Digestive System: Structure and Function Study Notes

Digestive System Overview

Main Organs of the Digestive System

The digestive system is composed of organs that process food, absorb nutrients, and eliminate waste. These organs are categorized as part of the digestive tract or as accessory organs.

• Digestive Tract: Mouth, Pharynx, Esophagus, Stomach, Small intestine, Large intestine

• Accessory Organs: Teeth, Tongue, Salivary glands, Liver, Gallbladder, Pancreas

Layers of the Digestive Tract Wall

The wall of the digestive tract is organized into four distinct layers, from the innermost to the outermost:

• Mucosa: The innermost lining, involved in secretion and absorption.

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

• Muscularis externa: Smooth muscle responsible for peristalsis and segmentation.

• Serosa: The outermost layer, providing structural support and protection.

Muscular Layers of the Digestive Tract

Most digestive tract organs have two layers of smooth muscle:

• Inner layer: Circular muscle

• Outer layer: Longitudinal muscle

• Stomach: Has an additional oblique muscle layer for enhanced churning.

Mesenteries

The mesenteries are double layers of peritoneum that connect the visceral and parietal peritoneum, supporting the intestines and providing a pathway for blood vessels, nerves, and lymphatics.

Pathway of Food Through the Digestive Tract

Order of Organs

Food passes through the following organs in sequence:

1. Mouth

2. Pharynx

3. Esophagus

4. Stomach

5. Small intestine

6. Large intestine

7. Rectum

Oral Cavity and Associated Structures

Functions of the Oral Cavity

• Ingestion and initial mechanical processing of food

• Mixing food with saliva to form a bolus

• Enzymatic digestion of carbohydrates (salivary amylase)

• Sensory analysis (taste, texture, temperature)

• Speech production and articulation

Salivary Glands

There are three pairs of major salivary glands:

• Parotid glands: Located in front of and below the ears

• Sublingual glands: Located beneath the tongue

• Submandibular glands: Located beneath the lower jaw or mandible

Teeth

• 20 deciduous (primary) teeth

• 32 permanent teeth

Pharynx

• Three divisions: Nasopharynx, Oropharynx, Laryngopharynx

• Primary function: Moves food from the mouth to the esophagus during swallowing

Esophagus

• Muscular tube connecting the oral cavity to the stomach

Stomach

Regions of the Stomach

• Cardia

• Fundus

• Body

• Pylorus

Stomach Mucosa and Secretory Cells

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

• Chief cells: Secrete pepsinogen (converted to pepsin for protein digestion)

• G cells: Secrete gastrin, a hormone that stimulates gastric juice secretion and motility

Small Intestine

Regions of the Small Intestine

• Duodenum

• Jejunum

• Ileum

Villi and Absorption

• Villi contain blood capillaries (for nutrient absorption) and lacteals (lymphatic capillaries for fat absorption)

• The brush border increases surface area and contains enzymes for digestion

• Peyer's patches: Lymphatic nodules in the ileum for immune defense

Large Intestine

Regions and Subregions

• Cecum

• Colon

• Rectum

Primary Functions

• Water and electrolyte absorption

• Feces formation and storage

• Bacterial fermentation of undigested material

Sphincters of the Anal Canal

• External anal sphincter: Skeletal muscle (voluntary control)

• Internal anal sphincter: Smooth muscle (involuntary control)

Liver

Functions of the Liver

• Produces bile for fat digestion

• Stores nutrients (glycogen, vitamins)

• Detoxifies blood

• Metabolizes carbohydrates, proteins, and fats

Blood Supply to the Liver

• Hepatic artery proper: Delivers oxygen-rich blood from the aorta

• Hepatic portal vein: Delivers nutrient-rich, oxygen-poor blood from the small intestine

Portal Triads

• Each triad contains: branch of the hepatic artery, branch of the hepatic portal vein, and bile duct

Bile Pathways

• Bile leaves the liver via the duodenum or is stored in the gallbladder

Gallbladder

• Stores and concentrates bile

• Release of bile is stimulated by the hormone cholecystokinin (CCK)

• Bile leaves the gallbladder through the cystic duct

Pancreas

Endocrine and Exocrine Functions

• Endocrine: Pancreatic islets produce insulin and glucagon (regulate blood glucose)

• Exocrine: Acinar cells produce digestive enzymes

Hormones

• Insulin: Lowers blood glucose

• Glucagon: Raises blood glucose

Summary Table: Digestive Tract Layers

Layer	Main Components	Function
Mucosa	Epithelial lining, lamina propria, muscularis mucosae	Secretion, absorption, protection
Submucosa	Connective tissue, blood vessels, nerves	Support, nutrient supply
Muscularis externa	Circular and longitudinal muscle layers	Motility (peristalsis, segmentation)
Serosa	Connective tissue, mesothelium	Protection, structural support

Key Equations and Terms

• Peristalsis: Involuntary wave-like muscle contractions that move food along the digestive tract.

• Segmentation: Contractions that mix and break down food in the intestines.

• Gastrin: $\text{G cells secrete gastrin} \rightarrow \text{stimulates gastric juice secretion}$

• Insulin and Glucagon: $\text{Insulin} \downarrow \text{blood glucose}$, $\text{Glucagon} \uparrow \text{blood glucose}$