The Digestive System: Structure, Function, and Physiology

Digestive System Overview

Major Functions and Organization

The digestive system is responsible for breaking down food into nutrients, which are then absorbed into the bloodstream and delivered to cells throughout the body. It consists of the gastrointestinal (GI) tract—a continuous tube with two openings—and several accessory organs that aid in digestion but are not part of the GI tract itself.

• GI Tract Segments: Mouth, esophagus, stomach, small intestine, large intestine

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

• Main Functions: Ingestion, secretion, propulsion (e.g., peristalsis), mechanical and chemical digestion, absorption, and defecation (elimination)

Functional Processes of Digestion

• Ingestion: Taking in food via the mouth

• Secretion: Release of water, acid, buffers, and enzymes into the lumen

• Propulsion: Movement of food through the GI tract (e.g., peristalsis)

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning)

• Chemical Digestion: Enzymatic breakdown of macromolecules

• Absorption: Movement of nutrients into blood or lymph

• Defecation: Elimination of indigestible substances as feces

Histological Structure of the GI Tract

Four Layers of the Alimentary Canal Wall

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

• Mucosa: Innermost layer; consists of epithelium, lamina propria, and muscularis mucosae. Functions in secretion and absorption.

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

• Muscularis: Two to three layers of smooth muscle responsible for movement and mixing of contents.

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

The Peritoneum and Its Folds

The peritoneum is a serous membrane lining the abdominopelvic cavity, consisting of two layers:

• Parietal Peritoneum: Lines the inner wall of the cavity

• Visceral Peritoneum: Covers the organs and is continuous with the serosa

• Major Folds: Mesenteries, greater omentum, lesser omentum (support and house blood/nerve supply)

The Oral Cavity, Teeth, and Salivary Glands

Oral Cavity Structure and Function

The oral cavity (mouth) is formed by the cheeks, lips, tongue, and palate. It contains the teeth and salivary glands and is the site where ingestion, secretion, mixing, propulsion, and chemical digestion begin.

Teeth: Structure and Types

• Regions: Crown, neck, root

• Dentitions: Primary (deciduous) and secondary (permanent)

• Types: Incisors (cutting), canines (shearing), premolars and molars (crushing and grinding)

• Function: Mastication increases surface area for enzymatic activity

Salivary Glands

• Three Paired Glands: Parotid, submandibular, sublingual

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

• Functions: Lubricates and moistens food, begins chemical digestion

Pharynx and Esophagus

Pharynx

The pharynx (throat) connects the oral cavity to the esophagus and is involved in propulsion of food. It is divided into three regions: upper (skeletal muscle), middle (mixed), and lower (smooth muscle).

Esophagus and Deglutition (Swallowing)

• Structure: Muscular tube with upper and lower esophageal sphincters

• Function: Propels food to the stomach via peristalsis

• Deglutition: Three stages—voluntary (tongue pushes food), pharyngeal (epiglottis closes airway), esophageal (peristalsis moves food)

The Stomach

Structure and Regions

The stomach is a J-shaped organ bounded by the esophageal and pyloric sphincters. It serves as a mixing and holding area for food and is divided into four regions: cardia, fundus, body, and pylorus. The stomach wall contains three layers of smooth muscle for churning food.

Histology of the Stomach

• Mucosa: Simple columnar epithelium, folded into gastric pits and glands

• Gastric Pits: Contain mucous cells (secrete mucus)

• Gastric Glands: Contain parietal cells (secrete H+, Cl-, intrinsic factor), chief cells (secrete pepsinogen, gastric lipase), and enteroendocrine cells (e.g., G cells secrete gastrin)

Stomach Digestion and Regulation

• Mechanical Digestion: Peristaltic contractions mix food

• Chemical Digestion: Pepsin (protein digestion), gastric lipase (lipid digestion)

• Minimal Absorption: Alcohol, aspirin, some electrolytes

Phases of Digestion

• Cephalic Phase: Initiated by sensory input; stimulates gastric secretion and motility

• Gastric Phase: Triggered by food in the stomach; increases secretion and motility

• Intestinal Phase: Begins as chyme enters the small intestine; regulates further gastric activity

The Small Intestine

Structure and Regions

The small intestine is the primary site for chemical digestion and nutrient absorption. It extends from the pyloric sphincter to the ileocecal sphincter and is divided into three regions: duodenum, jejunum, and ileum.

Histology and Surface Area Adaptations

• Circular Folds: Permanent ridges in mucosa and submucosa that increase surface area

• Villi: Fingerlike projections of mucosa; each contains a lacteal and blood supply

• Microvilli: Extensions of enterocyte membranes forming the brush border; contain digestive enzymes

• Submucosal Glands: Secrete alkaline mucus to neutralize chyme

• MALT (Peyer’s patches): Lymphatic tissue in the ileum

Cell Types and Motility

• Absorptive Cells (Enterocytes): Absorb nutrients; have microvilli

• Goblet Cells: Secrete mucus

• Enteroendocrine Cells: Secrete hormones (secretin, CCK, GIP) that regulate stomach and intestinal activity

• Motility: Segmentation (mixing), peristalsis (propulsion)

The Large Intestine

Structure and Function

The large intestine completes absorption of water, produces certain vitamins, forms, and expels feces. It extends from the ileocecal sphincter to the anus and is divided into the cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal. The appendix is attached to the cecum.

• Histology: Mucosa with many goblet cells, no villi; muscularis layer forms taeniae coli, creating haustra (pouches)

Defecation Reflex

• Stimulus: Stretch receptors in the sigmoid colon

• Process: Voluntary contraction of diaphragm and abdominal muscles, relaxation of external anal sphincter

Accessory Organs: Pancreas, Liver, and Gallbladder

Pancreas

• Location: Posterior to the stomach

• Function: 1% endocrine (islets), 99% exocrine (acini)

• Digestive Enzymes: Pancreatic amylase (starch), trypsin/chymotrypsin/carboxypeptidase (proteins), pancreatic lipase (fats), ribonuclease/deoxyribonuclease (nucleic acids)

Liver and Gallbladder

• Liver: Produces bile (emulsifies fats), processes nutrients, detoxifies substances, stores glycogen

• Gallbladder: Stores and releases bile

• Bile Ducts: Common bile and pancreatic ducts empty into the duodenum at the duodenal papilla

Liver Histology and Blood Supply

• Hepatocytes: Arranged in lobules around a central vein

• Portal Triad: Contains bile duct, hepatic arteriole (O2-rich blood), hepatic portal venule (nutrient-rich, O2-poor blood)

• Blood Flow: All blood drains into hepatic sinusoids and then to the central vein

Nutrient Digestion and Absorption

Enzymatic Hydrolysis and Chemical Digestion

• Carbohydrates: Digested into monosaccharides (e.g., glucose, galactose)

• Proteins: Digested into amino acids

• Lipids: Digested into fatty acids and monoglycerides

Lipid Digestion and Absorption

1. Lipids are broken apart by stomach churning and gastric lipase

2. Emulsified by bile salts in the small intestine

3. Pancreatic lipase digests lipids into free fatty acids and monoglycerides

4. Bile salts form micelles with digested lipids

5. Absorbed into lacteals as chylomicrons, bypassing the liver initially

Absorption of Water, Electrolytes, and Vitamins

• Most water (>8L/day) is absorbed in the small intestine; remainder in the large intestine

• Electrolytes (Na+, K+, Ca2+) and vitamins are absorbed throughout the intestines

• Fat-soluble vitamins (A, D, E, K) are absorbed with micelles; water-soluble vitamins (B, C) are absorbed directly, except B12 which requires intrinsic factor

Summary Table: Structure and Function of Alimentary Canal Organs

Organ	Main Function(s)	Special Features
Mouth	Ingestion, mechanical and chemical digestion	Teeth, salivary glands, tongue
Esophagus	Propulsion	Peristalsis, sphincters
Stomach	Mixing, chemical digestion, limited absorption	Rugae, gastric glands, three muscle layers
Small Intestine	Chemical digestion, nutrient absorption	Circular folds, villi, microvilli
Large Intestine	Water absorption, feces formation		Haustra, taeniae coli, abundant goblet cells