Digestive System Lecture Notes

Overview

• The digestive system is a 30ft tube from the mouth to the anus.
• It involves multiple organs, including the stomach, small intestine, and accessory organs like the liver, gallbladder, and pancreas.

Stomach

• Location: Below the diaphragm and liver, near the heart.
• Functions:
  • Mechanical and chemical digestion.
  • Converts food into a soupy mixture called chyme.
• Structure:
  • Muscular sac with volume ranging from 50 mL to 4 liters.
  • J-shaped with regions: cardiac orifice, fundus, body, pylorus.
  • Innovated by parasympathetic and sympathetic fibers.
  • Blood leaves the stomach to the liver.

Stomach Layers

• Longitudinal, circular, and oblique muscles.
• Lining has gastric rugae for mechanical digestion.
• Composed of simple columnar epithelium.

Gastric Glands

• Cell Types:
  • Mucous cells: produce mucus.
  • Parietal cells: produce hydrochloric acid.
  • Chief cells: secrete pepsinogen, which is activated to pepsin by HCl.
  • Enteroendocrine cells: secrete hormones and paracrine messengers.

Hydrochloric Acid (HCl)

• Produced by parietal cells.
• Activates pepsin, breaks down connective tissues and cellulose, and acts as a defense mechanism.

Intrinsic Factor

• Produced by parietal cells.
• Essential for vitamin B12 absorption, crucial for red blood cell production.
• Deficiency can lead to the need for B12 injections in the elderly.

Gastric Phases of Secretion

• Cephalic Phase: Brain signals stomach to prepare for food.
• Gastric Phase: Food in stomach stimulates further secretion.
• Intestinal Phase: Intestines signal stomach to stop and prepare to receive food.

Accessory Organs

Liver

• Location: Largest organ, above the stomach.
• Functions:
  • Processes nutrients, toxins, hormones.
  • Produces bile stored in the gallbladder.

Gallbladder

• Stores bile produced by the liver.
• Releases bile into the small intestine to aid fat digestion.

Pancreas

• Location: Behind the stomach.
• Functions:
  • Endocrine: secretes hormones like insulin.
  • Exocrine: produces pancreatic juice with enzymes and bicarbonate.

Small Intestine

• Sections: Duodenum, jejunum, ileum.
• Functions:
  • Final stages of chemical digestion and nutrient absorption.
  • Large surface area due to circular folds, villi, and microvilli.

Digestive Enzymes

• Zymogens: Inactive forms activated in the intestine.
  • Examples: trypsinogen, chymotrypsinogen.
• Enzymes:
  • Amylase: carbohydrate digestion.
  • Lipase: fat digestion.
  • Ribonuclease and deoxyribonuclease: digest nucleic acids.

Key Concepts

• Digestion involves both mechanical and chemical processes.
• Regulation involves nervous and hormonal signals.
• Accessory organs play vital roles in aiding digestion with enzymes and bile.
• Integration and regulation occur between all digestive phases and organs.