Digestive System: Structure, Function, and Organization (ANAT 100 Study Notes)

Digestive System Overview

Introduction

The digestive system is responsible for breaking down the food we eat and converting it into substances that the body's cells can use. It consists of a long tube called the gastrointestinal (GI) tract and several accessory organs that aid in digestion.

• Gastrointestinal (GI) Tract: Also known as the digestive tract or alimentary canal, this is a continuous tube running from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, anal canal, and anus.

• Accessory Organs: These are not part of the tube but are essential for digestion. They include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Functions of the Digestive System

Major Functions

• Ingestion: Taking food and liquids into the mouth.

• Mixing and Propulsion: Mixing food with secretions and moving it along the GI tract through muscular contractions (peristalsis).

• Secretion: Cells in the GI tract secrete about 7 liters of water, acid, buffers, and digestive enzymes daily.

• Digestion: The breakdown of food into smaller molecules by mechanical and chemical processes.

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

• Defecation: Elimination of indigestible substances and waste as feces.

Digestion: Mechanical vs. Chemical

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning in the stomach, segmentation in the intestines).

• Chemical Digestion: Enzymatic breakdown of macromolecules into their building blocks (e.g., proteins to amino acids, carbohydrates to monosaccharides, fats to fatty acids and glycerol).

Absorption

• End products of digestion are absorbed through the epithelial lining of the GI tract into the blood or lymph.

• Proteins → amino acids

• Lipids → fatty acids and glycerol

• Carbohydrates → monosaccharides (e.g., glucose, fructose, galactose)

Defecation

• Elimination of waste products, indigestible food, bacteria, and sloughed cells as feces through the rectum and anus.

Structure of the GI Tract Wall

General Organization

The GI tract wall is composed of four basic layers, which are consistent throughout most of its length, though their specific structure may vary by region.

1. Mucosa: The innermost layer, consisting of epithelium (in direct contact with the lumen), connective tissue, and a thin layer of smooth muscle. Functions in protection, secretion, and absorption.

2. Submucosa: Areolar connective tissue containing blood vessels, lymphatics, and nerves (enteric nervous system). Binds the mucosa to the muscularis.

3. Muscularis: Typically two layers of muscle (circular and longitudinal), except in the stomach, which has an additional oblique layer. Responsible for peristalsis and mixing.

4. Serosa/Adventitia: The outermost layer. Above the diaphragm, it is called adventitia (connective tissue); below the diaphragm, it is serosa (a serous membrane also known as the visceral peritoneum).

The Peritoneum and Peritoneal Folds

Peritoneum

• Parietal Peritoneum: Lines the inside of the abdominal cavity.

• Visceral Peritoneum (Serosa): Covers the external surfaces of most digestive organs.

• Peritoneal Cavity: The space between the two layers, containing serous fluid to reduce friction.

Peritoneal Folds

• Mesentery: Double layer of visceral peritoneum anchoring the small intestine to the posterior abdominal wall.

• Mesocolon: Same tissue as mesentery, but anchors the large intestine/colon.

• Lesser Omentum: Connects the stomach and duodenum to the liver.

• Greater Omentum: Hangs in front of the abdominal organs, attaches to the greater curvature of the stomach and transverse colon, contains adipose tissue and immune cells, and helps with wound healing.

GI Tract Organs

Mouth/Oral Cavity

• Takes in food and mixes it with saliva to form a bolus.

• Mechanical Digestion: Chewing (mastication) breaks food into smaller pieces; tongue moves food for effective chewing.

• Chemical Digestion: Saliva contains salivary amylase, which begins carbohydrate digestion.

Salivary Glands

• Three pairs: parotid (anterior to ear), submandibular (under mandible), sublingual (under tongue).

• Produce 1–1.5 liters of saliva per day, which is slightly acidic and contains water, electrolytes, proteins, mucin, and salivary amylase.

Esophagus

• Collapsible tube lined with stratified squamous epithelium.

• Transports food from the mouth to the stomach via peristalsis (coordinated contraction and relaxation of muscularis layers).

• Passes through the diaphragm at the esophageal hiatus.

• Lower esophageal (cardiac) sphincter controls entry into the stomach and prevents reflux.

Stomach

• J-shaped organ connecting the esophagus to the duodenum (first part of the small intestine).

• Can hold up to 6.4 liters of food; most elastic part of the GI tract.

• Regions: cardia (entry), fundus (storage), body (main part), pylorus (exit to duodenum via pyloric sphincter).

• Wall has three muscle layers (longitudinal, circular, oblique) for enhanced mechanical digestion.

Stomach Mucosa

• Contains gastric glands with different cell types:

  • Surface mucous cells: Secrete mucus to protect the stomach lining.

  • Mucous neck cells: Secrete mucus.

  • Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor (for vitamin B12 absorption).

  • Chief cells: Secrete pepsinogen (inactive form of pepsin) and gastric lipase.

  • G cells: Secrete the hormone gastrin.

• HCl creates an acidic environment, converting pepsinogen to pepsin (active protease).

Stomach Functions

• Temporary storage of food.

• Secretes gastric juice; mixes with food to form chyme.

• Mechanical and chemical digestion (proteins and lipids).

• Moves chyme into the duodenum.

• Absorbs some substances (water, alcohol, certain drugs).

Small Intestine

• About 3 meters long, 2.5 cm in diameter.

• Extends from the pyloric sphincter to the ileocecal sphincter (junction with large intestine).

• Three parts: duodenum, jejunum, ileum.

• Mucosa is highly folded (villi and microvilli) to increase surface area for absorption.

• Major function: absorption of nutrients.

Large Intestine/Colon

• About 1.5 meters long, 6.5 cm in diameter.

• Extends from the ileocecal sphincter to the anus.

• Parts: cecum (with appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal.

• Functions: absorption of water and electrolytes, formation and elimination of feces, bacterial fermentation of undigested carbohydrates, production of certain vitamins (e.g., vitamin K, some B vitamins).

Accessory Organs

Liver and Gallbladder

• Liver: Large organ in the right upper quadrant; produces bile, processes nutrients, detoxifies substances, stores vitamins and minerals.

• Gallbladder: Small, pear-shaped organ under the liver; stores and concentrates bile, releases it into the duodenum to aid in fat digestion.

Bile

• Produced by the liver, stored in the gallbladder.

• Contains bile salts (from cholesterol), bile pigments (from hemoglobin breakdown), and other substances.

• Emulsifies fats, aiding in their digestion and absorption.

Pancreas

• Both an endocrine and exocrine gland.

• Exocrine function: produces pancreatic juice containing digestive enzymes (amylase, proteases, lipase) and bicarbonate to neutralize stomach acid.

• Pancreatic juice is secreted into the duodenum via the pancreatic duct, which joins the common bile duct at the hepatopancreatic ampulla.

Integration: How It All Fits Together

Hepatic Portal System

• Nutrients absorbed from the GI tract are transported to the liver via the hepatic portal vein for processing before entering the systemic circulation.

• Main vessels: superior mesenteric vein, splenic vein, hepatic portal vein.

Summary Table: Layers of the GI Tract

Layer	Main Components	Function
Mucosa	Epithelium, connective tissue, smooth muscle	Protection, secretion, absorption
Submucosa	Areolar connective tissue, blood/lymph vessels, nerves	Support, binds mucosa to muscularis
Muscularis	Smooth muscle (circular, longitudinal; oblique in stomach)	Peristalsis, mixing
Serosa/Adventitia	Connective tissue, epithelium (serosa)	Protection, reduces friction (serosa); anchoring (adventitia)

Key Terms and Definitions

• Peristalsis: Wave-like muscular contractions that move food along the GI tract.

• Chyme: Semi-fluid mass of partially digested food and gastric juice in the stomach.

• Bolus: Chewed food mixed with saliva, ready to be swallowed.

• Sphincter: Circular muscle that regulates passage of material between GI tract regions.

• Emulsification: Breakdown of large fat globules into smaller droplets, increasing surface area for enzyme action.