The Intestines

Question 1

Describe the macroscopic anatomy of the intestines and how it links to their function

Answer 1

• Needs large surface area
  
  • Mucosa folded with villi
  • Surface is covered with microvilli (brush border)
• Slow movement of contents
• Plicae circulares - permanent folds

Question 2

Describe the microscopic anatomy of the intestines

Answer 2

• Epithelial cells - enterocytes
• Goblet cells - mucous producing
• Mucosa constantly shed - 3-6 days
• Intestinal gland (crypt)
  - Stem cells, enteroendocrine glands, paneth cells

Question 3

Describe the breakdown of carbohydrates in the intestines

Answer 3

• Only monosaccharides can be absorbed (fructose, galactose, glucose)
• Final breakdown occurs in brush border by brush border hydrolases
• Amylose - straight chain polysaccharide bound by α 1-4 glycosidic bonds
  
  • Amylase breaks α 1-4 bonds to form glucose
  • Can also be broken to form disaccharide (maltose)
• Amylopectin - straight chains of amylose with α 1-6 glycosidic bonds connecting chains together
  
  • Amylase cleaves amylopectin into short-chained branched glucose polysaccharides called alpha dextrins
  • Isomaltase breaks α 1-6 glycosidic bonds to form single chained polypeptides (amylose)
• So in the end, starch is broken into glucose, maltose and alpha dextrins

Question 4

Describe how monosaccharides are reabsorbed in the intestines

Answer 4

• Na/K ATPase on basolateral membrane maintains low intracellular Na
• SGLT-1 binds Na to allow glucose binding
  
  • Na and glucose moves into cell
• GLUT2 transports glucose out of enterocyte
  
  • Diffuses down gradient into capillary blood
• Fructose uses GLUT5 transporter to enter enterocyte

Question 5

Describe how protein breakdown occurs

Answer 5

• Only amino acids, dipeptides and tripeptides absorbed
• In stomach:
  
  • Pepsinogen released from chief cell and converted to pepsin in the presence of HCl
  • Pepsin breaks protein into oligopeptides and amino acids
• Pancreas releases proteases as zymogens
  
  • Trypsinogen converted to trypsin by enteropeptidase (enterokinase)
    
    • Trypsin then activates other proteases
• Major proteases
  
  • Endopeptidases break bonds in middle of polypeptide to produce shorter polypeptides
    
    • Trypsin, chymotrypsin, elastase
  • Exopeptidases break bonds at ends of polypeptide to produce dipeptides or amino acids
  • Carboxypeptidase (A & B)

Question 6

Describe how absorption of protein products occurs

Answer 6

• Amino acids are transported into cell (similar to glucose)
  
  • Na-amino acid co-transporters
• Most protein products are ingested as dipeptides/tripeptides
• Dipeptides/tripeptides moved by H co-transporter
  
  • Peptide transporter 1 (PepT1)
  • Inside cell, these are converted to amino acids

Question 7

Describe how sodium and water reuptake occurs in the intestines

Answer 7

• Na moved by active transport out of cell on basolateral membrane
• Na diffuses into epithelial cells
  
  • Water can also move into intercellular space
• Fluid water reabsorbed is isosmotic
• Small intestine vs large intestine
  
  • Both have Na/K ATPase on basolateral membrane
  • Apical membrane
    
    • Small intestine - Na is co-transported
    • Large intestine - Na channels
      - Induced by aldosterone

Question 8

Outline how calcium reuptake occurs

Answer 8

• When calcium intake is low
  
  • Active transcellular absorption
    
    • Enters cell via facilitated diffusion
    • Ca ATPase removes Ca from basolateral membrane
  • Process requires Vitamin D
    
    • Calbindin binds to calcium and moves it to the Ca ATPase
    • Stimulated by parathyroid hormone
• When calcium intake is normal/high
  
  • Passive paracellular absorption

Question 9

Describe the basis of oral rehydration therapy

Answer 9

• Uptake of Na generates osmotic gradient
  
  • Water follows
• Glucose uptake stimulates Na uptake
• Mixture of glucose and salt will stimulate maximum water uptake

Question 10

List the signs and symptoms of coeliac disease

Answer 10

• Diarrhoea
• Flatulence - gas in gut tract
• Weight loss
• Sensory loss in hands
• Fatigue
• Abdominal pain
• Bloating

Question 11

Describe the pathogenesis of coeliac disease

Answer 11

• Intolerance of the gliadin fraction of gluten
  
  • Found in wheat, rye and barley
• Results in immune response
  
  • Damages mucosa of intestines
    
    • Absence of intestinal villi
    • Lengthening of intestinal crypts
    • Lymphocytes infiltrate epithelium
      
      • Impaired digestion/malabsorption
        
        • Osmotic content in gut
        • Draws water into gut - diarrhoea

Question 12

Describe the investigations and treatment for coeliac disease

Answer 12

• Upper GI endoscopy + biopsies (duodenum)
  
  • Mucosal pathology
  • Villi are reduced or absent
  • Lymphocyte levels
  • Crypt structure
• Bloods
  
  • Serology
  • Electrolyte imbalances
    
    • Anaemia
• Treatment - reduce gluten from diet