PHYSIO: Module 8.1

I. Introduction to the Digestive System

A. Introduction

  1. Basics of Nutrient Requirement:
    • Humans need basic organic molecules derived from food for various biological functions:
      • To produce ATP, building tissues, and acting as cofactors/coenzymes.
  2. Process of Digestion:
    • Digestion involves breaking down polymers like carbohydrates, fats, and proteins into monomers via hydrolysis reactions.
  3. Absorption:
    • The resulting monomers are absorbed into the bloodstream to be utilized by cells.

II. Digestive Tract Functions

A. Overview of Functions

  1. Motility: Movement of food through the digestive tract.
    • Ingestion: Taking food into the mouth.
    • Mastication: Chewing and mixing food with saliva.
    • Deglutination: Swallowing.
    • Peristalsis: Wave-like, one-way movement through the tract.
    • Segmentation: Churning and mixing while moving forward.
  2. Secretion: Release of substances to aid digestion.
    • Exocrine: Digestive enzymes, hydrochloric acid, mucus, water, and bicarbonate are secreted.
    • Endocrine: Hormones are released to help regulate digestion.
  3. Digestion: Physically and chemically breaking food down into smaller units.
  4. Absorption: Transferring broken-down food into blood or lymph.
  5. Storage and Elimination: Temporary storage of undigested food followed by elimination.
  6. Immune Barrier:
    • The digestive tract contains simple columnar epithelium with tight junctions to prevent pathogens from entering the body.
    • Immune cells in connective tissue promote immune responses.

III. Organs of the Digestive System

A. Comprehensive List of Digestive Organs

  • Parotid gland
  • Submandibular gland
  • Sublingual gland
  • Oral cavity
  • Teeth
  • Tongue
  • Pharynx
  • Esophagus
  • Stomach
  • Duodenum
  • Small intestine
  • Cecum
  • Appendix
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon
  • Rectum
  • Anal canal
  • Anus
  • Common bile duct
  • Diaphragm
  • Liver
  • Gallbladder
  • Pancreas

IV. Layers of the Digestive Tract

A. Organizational Structure

  1. Layers:
    • Serosa
    • Mucosa
    • Submucosa
    • Muscularis externa: Includes longitudinal and circular muscle layers.
    • Muscularis mucosae: Thin layer under the mucosa.
    • Intestinal crypts (of Lieberkühn) and Lamina propria: Supporting structures within the mucosa.
  2. Nervous Elements:
    • Myenteric plexus and Submucosal plexus: Control gut motility and secretions.
  3. Lymphatic Structures:
    • Lymph nodules and Lacteals: Involved in immune function and fat absorption.
  4. Blood Supply: Related to arteries and veins servicing the tract.

V. Regulation of the GI Tract

A. Neural Regulation

  1. Parasympathetic Division (Extrinsic)
    • Stimulates functions of the esophagus, stomach, small intestine, pancreas, gallbladder, and the first part of the large intestine through the vagus nerve.
    • Sacral spinal nerves stimulate the lower large intestine.
    • Preganglionic neurons synapse on submucosal and myenteric plexuses.
  2. Sympathetic Division (Extrinsic)
    • Inhibits peristalsis and secretion.
    • Stimulates contraction of sphincters.
  3. Hormonal Regulation:
    • Hormones influence digestive functions, sourced from the brain or digestive organs.
  4. Intrinsic Regulation:
    • Intrinsic sensory neurons in the gut wall facilitate regulation via the enteric nervous system.
    • Paracrine signals regulate local actions.

VI. From Mouth to Stomach

A. Mouth

  1. Mastication: Chewing breaks food into smaller pieces for deglutition and mixes it with saliva.
  2. Saliva Composition: Contains mucus, antimicrobial agents, and salivary amylase initiating starch digestion.
  3. Deglutition: Involves a coordinated contraction of 25 pairs of muscles and has three parts:
    • Oral Phase: Voluntary; forming a bolus with the tongue and mouth muscles.
    • Pharyngeal Phase: Involuntary; initiated by receptors in the posterior oral cavity and oropharynx.
      • Uvula lifts to cover the nasopharynx and epiglottis covers vocal cords; upper esophageal sphincter relaxes.
    • Esophageal Phase: Automatic; bolus moves to the stomach via peristalsis.
Peristalsis in the Esophagus
  • The esophagus is about 10 inches long, passes through the diaphragm via the esophageal hiatus.
  • The upper region is lined with nonkeratinized stratified squamous epithelium.
  • The upper esophagus features skeletal muscles innervated by somatic motor neurons, while the lower esophagus contains smooth muscle controlled by the autonomic nervous system.
  • The lower esophageal sphincter opens for food passage into the stomach and remains closed to prevent regurgitation.

VII. Stomach

A. Stomach Functionality

  1. Functions:
    • Stores food.
    • Churns food to mix with gastric secretions.
    • Initiates protein digestion.
    • Kills bacteria in food (due to acidic environment).
    • Moves food into the small intestine as chyme.

B. Structure of the Stomach

  1. Regions:
    • Cardia
    • Fundus
    • Body
    • Pylorus: Guides chyme into the duodenum via the pyloric sphincter.
  2. Muscular Layers:
    • Includes longitudinal, circular, and oblique muscle layers.
  3. Gastric Pits and Glands:
    • Gastric pits lead to glands with multiple cell types:
      • Mucus Neck Cells: Secrete mucus to protect the stomach lining from acid.
      • Parietal Cells: Secrete HCl and intrinsic factor for vitamin B12 absorption.
      • Chief Cells: Secrete pepsinogen, an inactive protein-digesting enzyme.
      • ECL Cells: Secrete histamine and serotonin.
      • G Cells: Produce gastrin (a hormone).
      • D Cells: Produce somatostatin (a hormone).
      • Ghrelin: Signals the brain to regulate hunger.

VIII. Pepsin and HCl Secretion

A. Formation of HCl

  1. Primary Active Transport:
    • H+ is secreted via H+/K+ ATPase pumps.
  2. Facilitated Diffusion of Cl−:
    • Coupled with the movement of bicarbonate into the blood.
  3. Secretion:
    • Parietal cells secrete both Cl- and H+ into gastric juice and bicarbonate into blood.

B. Stimulation of HCl Secretion

  1. Gastrin: Produced in G cells; stimulates HCl release and ECL cell histamine production.
  2. Histamine: Stimulates parietal cells via H2 histamine receptors.
    • Example Drugs: Tagamet and Zantac block H2 receptors.
  3. Parasympathetic Neurons and ACh: Stimulate both parietal and ECL cells.

C. Functions of HCl

  1. Drops stomach pH to 2.
  2. Denatures ingested proteins for enzyme access.
  3. Converts pepsinogen into active pepsin for protein digestion.
  4. Optimal pH for pepsin activity.

D. Stomach Defenses

  1. Mucosal Defenses
    • Adherent layer of mucus containing alkaline bicarbonate.
    • Tight junctions between epithelial cells.
    • Rapid epithelial mitosis replacing the epithelium every three days.

E. Digestion and Absorption in the Stomach

  1. Protein digestion begins in the stomach.
  2. Starch digestion initiated in the mouth stops in the stomach due to pH 2.
  3. Alcohol and NSAIDs (e.g., aspirin) are absorbed, typical of their high lipid solubility.

IX. Peptic Ulcers

A. Overview

  1. Peptic Ulcers: Erosions in the stomach/duodenum mucosa due to HCl.
  2. Helicobacter pylori: A bacterium that affects mucosal barriers and contributes to ulcer formation.
  3. Treatment: Combination of K+/H+ pump inhibitors (Prilosec) and antibiotics for H. pylori.

B. Acute Gastritis

  1. Definition: Inflammation of the submucosa due to acid exposure.
  2. Histamine released can further stimulate acid secretion.
  3. Prostaglandins promote protective alkaline mucus secretion, inhibited by NSAIDs.

C. Duodenal Ulcers

  1. Protected by an alkaline mucus layer.
  2. Brunner’s Glands: Secrete bicarbonate.
  3. Acidic chyme is neutralized by bicarbonate from the pancreas.
  4. Zollinger-Ellison Syndrome: Ulcers linked to high gastrin secretion due to tumors.

X. Small Intestine

A. Structure of the Small Intestine

  1. Anatomy: Starts at pyloric sphincter and ends at ileocecal valve.
  2. Three Sections:
    • Duodenum: First 10 inches.
    • Jejunum: Middle 2/5.
    • Ileum: Last 3/5.
  3. Surface Anatomy:
    • Mucosa and submucosa are folded into plicae circulares; mucosa further into villi; and epithelial membranes folded into microvilli, increasing surface area for nutrient absorption.

Small Intestine Structure

A. Components

  1. Villi: Finger-like projections with simple columnar epithelium, lacteals, and a capillary network for nutrient absorption.

B. Functions of the Small Intestine

  1. Complete the digestion of carbohydrates, proteins, and fats.
  2. Absorption of nutrients including sugars, lipids, amino acids, calcium, and iron in the duodenum and jejunum.
  3. The ileum is responsible for absorbing bile salts, vitamin B12, water, and other electrolytes; these processes are facilitated by the high surface area provided by villi and microvilli.

XI. Intestinal Enzymes

A. Overview

  1. Brush Border Enzymes: Enzymes that are not released into the lumen but remain attached to the plasma membrane of epithelial cells with active sites exposed to chyme.
  2. Function: Hydrolyze disaccharides, polypeptides, and other substrates into simple nutrient molecules.

B. Major Brush Border Enzymes Table

  • Enzyme Types:
    • Disaccharidases: e.g., Sucrase (digests sucrose), Maltase (digests maltose), Lactase (digests lactose).
    • Peptidases: e.g., Aminopeptidase (produces free amino acids), Enterokinase (activates trypsinogen), etc.

XII. Intestinal Contractions and Motility

A. Overview

  1. Peristalsis: Movement of food is slower and weaker due to pressure at the pyloric end.
  2. Segmentation: Stronger contractions that mix the chyme.
  3. Smooth Muscle Contractions: Occur automatically due to endogenous pacemaker activity.
    • Graded depolarizations referred to as slow waves produced by pacemaker cells called intestinal cells of Cajal.

XIII. Large Intestine

A. Structure of the Large Intestine

  1. Regions of the Large Intestine:
    • Cecum → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum → Anal canal → Anus.
  2. Mucosa Structure: Columnar epithelial cells with goblet cells, crypts, lymphatic nodules, and notably, no villi.
  3. Outer Surface: Forms pouches called haustra.

B. Functions of the Large Intestine

  1. Absorbs water, electrolytes, vitamin K, and some B vitamins.
  2. Produces vitamin K and B vitamins via microbial organisms.
  3. Stores feces until elimination.

XIV. Intestinal Microbiota

A. Overview

  1. Microbiota: Host several hundred species of bacteria in the large intestine (commonly known as microflora).
    • Commensal: Some are non-harmful.
    • Mutualistic: Both bacteria and host benefit.
    • Mostly anaerobic bacteria.
    • An infant acquires initial bacteria from the mother during birth.

B. Benefits from Microbes

  1. Production of vitamins K and B
  2. Generate short-chain fatty acids from cellulose which aid in electrolyte absorption.
  3. Outcompete harmful bacteria.
  4. Disruption in normal microflora can lead to inflammatory bowel disease.

C. Protection from Intestinal Bacteria

  1. Involvement of mucus secreted from goblet cells.
  2. Defensins produced by Paneth cells.
  3. Immunoglobulin A (IgA) antibodies secreted by plasma cells.

D. Role in Healthy Epithelial Barrier

  1. Maintains health of the epithelial barrier, controls inflammation, and promotes damagerepair.

XV. Fluid and Electrolyte Absorption

A. Overview

  1. Most absorption occurs in the small intestine, but some also occurs in the large intestine.
  2. Approximately 200 ml of water is left unabsorbed daily and is excreted with feces.
  3. Absorption is passive, following an osmotic gradient established by active Na+/K+ pumps.
    • Aldosterone promotes greater sodium and water absorption.

XVI. Defecation

A. Overview

  1. Pressure increases as material transitions to the rectum, causing relaxation of the internal anal sphincter, signaling the need to defecate.
  2. The external anal sphincter controls voluntary defecation.
  3. During defecation, longitudinal muscle contractions of the rectum increase pressure while anal sphincters relax.
  4. Additionally assisted by abdominal and pelvic skeletal muscles through the Valsalva maneuver.

XVII. Liver, Gallbladder, and Pancreas

A. Liver

1. Structure
  • Largest abdominal organ, located underneath the diaphragm mainly on the right side.
  • Notable regenerative abilities attributed to hepatocyte mitosis.
  • Composed of hepatocytes arranged in hepatic plates separated by sinusoids:
  1. Capillaries have no diaphragm or basement membrane, allowing permeability for proteins, fats, and cholesterol.
  • Kupffer cells reside in sinusoids contributing to immunological function.
  • Damage due to alcohol or viral hepatitis can cause liver fibrosis leading to cirrhosis.
2. Hepatic Portal System
  • Digestion products are transported to the liver through the hepatic portal vein.
  • Combines blood from pancreas, gallbladder, stomach, omentum, and spleen for processing.
  • Circulation allows liver capillaries to clear the blood of toxins before releasing it into the systemic circulation.
3. Liver Lobules
  • Hepatic plates organized into lobules with associated hepatic arteries, portal veins, and central veins.
  • Bile produced by hepatocytes is excreted into bile canaliculi, draining into bile ducts.
4. Enterohepatic Circulation
  • Besides bile, liver secretes substances into bile ducts for blood clearance.
  • Some released molecules are reabsorbed in the small intestine and returned to the liver, also characterized as enterohepatic circulation.

XVIII. Liver Functions

A. Bile Production and Secretion

  1. Liver secretes 250-1,500 ml of bile daily, including
    • Bile pigment (bilirubin),
    • Bile salts,
    • Cholesterol,
    • Inorganic ions.
  2. Bilirubin
    • Derives from hemoglobin breakdown from heme; not water-soluble.
    • Conjugated bilurubin is water-soluble and secreted into bile, eventually aiding in fecal color.
B. Detoxification of Blood
  • Hormones, drugs, and metabolic waste are processed via three methods:
  1. Excretion into bile.
  2. Phagocytosis by Kupffer cells.
  3. Chemical conversion by hepatocytes.
Urea is produced and returned to the blood for kidney filtration.
C. Glucose, Triglycerides, and Ketone Bodies Secretion
  • Liver ensures blood glucose homeostasis.
    • Stores glucose as glycogen and creates glucose from non-carbohydrates when needed.
    • Produced triglycerides and ketones from fatty acids as necessary.
D. Production of Plasma Proteins
  • Key proteins including albumin (for osmotic pressure), clotting factors and proteins like angiotensinogen are synthesized.

XIX. Gallbladder

A. Structure and Function

  1. Storage: Sac-like organ that stores and concentrates bile.
    • Sequence: Liver → Bile ducts → Hepatic duct → Cystic duct → Gallbladder → Common bile duct → Duodenum.
  2. Gallstones: Potential blockages that form due to bile constituents.

XX. Pancreas

A. Structure and Functions

  1. Endocrine and Exocrine Functions:
    • Endocrine cells (Islets of Langerhans) secrete insulin and glucagon.
    • Exocrine acinar cells produce pancreatic juices containing digestive enzymes and bicarbonate.
B. Pancreatic Juice Components
  1. Composed of bicarbonate and approximately 20 digestive enzymes pertinent to macromolecule digestion.
C. Bicarbonate Formation
  • Produced from CO2; affecting pH balance in the duodenum.
  1. Patients with cystic fibrosis might have trouble secreting bicarbonate due to chloride transporter dysfunction.
D. Activation of Pancreatic Enzymes
  1. Most enzymes are initially inactive (zymogens) and activated upon reaching the small intestine.
    • Enterokinase activates trypsinogen into trypsin, subsequently activating other enzymes.