Human Physiology: The Digestive System

Human Physiology: The Digestive System

Introduction

  • Presented by Dr. Steven Patterson (Steven.Patterson@gcu.ac.uk).
  • Covers the organs of the gastrointestinal (GI) tract and their functions.

Learning Outcomes

  • Identify organs of the GI tract and accessory organs, and describe their basic functions.
  • Describe the four layers forming the GI tract wall.
  • Explain how the epithelial layer's structure and function vary in different parts of the GI tract.
  • Name and identify the locations of salivary glands and the functions of their secretions.
  • Describe the location, structure, and function of the esophagus, explaining how a food bolus is moved via peristalsis.
  • Describe the location, structure, and functions of the stomach.
  • Describe different cell types in gastric glands and their secretions' roles.
  • Describe the functions of stomach acid and the consequences of overproduction.
  • Give an account of digestion and absorption of nutrients in the stomach.
  • Describe the small intestine's location, structure, and function, naming the three sections and their specific roles.
  • Explain how plicae circulares, villi, and microvilli increase the surface area for digestion and absorption.
  • Account for the digestion and absorption mechanisms of different nutrients (proteins, fats, and carbohydrates) in the small intestine.

Overview of the Digestive System

  • Organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus.
  • Accessory Organs: Salivary glands, liver, gallbladder, pancreas.

Important Factors of the Digestive System

  • Large surface area
  • Absorption and secretion
  • Exposure to diverse chemicals and microbes
  • Motility
  • Sensory input and regulation
  • Immunity
  • Regeneration and cell turnover
  • Removal of waste

Functions of the Digestive System

  • Processes food to extract nutrients for energy, growth, and tissue repair.
  • Regulates body processes.
  • Nutrients:
    • Macronutrients: carbohydrates (CHO), fats, proteins
    • Micronutrients: vitamins (water- and fat-soluble), minerals (Ca, Zn, Fe, Na, K, etc.)
    • Water
    • (Alcohol is not essential and thus not a nutrient).

Overall Process

  • Food (CHO, fats, protein) is ingested.
  • Digestion and absorption occur.
  • Results in glucose, fatty acids, and amino acids.
  • These are metabolized in cells (Krebs Cycle, etc.) to produce ATP for biological processes like muscle contraction.

Components of the Digestive System and Their Functions

  • Oral Cavity/Mouth: Mechanical processing, moistening, mixing with salivary secretions.
  • Teeth & Tongue: Mechanical processing.
  • Salivary Glands: Secrete lubricating fluid with enzymes to break down carbohydrates.
  • Pharynx: Muscular propulsion of materials into the esophagus.
  • Esophagus: Transports materials to the stomach.
  • Stomach: Chemical breakdown via acid and enzymes; mechanical processing through muscular contractions.
  • Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones.
  • Liver: Secretion of bile (for lipid digestion); nutrient storage; many other vital functions.
  • Gallbladder: Storage and concentration of bile.
  • Small Intestine: Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
  • Large Intestine: Dehydration and compaction of indigestible materials for elimination.
  • Anus: Elimination of waste.

Basic Processes of the GI Tract

  • Ingestion: Eating
  • Mechanical Digestion: Chewing, peristalsis, churning, etc.
  • Chemical Digestion: Breakdown by enzymes
  • Secretion: Bile, bicarbonate, enzymes
  • Absorption: From GI tract into blood/lymph
  • Excretion: Elimination of waste
  • Transit time: 36-48 hours (variable).

Detail on Digestion and Absorption

  • Digestion:
    • Mechanical: Chewing, mixing, and propulsion (smooth muscle contraction)
    • Chemical: Large molecules are broken down into small molecules by enzymes.
  • Absorption:
    • Products of digestion move from epithelial cells into the blood or lymph, then to tissues.

Layers of the GI Tract Wall

  • 1. Mucosa:
    • Inner layer of epithelium (absorptive & secretory function)
    • Lamina propria (connective tissue, blood vessels)
    • Muscularis mucosae (inner muscular layer)
  • 2. Submucosa:
    • Connective tissue
    • Large blood and lymphatic vessels
    • Submucosal plexus (Meissner's plexus): network of neurons controlling secretions of the GI tract
  • 3. Muscularis:
    • Smooth muscle layer
    • Inner circular layer (contraction narrows the gut)
    • Outer longitudinal layer (contraction shortens the gut)
    • Myenteric plexus (Auerbach's plexus) controls motility.
  • 4. Serosa:
    • Outer layer of the gut
    • Forms the mesentery where nerves and blood vessels enter and leave the gut

Digestive Epithelium

  • Simple (1 layer) or stratified (several layers), depending on location, function, and stresses.
  • Oral cavity, pharynx, and esophagus:
    • Lined by stratified squamous epithelium for protection against mechanical stresses.
  • Stomach, small intestine, and most of the large intestine:
    • Simple columnar epithelium with goblet cells for secretion and absorption.

Control of GI Tract

  • Neuronal:
    • Myenteric plexus: Part of the enteric nervous system ('2nd brain') that controls motility.
    • Submucosal plexus: Regulates mucosal secretion
  • Hormonal:
    • Gastrin, cholecystokinin (CCK), and others

Mouth (Oral Cavity) Functions

  • Food is chewed (mastication) for mechanical digestion.
  • Food is mixed with saliva from three pairs of salivary glands (99.5% water, digestive enzymes like amylase, mucus, ions, etc.).
  • Food is formed into a bolus for swallowing (deglutition).

Salivary Glands

  • Three Pairs: controlled by the autonomic nervous system (ANS)
  • Produce 1.0–1.5 liters of saliva/day
    • 70% by submandibular glands (mixed serous & mucus)
    • 25% by parotid glands (serous, watery secretion + amylase, very little mucus)
    • 5% by sublingual glands (mainly mucus)
  • Composition: 99.5% water + 0.5% salivary amylase + mucus + antibacterial enzymes.

Functions of Saliva

  • Lubricates the mouth (easier to swallow; speech)
  • Enzyme lysozyme kills bacteria; IgA antibodies are also protective
  • Dissolves chemicals (stimulates taste buds)
  • Buffers acidic foods (salivary pH ranges between 6.35 – 8.0)
  • Starts digestion of carbohydrates (starch) by the enzyme salivary amylase into maltose (disaccharide).

Swallowing (Deglutition)

  • Process involves the tongue, pharynx, and esophagus to move the bolus to the stomach.

Esophagus

  • Collapsible muscular tube (about 25cm)
  • Transports food to the stomach
  • Sphincters at either end
  • No digestion or absorption
  • Goblet cells secrete mucus for lubrication and protection.
  • The bolus is propelled down by PERISTALSIS.
    • Upper 1/3rd of muscularis: skeletal muscle
    • Middle 1/3 of muscularis: skeletal & smooth
    • Lower 1/3rd of muscularis: smooth muscle

Peristalsis

  • Contraction & relaxation of muscle.
  • Wave of contraction propels food (4-8 secs).
  • Controlled & co-ordinated by myenteric plexus.
    • Circular muscle:
    • Contracts behind the bolus
    • Relaxes in front

Lower Esophageal Sphincter (Cardiac Sphincter)

  • Controls entrance to the stomach.
  • Ring of smooth muscle (normally contracted).
  • Relaxes during swallowing.
  • Sphincter contraction prevents reflux of stomach acid into the esophagus.
  • Dysfunction leads to gastro-oesophageal reflux disease (GORD/GERD) and heartburn.

Clinical Conditions: Hiatus Hernia (Diaphragmatic Hernia)

  • Occurs when part of the stomach protrudes through the diaphragm.

Stomach Functions

  • Stores food.
  • Mechanical digestion: softens food and mixes (3 layers of smooth muscle).
  • Chemical digestion: secretes enzymes & HCl.
  • Production of intrinsic factor: essential for Vitamin B12 absorption in the small intestine (needed for RBC synthesis); deficiency causes pernicious anaemia.

Stomach Anatomy

  • Mucosa in folds (rugae) allows for expansion.

Stomach Regions

  • Cardia
  • Fundus
  • Body
  • Pylorus (controls exit from the stomach)

Stomach Lining

  • Contains various cells in gastric glands.
    • Mucous cells: protect.
    • Parietal cells: produce HCl and intrinsic factor.
    • Chief cells: secrete digestive enzymes (inactive pepsinogen converted to active pepsin by HCl).
    • G cells: secrete gastrin (stimulates gastric juice secretion, ~1.0-1.5 L/day).

Functions of Acid in Stomach

  • Kills most bacteria in food.
  • Denatures proteins in food.
  • Breaks down connective tissue in meat.
  • Creates a very acidic environment (pH 1.5-2.0) for pepsin activation.
  • Too much acid leads to peptic ulcers.

Peptic Ulcers

  • Can be caused by H. pylori, aspirin, etc.
  • May lead to bleeding or perforation (emergency situations).

Digestion & Absorption in Stomach

  • Digestion:
    • Pepsinogen (inactive) converts to pepsin (active) via HCl.
    • Pepsin digests proteins into peptides.
    • Salivary enzymes are active until low pH denatures them (~1 hour).
  • Absorption:
    • No nutrients are absorbed.
    • Some H2O, salts are absorbed.
    • Some drugs (alcohol, aspirin) are absorbed.

Small Intestine

  • Segments: Duodenum, Jejunum, Ileum
  • Length: ~6m / ~20ft long; 3 cm/ 1in wide
    -Duodenum (25cm/10in)
    -Jejunum (2.5m/8ft)
    -Ileum (3.5m/11ft)

Small Intestine Function

  • Digestion & absorption of nutrients and water (90% of absorption occurs here).
  • Duodenum: mixes chyme with pancreatic & liver/gallbladder secretions; neutralizes stomach acid.
  • Jejunum: most digestion and absorption of nutrients takes place here.
  • Ileum: specialist function - absorbs vitamin B12 & bile salts; has spare capacity.

Adaptations for Large Surface Area in Small Intestine

  • Length: ~ 6m (20 ft)
  • Plicae circulares (circular folds): transverse folds in the mucosa (permanent).
  • Villi: finger-like projections increasing surface area for absorption (not found in stomach or colon).
  • Microvilli: Further increase surface area (brush border; 250 m^2 = tennis court).

Intestinal Glands (Crypts of Lieberkuhn)

  • Located at the bases of villi.
  • Secrete intestinal juice (1-2 L/day): water & mucus.
  • Slightly alkaline.
  • Contain endocrine cells that secrete hormones into the blood (CCK & secretin).

Brunner’s Glands

  • Submucosal glands of the duodenum.
  • Produce alkaline mucus when acidic chyme arrives from the stomach.
  • Only in the duodenum (not jejunum or ileum).

Coeliac Disease

  • Autoimmune disease leading to flattened villi.
  • Reduced absorptive surface results in malabsorption of nutrients.

Steatorrhea

  • Excessive fat in stools due to fat malabsorption.

Brush Border Enzymes

  • Integral membrane proteins on the surfaces of intestinal microvilli.
  • Break down materials in contact with the brush border.
  • Examples: maltase, lactase, sucrase (break down disaccharides to monosaccharides).
  • Lactose intolerance: lack of lactase -> undigested lactose -> bloating, pain, diarrhea.

Mechanical Digestion in Small Intestine

  • Peristalsis: Propulsive contractions.
  • Segmentation: Churns and fragments the bolus; non-propulsive; mixes contents with intestinal secretions.

Chemical Digestion in Small Intestine: CHO Digestion

  • Polysaccharides (starch) are broken down into disaccharides (maltose) by amylase (salivary glands & pancreas).
  • Disaccharides are broken down into monosaccharides (glucose, fructose, galactose) by brush border enzymes.

Brush Border Enzymes for CHO Digestion

  • Maltase: Breaks down maltose into 2 molecules of glucose.
  • Sucrase: Breaks down sucrose into glucose and fructose.
  • Lactase: Breaks down lactose into glucose and galactose.

Digestion of Proteins

  • Proteins and large polypeptides are broken down into peptides by pepsin (stomach) and pancreatic proteases.
  • Peptides are further broken down into amino acids by brush border enzymes.

Proteases

  • Pancreatic proteases: trypsin, chymotrypsin, carboxypeptidase, elastase.
  • Must be released in inactive forms to prevent digestion of the pancreas itself.

Fat (Lipid) Digestion

  • Dietary fats (triglycerides) are not water-soluble.
  • Mechanical processing (stomach) creates large fat droplets.
  • Pancreatic lipase, being water-soluble, can only act at the surface of the lipid droplet.
  • Bile salts are needed to break down the droplet into smaller droplets (emulsification).

Role of Bile Salts (1st Role)

  • Bile salts break droplets apart (emulsification), creating tiny emulsion droplets coated with bile salts.
  • Increases the surface area for lipase to act.

Digestion of Fats - Triglycerides

  • Triglycerides are broken down by pancreatic lipase into monoglycerides + 2 free fatty acids.

Absorption Mechanisms

  • 90% of absorption occurs in the small intestine (mainly jejunum).
  • Very large surface area due to:
    • Length
    • Presence of circular folds (plicae circulares)
    • Villi
    • Microvilli (a 600-fold increase in surface area)

Transport Mechanisms

  • Through luminal surface
  • Through cytoplasm
  • Through basolateral membrane.
  • Uptake into blood stream

Paracellular Transport

  • Water and other small water-soluble molecules may also cross via paracellular transport (i.e. between tight junctions).

Simple Diffusion

  • Molecules move down their concentration gradient.
  • Fatty acids, alcohol, and dissolved gases diffuse through the lipid portion of the cell membrane.
  • Water & some ions pass through tiny membrane channels.
  • Bigger water-soluble molecules (e.g., glucose, amino acids) are too big to fit through channels and use carrier proteins (facilitated diffusion).

Facilitated Diffusion

  • Uses carrier proteins to move molecules down their concentration gradient.

Co-transport (Secondary Active Transport)

  • Both receptor sites must be filled for transport to occur.
  • Na+ diffuses down its concentration gradient and is then actively pumped back out of the cells.
  • SGLT1 is an example.

Absorption of CHO & Amino Acids

  • Glucose/galactose: Co-transport + Na+ (BOTH must bind to the carrier).
  • Fructose: Facilitated diffusion (carrier-mediated; moves down the concentration gradient).
  • Amino acids: Absorbed by both facilitated diffusion and co-transport.

Absorption of fats (2nd role of Bile salts)

  • Long-chain fatty acids & monoglycerides + Bile salts form micelles.
  • Micelles: water-soluble; diffuse to mucosa; FA & monoglyceride diffuse into epithelial cell by simple diffusion.
  • Short/medium-chain FAs go directly into villus blood.

Summary of Fat Absorption

  • Within epithelial cells, triglycerides are re-formed from monoglycerides & free fatty acids.
  • Coated with protein (mostly apoB48) to form chylomicrons.
  • Chylomicrons: are too big to enter capillaries so they enter lacteals (lymphatics) and then the systemic circulation (left subclavian vein).
  • The blood has a ‘milky’ appearance.

Cholesterol Absorption

  • Cholesterol esters are de-esterified by enzymes in the small intestine to yield free cholesterol.
  • Forms micelles with bile salts and phospholipids.
  • Micelle reaches the brush border.
  • Neimann-Pick C1 Like-1 (NPC1L1) is a membrane transport protein for cholesterol and other sterols.
  • Inside cells, cholesterol is re-esterified.

Absorption of Products of Digestion

  • Monosaccharides and amino acids are absorbed into blood capillaries and transported via the hepatic portal vein to the liver.
  • Short-chain fatty acids are absorbed into blood capillaries.
  • Chylomicrons are absorbed into lacteals and transported via the lymphatic system, eventually entering the bloodstream at the left subclavian vein.