BIOL 252 Lesson 20 Notes

General Structure of the Digestive System

  • Gastrointestinal Tract: Also known as the alimentary canal, it is the continuous passage through which food moves.
  • Accessory Organs: Assist in digestion but usually do not come into direct contact with food.

Functions of the Digestive System

  1. Ingestion: Taking in food and fluids.
  2. Secretion: Hormones, enzymes, acids, mucus, etc., from endocrine and exocrine glands aid digestion.
  3. Propulsion: Rhythmic contractions (peristalsis) move food through the alimentary canal.
  4. Digestion: Breakdown of food via:
    • Mechanical means (e.g. chewing)
    • Chemical means (e.g. enzymatic actions)
  5. Absorption: Transfer of nutrients from the digestive tract into blood or lymphatic vessels.
  6. Defecation: Elimination of indigestible substances and metabolic waste.

Stomach Anatomy and Functions

  • Muscular Layers: The stomach has a third layer of smooth muscle (muscularis externa) for churning food.
  • Chyme Formation: Mixture of food and gastric juices.
Cellular Composition
  • Goblet Cells: Produce mucus to protect the stomach lining.
  • Chief Cells: Secrete pepsinogen (inactive form of pepsin).
  • Parietal Cells: Produce HCl and intrinsic factor.

Gastric Juice Components

  • Hydrochloric Acid (HCl):
    • Functions:
      • Activates pepsinogen to pepsin for protein digestion
      • Provides an acidic environment for digestion
      • Defends against pathogens
  • Pepsin: Active enzyme that digests proteins.

Phases of Gastric Acid Production

  1. Cephalic Phase: Initiated by sight, smell, or thought of food; involves acetylcholine release.
  2. Gastric Phase: Triggered by food entering the stomach; releases gastrin and increases acid secretion.
  3. Intestinal Phase: Regulates gastric secretion via partially digested food in the small intestine; secretin slows gastric activity.
Receptive Relaxation
  • Accommodates food as it enters the stomach; mediated by the parasympathetic nervous system via the vagus nerve.

Small Intestine Structure and Function

  • Regions: Duodenum, Jejunum, and Ileum.
  • Nutrient Absorption: Primarily occurs in the jejunum and ileum.
  • Surface Area Increase: Achieved through circular folds, villi, and microvilli.
Cell Types
  • Enterocytes: Special epithelial cells with microvilli that increase absorption surface area.
  • Lacteals: Lymphatic vessels in the villi that absorb lipids.

Digestion of Nutrients

Carbohydrates
  • Digestion Process:
    • Begins in the mouth with salivary amylase.
    • Continues in the small intestine with pancreatic enzymes.
  • Monosaccharide Absorption: Via facilitated diffusion across the basolateral membrane.
Proteins
  • Breakdown: Proteins are reduced to amino acids by:
    • Pepsin in the stomach
    • Pancreatic enzymes in the small intestine.
  • Transport: Amino acids absorbed through secondary active transport facilitated by Na+/K+ pumps.
Lipids
  • Emulsification: Bile salts break down fats into smaller droplets in the small intestine.
  • Micelle Formation: Lipids are surrounded by bile salts to facilitate absorption into enterocytes.
  • Chylomicron Formation: Lipids are repackaged into chylomicrons before entering the lymphatic system.

Pancreatic Function

  • Endocrine and Exocrine: Secretes pancreatic juices rich in digestive enzymes regulated by CCK and secretin.

Liver and Gallbladder Functions

  • Bile Production: Assists in lipid digestion and is stored in the gallbladder.
  • Other Functions:
    • Storage of carbohydrates (glycogen)
    • Protein production (e.g., albumin)
    • Detoxification of harmful substances.

Defecation

  1. Stretch Receptors: Detect fecal matter in the distal colon, sending signals to the spinal cord.
  2. Relaxation of Smooth Muscles: Results in the contraction of the sigmoid colon and rectum.
  3. Voluntary Control: Adults control anal sphincter relaxation through the cerebral cortex.

Bristol Stool Chart

  • Types of Stools: Ranges from separate hard lumps (Type 1) to liquid consistency (Type 7) indicating digestive health issues such as constipation or diarrhea.

Summary

  • Understanding the components of gastric juices, mechanisms of digestion for nutrients, and the processes involved in absorption are crucial for grasping the broader functions of the digestive system.