Chapter 24: The Digestive System – Structure, Function, and Regulation

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Overview of the Digestive System

Introduction

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It consists of the digestive tract (alimentary canal) and accessory organs that aid in digestion and absorption.

Digestive tract: A continuous tube from mouth to anus, including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
Accessory organs: Structures such as the liver, gall bladder, pancreas, salivary glands, teeth, and tongue that assist in digestion.

Functions of the Digestive System

Ingestion: Taking in food and liquids.
Mechanical Processing: Physical breakdown of food (chewing, mixing, churning).
Digestion: Chemical breakdown of food into absorbable molecules.
Secretion: Release of water, acids, enzymes, and buffers.
Absorption: Movement of nutrients, water, and electrolytes into the blood or lymph.
Excretion: Elimination of indigestible substances and waste products.

General Structure of the Digestive System

Anatomy

Digestive Tract: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine.
Accessory Organs: Liver, gall bladder, pancreas, salivary glands, teeth, tongue, gastric & intestinal glands.

Histological Organization

The wall of the digestive tract is organized into four main layers:

Mucosa: Innermost layer; contains epithelium, lamina propria, and muscularis mucosae. Specialized for secretion and absorption.
Submucosa: Dense connective tissue with blood vessels, lymphatics, and nerves.
Muscularis externa: Smooth muscle responsible for peristalsis and segmentation; typically has inner circular and outer longitudinal layers.
Serosa: Outermost layer; a serous membrane (visceral peritoneum) in most of the tract.

Movement of Digestive Materials

Mixing and Propulsion

Segmentation: Localized contractions that mix and divide contents, enhancing digestion and absorption.
Peristalsis: Coordinated, wave-like contractions of the muscularis externa that propel material through the digestive tract.

Example: Peristalsis moves a bolus of food from the esophagus to the stomach.

Moving Through the Digestive Tract

The Oral Cavity

Structures: Includes lips, palate, tongue, teeth, salivary glands, and associated ducts.
Functions: Ingestion, mechanical processing (mastication), lubrication, and initial digestion of carbohydrates by salivary amylase.

Swallowing (Deglutition)

Buccal Phase: Voluntary; tongue pushes bolus into oropharynx.
Pharyngeal Phase: Involuntary; soft palate and uvula close nasopharynx, epiglottis covers larynx.
Esophageal Phase: Involuntary; peristalsis moves bolus down esophagus.
Bolus enters stomach.

The Stomach

Regions: Cardia, fundus, body, pylorus.
Functions: Storage, mechanical breakdown, chemical digestion (especially proteins), and production of intrinsic factor for vitamin B12 absorption.

Factors Controlling Stomach Motility

Increased motility: Gastric distension and certain hormones (e.g., gastrin) promote stomach emptying.
Decreased motility: Duodenal distension and hormones (e.g., secretin, cholecystokinin) slow gastric emptying.

The Small Intestine

Movements: Segmentation (mixing) and peristalsis (propulsion).
Reflexes: Gastroenteric reflex (stimulates motility and secretion along the small intestine) and gastroileal reflex (triggers opening of the ileocecal valve).

The Large Intestine

Movements: Haustral churning (mixing), mass movements (powerful peristaltic contractions).
Defecation: Elimination of feces; controlled by the defecation reflex.

The Defecation Reflex

Stimulus: Stretching of the rectal wall by feces.
Response: Contraction of rectal muscles, relaxation of internal anal sphincter, voluntary relaxation of external anal sphincter.
Consequence: Defecation (expulsion of feces).

Chemical Digestion

Basic Strategy

Proteins → protein fragments (short polypeptides)
Carbohydrates → small fragments (di- & tri-saccharides)
Fats → triglycerides → fatty acids and monoglycerides

Chemical Digestion in the Mouth

Saliva: Contains salivary amylase, which begins starch digestion.
Regulation: Salivary secretion is controlled by autonomic nervous system reflexes.

Chemical Digestion in the Stomach

Gastric Juice: Secreted by gastric glands in the stomach lining.

Cell Type	Location	Secretion
Mucous Cells	Gastric pit	Mucus (protects lining)
Parietal Cells	Gastric gland	Hydrochloric acid (HCl), intrinsic factor
Chief Cells	Gastric gland	Pepsinogen (inactive enzyme)
G Cells (pylorus)	Gastric gland	Gastrin (hormone)

Chemical Digestion: Accessory Glands

Liver: Produces bile, which emulsifies fats for digestion and absorption.
Gall Bladder: Stores and concentrates bile; releases bile in response to chyme in the duodenum.
Pancreas: Secretes pancreatic juice containing digestive enzymes and bicarbonate to neutralize stomach acid.

Composition of Pancreatic Juice

Water
Bicarbonate & phosphate buffers
Enzymes:
- Pancreatic alpha-amylase (digests starch)
- Pancreatic lipase (digests fats)
- Nucleases (digest nucleic acids)
- Proteolytic enzymes (secreted as inactive proenzymes):
  - Trypsinogen → Trypsin
  - Chymotrypsinogen → Chymotrypsin
  - Procarboxypeptidase → Carboxypeptidase
  - Proelastase → Elastase

Proenzyme	Active Enzyme	Enzyme Responsible for Conversion	Function of Active Enzyme
Trypsinogen	Trypsin	Enteropeptidase	Protein digestion
Chymotrypsinogen	Chymotrypsin	Trypsin	Protein digestion
Procarboxypeptidase	Carboxypeptidase	Trypsin	Protein digestion
Proelastase	Elastase	Trypsin	Protein digestion

Regulation of Pancreatic Secretion

Stimulus: Presence of chyme in the duodenum.
Hormones released:
- Secretin
- Cholecystokinin (CCK)
- Gastric inhibitory peptide (GIP)

Chemical Digestion in the Small Intestine

Brush Border Enzymes:
- Protein-digesting: Enteropeptidase, dipeptidase/peptidase
- Carbohydrate-digesting: Sucrase, maltase, lactase

Absorption

Sites and Mechanisms

Plica circulares: Folds of the small intestine that increase surface area.
Villi: Finger-like projections of the mucosa that contain capillaries and lacteals.
Microvilli: Microscopic extensions of epithelial cells, forming the brush border.

Absorption of Water

Water is absorbed throughout the digestive tract, primarily in the small and large intestines, by osmosis.

Absorption into Bloodstream and Lymphatics

Blood: Capillary beds in the villi absorb monosaccharides, amino acids, and small peptides.
Lymphatics: Lacteals absorb chylomicrons (lipid-protein complexes) formed from digested fats.

Absorption of Carbohydrates, Proteins, and Lipids

Macronutrient	Lumen	Epithelial Cell of Villus	Bloodstream
Carbohydrate	Polysaccharides → disaccharides	Monosaccharides (via brush border enzymes)	Transported via capillaries
Protein	Proteins → peptides	Amino acids (via peptidases)	Transported via capillaries
Lipid	Triglycerides → fatty acids, monoglycerides	Reassembled into triglycerides, packaged as chylomicrons	Transported via lacteals (lymphatics)

Key Terms and Concepts

Gastrointestinal tract: The continuous tube from mouth to anus.
Mucosa, submucosa, muscularis externa, serosa: Layers of the digestive tract wall.
Segmentation, peristalsis: Types of muscular movements in the digestive tract.
Gastric glands, parietal cells, chief cells, G cells: Specialized cells in the stomach lining.
Bile, pancreatic juice, brush border enzymes: Secretions aiding in digestion and absorption.
Plica circulares, villi, microvilli: Structural adaptations for absorption in the small intestine.

Summary Table: Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Salivary amylase	Salivary glands	Starch	Maltose, dextrins
Pepsin	Stomach (chief cells)	Proteins	Peptides
Pancreatic amylase	Pancreas	Starch	Maltose, dextrins
Trypsin, chymotrypsin, carboxypeptidase	Pancreas	Proteins, peptides	Peptides, amino acids
Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
Sucrase, maltase, lactase	Small intestine (brush border)	Disaccharides	Monosaccharides
Dipeptidase	Small intestine (brush border)	Dipeptides	Amino acids

Additional Info

The liver performs over 200 functions, including metabolic regulation (e.g., glucose storage, detoxification) and hematologic regulation (e.g., plasma protein synthesis).
Hormonal and neural mechanisms tightly regulate digestive secretions and motility.