Digestive System Study Guide

Overview of the Digestive System

• The digestive system breaks down food into nutrients for absorption into the bloodstream.

• It comprises the alimentary canal (GI tract) and accessory digestive organs.

Primary Functions of the Digestive System

• Ingestion: Taking in food.

• Propulsion: Moving food through the GI tract.

  • Includes swallowing and peristalsis.

• Mechanical Digestion: Physical breakdown of food.

  • Examples: Chewing, churning, and segmentation.

• Chemical Digestion: Enzymatic breakdown into small molecules.

• Absorption: Uptake of nutrients into blood or lymph.

• Defecation: Elimination of indigestible substances as feces.

Layers of the GI Tract Wall

1. Mucosa:

   • Inner layer.

   • Functions: Secretes mucus, enzymes, and hormones; absorbs nutrients.

2. Submucosa:

   • Connective tissue layer.

   • Contains blood vessels, nerves, and lymphatics.

3. Muscularis:

   • Smooth muscle layer (inner circular and outer longitudinal).

   • Responsible for motility.

4. Serosa:

   • Protective outermost layer.

   • Also known as the visceral peritoneum.

Major Digestive Organs and Functions

Esophagus

• Function: Moves food from the pharynx to the stomach via peristalsis.

Stomach

• Muscular sac that churns food and initiates protein digestion.

• Secretions:

  • Pepsinogen (converted to pepsin for protein digestion).

  • HCl (Hydrochloric acid).

  • Intrinsic factor.

  • Mucus.

• Regions: Cardia, fundus, body, pylorus.

Small Intestine

• Main site of digestion and absorption.

• Sections:

  • Duodenum: Receives chyme, bile, and pancreatic juice.

  • Jejunum: Site of most nutrient absorption.

  • Ileum: Absorbs bile salts and remaining nutrients.

• Structural features: Villi and microvilli to increase surface area for absorption.

Liver

• Function: Produces bile for fat emulsification, processes nutrients, detoxifies blood, and stores vitamins.

Gallbladder

• Function: Stores and concentrates bile; releases it into the duodenum during digestion.

Pancreas

• Function: Produces digestive enzymes (amylase, lipase, proteases) and bicarbonate to neutralize stomach acid.

Large Intestine

• Function: Absorbs water and electrolytes, forms feces.

• Sections: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.

Key Enzymes and Their Functions

• Salivary amylase: Breaks down carbohydrates in the mouth.

• Pepsin: Digests proteins in the stomach.

• Pancreatic amylase: Continues carbohydrate digestion in the small intestine.

• Trypsin/chymotrypsin: Protein digestion in the small intestine.

• Pancreatic lipase: Digests fats with help from bile.

Digestive Hormones

• Gastrin: Stimulates acid secretion and gastric motility.

• Secretin: Stimulates bicarbonate release from the pancreas.

• Cholecystokinin (CCK): Triggers bile release and enzyme secretion from the pancreas.

Macronutrients

Macronutrients include carbohydrates, proteins, and lipids, which are required in large amounts to provide energy and support body functions.

• Carbohydrates – broken down into monosaccharides (mainly glucose).

  - Digestion begins in the mouth with salivary amylase, continues in the small intestine with pancreatic amylase.

  - Absorption occurs in the small intestine (mostly jejunum) via active transport into blood capillaries.

• Proteins – broken down into amino acids.

  - Digestion begins in the stomach (pepsin), continues in the small intestine with pancreatic proteases (trypsin, chymotrypsin).

  - Absorbed in the small intestine (duodenum and jejunum) via active transport into blood.

• Lipids – broken down into fatty acids and monoglycerides.

  - Emulsified by bile salts in the small intestine.

  - Pancreatic lipase breaks triglycerides into absorbable units.

  - Absorbed in the small intestine (mostly ileum) into lymphatic vessels via micelles and chylomicrons.

Micronutrients

Micronutrients include vitamins and minerals, which are required in smaller amounts but are essential for many physiological functions.

• Fat-soluble vitamins (A, D, E, K) – absorbed in the small intestine with dietary fats; stored in body fat.

• Water-soluble vitamins (B-complex, C) – absorbed directly into the bloodstream in the small intestine.

• Major minerals (e.g., calcium, sodium, potassium) – absorbed in the small intestine; calcium absorption is enhanced by vitamin D.

• Trace minerals (e.g., iron, zinc, iodine) – absorbed in the small intestine in smaller quantities.