palate elevates to seal nasopharynx

2. Pharyngeal phase: Pharyngeal muscles contract, larynx elevates, epiglottis folds, and bolus moves into esophagus

3. Esophageal phase: Bolus triggers opening of lower esophageal sphincter (LES) and enters the stomach

Mesenteries and Stomach Structure

Mesenteries

• Greater omentum: Fatty tissue attached to stomach and transverse colon; provides padding and protection

• Lesser omentum: Connects stomach to liver; provides access for blood vessels and other structures

Stomach Regions and Wall Structure

• Fundus: Superior region

• Cardia: Borders gastroesophageal junction; secretes mucus to protect esophagus

• Body: Main mixing area

• Pylorus: Sharp curve; changes shape with mixing movements

The stomach wall has three layers of smooth muscle (circular, longitudinal, oblique) to strengthen the wall and assist in mixing and churning food into chyme. Rugae (temporary mucosal folds) allow the stomach to expand.

• Cardiac sphincter (LES): Controls entry from esophagus

• Pyloric sphincter: Regulates release of chyme into the duodenum

Stomach Functions and Gastric Glands

Functions of the Stomach

• Temporary storage of ingested food

• Mechanical digestion (mixing and churning)

• Chemical digestion (acid and enzymes)

• Production of intrinsic factor (for vitamin B12 absorption)

Stomach Wall and Gastric Glands

The mucosa (inner layer) is simple columnar epithelium, producing alkaline mucus to protect against acid and enzymes. Gastric epithelial cells have a short lifespan (3–7 days).

• Gastric glands: Secrete most of the acid (HCl) and enzymes for digestion; dominated by parietal and chief cells

• Pyloric glands: Secrete mucus and hormones to coordinate digestive activity

Cells of Gastric Glands

• Parietal cells: Secrete intrinsic factor (for B12 absorption) and hydrochloric acid (HCl) to maintain low stomach pH (1.5–2.0) for protein digestion

• Chief cells: Secrete pepsinogen, which is activated by HCl to form pepsin (digests proteins)

• G cells (enteroendocrine cells): Secrete hormones such as gastrin to regulate stomach acid

HCl Production

Hydrochloric acid is produced by the following process:

• H+ ions from carbonic acid (H2CO3) enter the lumen

• Cl- ions from interstitial fluid also enter the lumen

• H+ and Cl- combine in the lumen to form HCl

$\mathrm{CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-}$ $\mathrm{H^+}$ (from parietal cell) $+ \mathrm{Cl^-}$ (from interstitial fluid) $\rightarrow \mathrm{HCl}$ (in gastric lumen)