Chapter 23: The Digestive System – Structure, Function, and Physiology

The Digestive System: Overview

Introduction and Major Functions

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It consists of a long tube called the alimentary canal (gastrointestinal tract) and accessory organs that aid in digestion.

• Main responsibilities: Transfer nutrients, water, and electrolytes from food into the body’s internal environment.

• Alimentary canal (GI tract): Mouth, pharynx, esophagus, stomach, small intestine, large intestine (about 30 ft long).

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

The Digestive Tract: A Disassembly Line

Major Digestive Processes

• Ingestion: Taking food into the mouth.

• Propulsion/Motility: Moving food through the tract via peristalsis and segmentation.

• Mechanical Digestion: Physically breaking food into smaller pieces (e.g., chewing, churning).

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

• Secretion: Release of digestive juices and enzymes.

• Absorption: Transport of digested nutrients into blood or lymph.

• Defecation: Elimination of indigestible substances as feces.

Control of the Digestive System

Neural Regulation

• Enteric Nervous System (ENS): The GI tract’s own nervous system, also called the "gut brain." Contains more neurons than the spinal cord and controls motility and secretion.

• Gut-brain axis: Extensive communication between enteric neurons and the central nervous system.

• Vagus nerve: The primary parasympathetic nerve that increases digestive activity.

Digestive Processes of the Mouth

Functions and Phases

• Ingestion: Food enters the mouth.

• Mechanical digestion: Chewing breaks food into smaller pieces.

• Chemical digestion: Salivary amylase begins carbohydrate digestion.

• Exocrine function: Salivary glands secrete saliva to form a bolus.

• Propulsion: Swallowing moves food to the pharynx (no absorption occurs in the mouth).

Swallowing (Deglutition)

• Voluntary phase: Tongue pushes bolus against hard palate, moving it into the pharynx.

• Involuntary phase: Epiglottis blocks larynx, peristalsis propels bolus through esophagus, and mucus secretion aids movement.

Motility in the Digestive Tract

Peristalsis vs. Segmentation

	Peristalsis	Segmentation
Description	Directional wave of contraction and relaxation of smooth muscle	Coordinated, non-directional contraction and relaxation
Function	Propels food forward	Mixes food, mechanical digestion, and mixing with digestive juices
Location	Esophagus, stomach, small and large intestine, rectum	Small and large intestine

Digestive Processes in the Stomach

Major Functions

• Mechanical digestion: Churning converts food bolus to chyme.

• Chemical digestion: Enzymes (pepsin, lipases) break down proteins and some fats.

• Absorption: Minimal; mainly water, alcohol, and some drugs.

• Secretion: Intrinsic factor (IF) for vitamin B12 absorption; deficiency leads to pernicious anemia.

Stomach Epithelium and Microscopic Anatomy

• Gastric pits: Contain exocrine glands and cells (mucous, chief, parietal) that secrete various substances.

• Mucous cells: Secrete alkaline mucus for protection.

• Chief cells: Secrete pepsinogen and gastric lipase.

• Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.

• Enteroendocrine cells: Release hormones (e.g., gastrin, histamine, serotonin, somatostatin).

Activation of Pepsin

• Chief cells secrete pepsinogen (inactive form).

• Parietal cells secrete HCl, which converts pepsinogen to pepsin (active enzyme).

• Pepsin digests proteins and also activates more pepsinogen.

Clinical Relevance: Peptic or Gastric Ulcers

• Ulcers are erosions in the stomach wall, potentially leading to peritonitis and hemorrhage if perforated.

• Most ulcers are caused by Helicobacter pylori infection or NSAIDs (e.g., aspirin).

Stomach Mucosa and Gastric Glands Table

Type of Secretory Cell	Product Secreted	Stimuli for Secretion	Function(s)
Mucous cells	Alkaline mucus	Mechanical stimulation, irritation	Protects mucosa from acid/enzymes
Chief cells	Pepsinogen	ACh, gastrin	Begins protein digestion
Parietal cells	HCl, intrinsic factor	ACh, gastrin, histamine	Activates pepsinogen, absorbs vitamin B12
Enterochromaffin-like (ECL) cells	Histamine	ACh, gastrin	Stimulates parietal cells
G cells	Gastrin	Protein, ACh	Stimulates chief/parietal cells
D cells	Somatostatin	Acid	Inhibits gastric secretion

Regulation of Gastric Secretion

• Cephalic phase: Triggered by sight, smell, or thought of food; stimulates gastric secretion via vagus nerve.

• Gastric phase: Food in stomach increases secretion via stretch and chemical stimuli.

• Intestinal phase: Chyme entering duodenum initially stimulates, then inhibits gastric secretion.

The Small Intestine

Structure and Function

• Receives input from pancreas, gallbladder, and stomach.

• Divided into duodenum, jejunum, and ileum.

• Main site of digestion and absorption.

Pancreas: Endocrine and Exocrine Functions

• Endocrine: Islets of Langerhans secrete insulin (beta cells) and glucagon (alpha cells) to regulate blood sugar.

• Exocrine: Acinar cells secrete digestive enzymes and alkaline fluid into the duodenum.

Blood Glucose Regulation

• Insulin lowers blood glucose by promoting uptake into cells.

• Glucagon raises blood glucose by stimulating glycogen breakdown in the liver.

• Negative feedback loop maintains homeostasis.

Diabetes Mellitus (DM)

• Polyuria: Excessive urine output due to glucose in urine.

• Polydipsia: Excessive thirst from water loss.

• Polyphagia: Excessive hunger due to inability to use glucose.

• Fat metabolism produces ketones, which can cause ketoacidosis if they accumulate.

Regulation of Pancreatic Exocrine Secretion

• Regulated by hormones secretin (stimulates bicarbonate secretion) and cholecystokinin (CCK) (stimulates enzyme secretion).

• Pancreatic amylase: Digests carbohydrates.

• Proteases (e.g., trypsin): Digest proteins.

• Lipases: Digest fats, work with bile salts.

Liver and Gallbladder

• Bile: Produced by liver, stored in gallbladder, released into small intestine to emulsify fats.

• Bile salts: Derivatives of cholesterol, recycled via enterohepatic circulation.

• Bile is continuously secreted and stored until needed for fat digestion.

Emulsification and Micelle Formation

• Bile salts emulsify lipids, increasing surface area for enzyme action.

• Emulsification does not break chemical bonds; it forms micelles for lipid absorption.

Absorptive Structures: Villi and Microvilli

• Villi: Fingerlike projections that increase surface area for absorption.

• Microvilli: Tiny projections on enterocytes forming the brush border, further increasing surface area.

Cells of the Small Intestine

• Enterocytes: Absorptive cells.

• Goblet cells: Secrete mucus.

• Enteroendocrine cells: Release hormones regulating digestion.

• Paneth cells: Secrete defensins and lysozyme (antimicrobial).

• Stem cells: Differentiate into other cell types.

Absorption Mechanisms

• Occurs via active (requires ATP) and passive (diffusion) transport.

• Transport proteins in the enterocyte membrane facilitate uptake of nutrients.

Digestion and Absorption of Nutrients

Carbohydrates

• Digestion: Begins with salivary amylase, continues with pancreatic amylase, and finishes with brush border enzymes (e.g., sucrase).

• Absorption: Glucose and galactose absorbed via SGLT1 (Na+-dependent cotransport); fructose via facilitated diffusion (GLUT2).

Proteins

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

• Absorption: Amino acids absorbed via Na+-dependent cotransporters; small peptides also absorbed.

Lipids

• Digestion: Emulsified by bile salts, digested by pancreatic lipase into fatty acids and monoglycerides.

• Absorption: Fatty acids and monoglycerides diffuse into enterocytes, reassembled into triglycerides, packaged as chylomicrons, and transported via lymphatics.

The Large Intestine

Functions and Microbiome

• Absorbs remaining water and salts from food residue.

• Secretes alkaline mucus.

• Stores and eliminates feces.

• Hosts the gut microbiome: beneficial bacteria with metabolic and immune functions, influencing weight, disease susceptibility, and mood.

Summary Table: Digestion of Carbohydrates, Proteins, and Lipids

Carbohydrates	Proteins	Lipids
Ingested as polysaccharides, disaccharides, monosaccharides. Amylase breaks down polysaccharides. Brush border enzymes digest disaccharides. Monosaccharides absorbed by Na+-dependent active transport.	Digestion begins in stomach, continues in small intestine. Pancreatic proteases and brush border enzymes break proteins into amino acids. Absorbed by Na+-dependent cotransport; only small peptides and amino acids absorbed.	Emulsification by bile salts increases surface area. Lipases produce free fatty acids and monoglycerides. Form micelles, absorbed, reassembled into chylomicrons, transported via lymphatics.

Additional info: The notes above provide a comprehensive overview of the digestive system, including structure, function, regulation, and clinical relevance, suitable for college-level Anatomy & Physiology students.