Digestive System: Structure, Function, and Processes

Chapter 23: The Digestive System

Overview

The digestive system is responsible for breaking down food into nutrients, absorbing these nutrients into the body, and eliminating indigestible remains. It consists of the alimentary canal and accessory organs, each with specialized structures and functions.

Organs of the Digestive System

Alimentary Canal

• Definition: The alimentary canal (gastrointestinal tract, GI tract) is a continuous, muscular tube that digests and absorbs food.

• Main Organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus.

Accessory Organs

• Definition: Organs that assist digestion by producing secretions or aiding mechanical processing.

• Main Organs: Teeth, tongue, gallbladder, salivary glands, liver, pancreas.

• Functions: Produce saliva, bile, and digestive enzymes.

Essential Activities of Digestion

Six Essential Activities

• Ingestion: Taking food into the digestive tract.

• Propulsion: Moving food through the alimentary canal, including swallowing and peristalsis (waves of muscle contraction and relaxation).

• Mechanical Digestion: Physical breakdown of food (chewing, mixing, churning, segmentation).

• Chemical Digestion: Enzymatic breakdown of food molecules into their building blocks (monomers).

• Absorption: Transport of digested end products from the GI tract into blood or lymph.

• Defecation: Elimination of indigestible substances as feces.

Structure of the Alimentary Canal Wall

Basic Layers (Tunics)

• Mucosa: Innermost layer; secretes mucus, digestive enzymes, and hormones; absorbs nutrients; protects against infection.

• Submucosa: Contains blood vessels, lymphatics, nerve fibers, and elastic tissue.

• Muscularis Externa: Responsible for segmentation and peristalsis; typically has inner circular and outer longitudinal smooth muscle layers; forms sphincters.

• Serosa: Outermost protective layer.

Mesentery: A double layer of peritoneum that supports digestive organs, provides routes for vessels and nerves, and stores fat.

Hepatic Portal Circulation: Collects nutrient-rich blood from the digestive tract and delivers it to the liver for processing.

Digestive System Organs and Their Functions

Mouth

• Functions: Ingestion, mechanical digestion (chewing), initiation of swallowing, mixing food with saliva to form a bolus.

• Saliva: Contains water, electrolytes, salivary amylase (begins starch digestion), mucin, lysozyme, IgA, and metabolic wastes.

• Absorption: Essentially none occurs in the mouth.

Pharynx and Esophagus

• Function: Serve as passageways for food from the mouth to the stomach; involved in propulsion only.

Stomach

• Structure: Four regions—cardiac, fundus, body, pylorus.

• Functions: Temporary storage, mechanical and chemical breakdown of food, formation of chyme.

• Gastric Glands:

  • Parietal cells: Secrete HCl (acidifies stomach, activates pepsinogen, kills bacteria) and intrinsic factor (needed for vitamin B12 absorption).

  • Chief cells: Produce pepsinogen (inactive enzyme, activated to pepsin for protein digestion).

  • Enteroendocrine cells: Release hormones (e.g., gastrin) that regulate secretion and motility.

• Gastric Secretion Phases:

  • Cephalic phase: Triggered by sight, smell, taste, or thought of food; prepares stomach for digestion.

  • Gastric phase: Begins when food enters stomach; neural and hormonal mechanisms stimulate secretion.

  • Intestinal phase: Excitatory (stimulates gastric activity as chyme enters duodenum) and inhibitory (enterogastric reflex slows gastric activity as intestine fills).

• Motility: Peristaltic waves mix and propel chyme; pyloric sphincter regulates emptying into small intestine.

• Essential Function: Secretion of intrinsic factor for vitamin B12 absorption.

Small Intestine

• Subdivisions: Duodenum (~25 cm), jejunum (~2.5 m), ileum (~3.6 m).

• Functions: Completion of digestion and absorption of nutrients.

• Structural Adaptations:

  • Plicae circulares: Circular folds that slow chyme movement and increase absorption time.

  • Villi: Fingerlike projections containing capillaries and lacteals for nutrient absorption.

  • Microvilli: Tiny projections (brush border) with enzymes for final digestion of carbohydrates and proteins.

Accessory Organs: Liver, Gallbladder, Pancreas

• Liver:

  • Produces bile (fat emulsifier).

  • Processes nutrient-rich blood from GI tract.

  • Composed of lobules with hepatocytes, portal triads, and sinusoids containing Kupffer cells (macrophages).

• Gallbladder: Stores and concentrates bile; releases bile in response to cholecystokinin when fatty chyme enters duodenum.

• Pancreas:

  • Produces pancreatic juice (enzymes and bicarbonate, pH ~8.0) for digestion.

  • Enzymes include amylase, proteases (trypsin, chymotrypsin, carboxypeptidase), lipases, nucleases.

  • Endocrine function: produces insulin and glucagon.

Large Intestine

• Subdivisions: Cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, anal canal.

• Functions: Absorbs water and electrolytes, forms and eliminates feces.

• Structural Features: Lacks villi and plicae circulares; thick mucosa with many goblet cells for lubrication.

• Bacterial Flora: Ferment indigestible carbohydrates, synthesize B vitamins and vitamin K.

• Defecation Reflex: Triggered by rectal stretch; involves involuntary and voluntary muscle actions.

Chemical Digestion and Absorption

Carbohydrates

• Digestion: Begins in mouth (salivary amylase), continues in small intestine (pancreatic amylase, brush border enzymes).

• Absorption: Monosaccharides absorbed via facilitated diffusion into blood.

Proteins

• Digestion: Begins in stomach (pepsin), continues in small intestine (pancreatic proteases, brush border enzymes).

• Absorption: Amino acids absorbed via facilitated diffusion into blood.

Lipids

• Digestion: Begins in small intestine; bile salts emulsify fats, pancreatic lipase digests triglycerides.

• Absorption:

  • Short-chain fatty acids: absorbed by diffusion into capillary blood.

  • Long-chain fatty acids and monoglycerides: diffuse into intestinal cells, reassembled into triglycerides, packaged as chylomicrons, and transported via lymphatics.

Common Disorders of the Digestive System

Disorder	Description	Causes/Notes
Gastric Ulcers	Erosions in the stomach wall	Most commonly caused by Helicobacter pylori infection
Diverticulosis	Formation of small herniations (diverticula) in colon wall	Associated with low-fiber diet, increased colon pressure; inflamed diverticula = diverticulitis
Diarrhea	Watery stools due to rapid transit through large intestine	Leads to dehydration and electrolyte imbalance