Digestive System: Structure, Function, and Processes

Digestive System Overview

Introduction to the Digestive System

The digestive system is responsible for breaking down food into absorbable units to provide nutrients for cellular activity. It consists of the alimentary canal and accessory organs that aid in both mechanical and chemical digestion.

• Alimentary canal: Mouth, pharynx, esophagus, stomach, small intestine, large intestine

• Accessory organs: Salivary glands, teeth, tongue, liver, gallbladder, pancreas

Structure of Alimentary Canal Organs

Four Tunics of the Alimentary Canal

The walls of the alimentary canal are composed of four basic layers, each with specialized functions:

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

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

• Muscularis externa: Responsible for segmentation and peristalsis; consists of circular and longitudinal muscle layers.

• Serosa: Outermost layer; also known as the visceral peritoneum.

Organs of the Digestive System

Mouth, Pharynx, and Esophagus

• Mouth/Oral cavity: Site of ingestion, mechanical breakdown (mastication), and mixing with saliva.

• Pharynx: Passageway for air and food; food is propelled to the esophagus by peristalsis, involving longitudinal and circular muscle layers.

• Esophagus: Muscular tube (~10 inches) conducting food from pharynx to stomach via peristalsis; passageway for food only.

Stomach

Stomach Anatomy and Regions

The stomach is a muscular organ with several regions and specialized structures for digestion.

• Cardiac region: Near the heart

• Fundus: Expanded portion lateral to the cardiac region

• Body: Main central region

• Pylorus: Funnel-shaped terminal end leading to the duodenum

• Muscularis externa: Includes an additional oblique muscle layer for churning food

• Rugae: Folds in the mucosa that allow expansion

Small Intestine

Structural Modifications for Absorption

The small intestine is the primary site for nutrient absorption, with three key adaptations to increase surface area:

• Circular folds (plicae circulares): Deep folds of mucosa and submucosa

• Villi: Fingerlike projections of the mucosa

• Microvilli: Tiny projections on absorptive cells, forming a brush border

Relationship with Pancreas, Liver, and Gallbladder

The small intestine receives digestive enzymes from the pancreas and bile from the liver and gallbladder, essential for chemical digestion.

• Pancreatic ducts deliver enzymes

• Bile duct delivers bile for fat emulsification

Large Intestine

Structure and Function

The large intestine absorbs water and electrolytes, forming and storing feces.

• Cecum: Saclike first part

• Appendix: Lymphatic tissue attached to the cecum

• Colon: Ascending, transverse, descending, and sigmoid sections

• Rectum and anal canal: Terminal portions

Digestive Processes

Physical (Mechanical) Digestion

Physical digestion involves breaking food into smaller pieces without chemical change to the food molecules.

• Mastication: Chewing in the mouth

• Churning: Mixing in the stomach

• Segmentation: Rhythmic contractions in the small intestine

• Peristalsis: Propels food along the tract

Chemical Digestion

Chemical digestion uses enzymes to break down complex molecules into absorbable units.

• Enzymes are secreted by intrinsic and accessory glands into the digestive tract lumen.

• Hydrolysis is the main chemical process, where water is used to break covalent bonds in food molecules.

Summary Tables: Chemical Digestion and Absorption

Carbohydrate Digestion and Absorption

Foodstuff	Enzyme(s) and Source	Site of Action
Starch and disaccharides	Salivary amylase, Pancreatic amylase	Mouth, Small intestine
Lactose, Maltose, Sucrose	Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase)	Small intestine
Monosaccharides (glucose, galactose, fructose)	Absorbed into capillary blood in villi	Small intestine

Protein Digestion and Absorption

Foodstuff	Enzyme(s) and Source	Site of Action
Protein	Pepsin (stomach glands, with HCl)	Stomach
Large polypeptides	Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase)	Small intestine
Small polypeptides, peptides	Brush border enzymes (aminopeptidase, carboxypeptidase, dipeptidase)	Small intestine
Amino acids	Absorbed into capillary blood in villi	Small intestine

Fat Digestion and Absorption

Foodstuff	Enzyme(s) and Source	Site of Action
Unemulsified fats	Bile salts (from liver), Pancreatic lipase	Small intestine
Monoglycerides, fatty acids, glycerol	Absorbed into lacteals of villi	Small intestine

Nucleic Acid Digestion

• Enzymes: Pancreatic ribonuclease and deoxyribonuclease, brush border enzymes (nucleosidases, phosphatases)

• Absorption: Pentose sugars, nitrogenous bases, and phosphate ions are absorbed into capillary blood in villi and transported to the liver via the hepatic portal vein.

Microscopic Anatomy of Digestive Organs

Histological Features

• Esophagus: Stratified squamous epithelium; gastroesophageal junction marks transition to stomach epithelium.

• Stomach: Gastric pits, parietal cells (secrete HCl), chief cells (secrete pepsinogen), mucosa, submucosa, muscularis mucosae.

• Small intestine (Duodenum): Villi, Brunner’s glands (secrete alkaline mucus).

• Small intestine (Ileum): Peyer’s patches (lymphatic tissue), villi.

• Large intestine: Goblet cells (secrete mucus).

• Liver: Central vein, hepatic portal triad (hepatic portal vein, hepatic artery, bile duct).

• Pancreas: Pancreatic islets (endocrine), acinar cells (exocrine).

Additional info: Histological details are essential for understanding the specialized functions of each digestive organ, such as secretion, absorption, and immune defense.