Digestive System Overview

Alimentary Canal

• Begins in the mouth.
• Digestive Processes:
  • Ingestion
  • Physical digestion (mastication or chewing)
  • Chemical digestion (enzymes)
  • Propulsion (swallowing)

Mouth

• Teeth: Used for mastication.
• Tongue: Synthesizes enzymes; positions food into a bolus.
• Salivary Glands: Secrete saliva containing ions, glycoproteins, enzymes, waste, water, and lysosomes. Maintains a pH of 7 and lubricates the bolus.
• Enzymes:
  • Salivary amylase: Breaks down starch/glycogen. Active in mouth, pharynx, and esophagus; inactive in stomach.
  • Lingual lipase: Breaks down lipids. Inactive in mouth, pharynx, esophagus; active in stomach.

Pharynx and Esophagus

• Pharynx: Pathway for air and food.
• Esophagus: Moves food only.
  • Upper Esophageal Sphincter: Controls bolus entry.
  • Muscle Composition:
    • Upper ⅔: Smooth (involuntary) and skeletal (voluntary) muscle.
    • Lower ⅓: Only smooth muscle.
  • Lower Esophageal Sphincter: Prevents acidic stomach contents from entering esophagus.

Stomach

• Regions: Fundus, body, pylorus.
• Structures:
  • Rugae: Allows stomach expansion.
  • Pyloric Sphincter: Controls chyme movement into small intestine.
  • Surface Mucus Cells: Protect stomach surface with alkaline mucus.
  • Gastric Pits: Connect to gastric glands.
• Cells in Gastric Pits:
  • Parietal Cells: Secrete HCl and intrinsic factor for B12 absorption.
  • Mucus Neck Cells: Secrete unknown function mucus.
  • Chief Cells: Secrete pepsinogen (inactive pepsin for protein digestion).
  • Enteroendocrine Cells: Secrete hormones (e.g., gastrin).
• Digestive Processes:
  • Propulsion (peristalsis, gastric emptying)
  • Mechanical Digestion (gastric mixing)
  • Chemical Digestion (pepsin for proteins, lingual lipase for lipids, gastric lipase)
  • No absorption occurs in the stomach.

Small Intestine

• Regions: Duodenum, jejunum, ileum.
• Digestive Processes:
  • Propulsion (peristalsis)
  • Chemical Digestion (enzymes)
  • Mechanical Digestion (segmentation)
  • Absorption of nutrients.
• Structures and Cells:
  • Circular Folds, Villi, Microvilli: Increase surface area for absorption.
  • Goblet Cells: Secrete mucus for protection and lubrication.
  • Crypts of Lieberkühn: Secrete intestinal juices and buffers HCl.
• Lipid Digestion Molecules: Bile, pancreatic lipase, co-lipase.

Large Intestine

• Sphincters:
  • Ileocecal Sphincter: Separates small and large intestine.
  • Internal Anal Sphincter: Smooth muscle, involuntary.
  • External Anal Sphincter: Skeletal muscle, voluntary.
• Main Function:
  • Finish absorption.
  • Absorb water.
  • Form and defecate feces.
• Digestive Processes:
  • Some absorption.
  • Propulsion (mass movement, peristalsis).
  • Mechanical Digestion (haustral contractions).
  • Defecation.
  • No chemical digestion.

Accessory Organs

Liver/Gallbladder

• Functions:
  • Synthesizes bile.
  • Removes old RBCs.
  • Processes nutrients post-meal.
  • Secreted hormones and plasma proteins.
  • Eliminates wastes.
  • Stores vitamins, e.g., vitamin A.
• Bile Salts:
  • Synthesized in liver from cholesterol.
  • Emulsify fats, increasing surface area for digestion.

Pancreas

• Pancreatic Juice: Contains zymogens, bicarbonate buffer, co-lipase, enzymes (pancreatic lipase, amylase).
• Zymogens: Inactive proteases activated in the small intestine.
• Pathway: Acinar cells → Pancreatic duct → Ampulla of Vater → Sphincter of Oddi → Duodenum

Nutrient Metabolism

Carbohydrate Digestion and Absorption

• Enzymes: Amylase.
• Processes:
  • Starch/glycogen → Maltose → Glucose via dextrinase and maltase (brush border enzymes).
  • Na/K Pump: Establishes sodium gradient for absorption.
• Absorption:
  • Glucose and Galactose: Cotransport with Na, facilitated diffusion into blood.
  • Fructose: Facilitated diffusion.

Protein Digestion and Absorption

• Enzymes: Proteases (endopeptidases, exopeptidases).
• Processes:
  • Protein digestion begins in the stomach (pepsinogen to pepsin).
  • Continues in the small intestine with zymogen activation.
• Absorption:
  • Uses Na/K pump for gradient.
  • Di/Tri Peptides: Co-transport with hydrogen ions, broken into amino acids by brush border enzymes.

Lipid Digestion and Absorption

• Types:
  • 90% triglycerides.
• Enzymes: Lipases, secreted from various organs.
• Processes:
  • Bile salts emulsify lipids, increasing surface area for lipase action.
  • Micelles release lipophilic molecules for absorption.
• Absorption:
  • Micelles: Release lipophilic molecules.
  • Smooth ER: Reassembles triglycerides.
  • Golgi Apparatus: Packages triglycerides into chylomicrons.
  • Chylomicrons: Transported to lymphatic system.

Metabolic Pathways

Carbohydrate Metabolism

• Glycolysis: Occurs in cytoplasm; inputs glucose, outputs ATP and pyruvate.
• Pyruvate Oxidation (Linking Step): Converts pyruvate to Acetyl CoA in mitochondria.
• Krebs Cycle: Converts Acetyl CoA to ATP, NADH, FADH2 in mitochondria.
• Oxidative Phosphorylation: Electron transport chain and chemiosmotic coupling produce ATP from NADH/FADH2.

Lipid Metabolism

• Lipogenesis: Formation of lipids from Acetyl CoA.
• Lipolysis: Breakdown of triglycerides.
• Beta Oxidation: Produces ATP from fatty acids.
• Ketogenesis: Converts acetyl CoA into ketones.

Endocrine System

Hormonal Regulation

• Insulin (Anabolic): Released during fed state.
• Glucagon (Catabolic): Released during fasted state to maintain blood glucose levels.

Intercellular Communication

• Messenger Classification: Chemical and functional.
• Hormones: Released by glands, travel in blood to target organs.

Signal Transduction

• Intracellular vs Membrane Receptors:
  • Intracellular: Lipophilic hormones (steroids, thyroid hormone).
  • Membrane: Lipophobic hormones (amino acids, peptides/proteins, amines).

Thyroid Hormones

• Regulation: TRH from hypothalamus, TSH from pituitary, TH from thyroid gland.
• Disorders: Hypothyroidism and hyperthyroidism.

These notes cover the key points from the transcript, organized by major topics covered in the lecture.