Overview
This is a case-based introduction to the physiology of diarrhoea, built around a 21-year-old medical student in the Philippines who develops severe cholera-induced watery diarrhoea and becomes haemodynamically compromised. The lecture uses the case to frame the definition and fundamental problem of diarrhoea, the four underlying mechanisms, the detailed secretory mechanism by which cholera toxin drives fluid loss, the downstream colonic and biochemical consequences, and the physiological basis of fluid and oral rehydration management. Several slides pose open questions (”??”) that are left for later study rather than answered here.
The case
A 21-year-old medical student working under an aid scheme in a village in the Philippines develops sudden, severe, persistent watery diarrhoea. Over 8 hours she becomes faint on standing, with sunken eyes and loss of normal skin turgor.
- Pulse: 136/min
- Blood pressure (propped up in bed): 84/46 mmHg
- Weight: fell from 56 kg to 52 kg (a 4 kg, i.e. ~4 litre, fluid loss over 8 hours)
The causative organism is Vibrio cholerae, a comma-shaped, flagellated bacterium.
What diarrhoea is
- Diarrhoea: decreased stool consistency, with or without increased frequency.
- Classified as acute or chronic, with chronic defined as lasting more than 3 weeks.
- Fundamental problem: an increase in water and electrolyte volume in the intestinal lumen. This drives:
- Increased stretch -> increased motility (cramps, pain)
- Increased water and electrolyte loss -> volume depletion -> CVS compromise
- Consequent biochemical changes
- Problems arising from the underlying cause itself (e.g. fever from infection)
Four mechanisms of diarrhoea
Excess water/NaCl in the lumen arises by one of four mechanisms:
- Osmotic: secretions are kept in the lumen (not reabsorbed), e.g. fibre, MgSO4.
- Secretory: fluid is actively put into the lumen (increased secretion), e.g. cholera.
- Inflammatory: fluid leaks into the lumen via mucosal inflammation/disruption, e.g. Campylobacter, colitis.
- Motility: fluid is left in the lumen because high motility gives no time for absorption.
Cholera as secretory diarrhoea
Steps by which cholera produces secretory diarrhoea:
- Cholera adheres to intestinal cells and its toxin enters the secretory cell.
- The toxin triggers intracellular cAMP pathways.
- This increases apical Cl⁻ channel activity, increasing Cl⁻ secretion into the lumen.
- Na⁺ follows Cl⁻ down its electrochemical gradient (paracellularly).
- H2O follows down the resulting osmotic gradient.
- The increased volume of water/salt in the lumen increases stretch, increasing motility (cramps, pain, frequency).
- Because the excess secretions are not reabsorbed (due to both the motility and some toxic effect on absorptive cells), net volume loss follows. This loss is isosmotic, so it has a marked circulatory (CVS) effect.
Diagram: secretory epithelial cell model (lumen | cell | interstitial fluid)
- Basolateral membrane: a Na⁺/K⁺/2Cl⁻ cotransporter brings Na⁺, K⁺ and 2Cl⁻ into the cell from interstitial fluid; a Na⁺/K⁺-ATPase (using ATP) pumps Na⁺ out and K⁺ in.
- Apical membrane: a Cl⁻ channel releases Cl⁻ into the lumen (this is the channel cholera toxin upregulates via cAMP).
- Paracellular route: H2O and Na⁺ move from interstitial fluid into the lumen, following the Cl⁻ gradient.
Normal small intestinal absorption (for contrast)
The normal small intestinal enterocyte, by contrast, is built for net absorption:
- Apical membrane transporters: Na⁺/glucose cotransporter, Na⁺/amino acid cotransporter, a Na⁺/H⁺ exchanger, and a Cl⁻/HCO3⁻ exchanger.
- Basolateral membrane: Na⁺/K⁺-ATPase, a glucose transporter, an amino acid transporter, a K⁺ channel, and a Cl⁻ channel.
- Paracellular movement of Cl⁻ and H2O runs from lumen toward interstitial fluid, i.e. net absorption, the opposite direction to the cholera-driven secretory picture above.
The apical Na⁺/glucose cotransporter in this diagram is the transporter that remains functional in cholera and underlies oral rehydration therapy (see Management).
Downstream colonic and biochemical effects
- The colon normally exchanges Na⁺ for K⁺, and Cl⁻ for HCO3⁻.
- In cholera, increased amounts of Na⁺ and Cl⁻ are delivered to the colon (from the small intestine’s excess secretion), which increases the probability of these exchanges occurring.
- The net effect is increased loss of K⁺ and HCO3⁻ in the diarrhoea.
- Consequently, the patient becomes hypokalaemic and acidotic (from loss of base/HCO3⁻).
The transcript's source diagram for this step (two small reproduced figures labelled "SI absorption" and "colon", annotated "++Cl / ++Na / ++H2O") had labels that were largely illegible at rendered resolution. Only the compartment and exchange labels above were legible; no further diagram detail is available.
Management
- Volume depletion in cholera is an isosmotic loss, so first-line treatment is IV fluids (0.9% saline).
- Large volumes are required: this patient lost 4 litres over 8 hours, and replacement must also match ongoing losses until the illness resolves.
- Oral rehydration therapy works because the Na⁺/glucose cotransporter (see small intestine diagram above) is unaffected by cholera toxin and still functions, allowing Na⁺- and glucose-driven fluid absorption to continue alongside the toxin-driven secretion.
- The lecture leaves several management questions open rather than answering them here: how to maximise absorption, the role of antibiotics, antidiarrhoeals, and vaccination [gap: posed as discussion questions, not elaborated in this lecture].
Study tasks set in the lecture
The lecture closes with study questions to guide further learning (answers not given in this lecture):
- Describe small intestinal secretion, its mechanisms, and how cholera changes these processes.
- Describe the mechanisms of absorption of ions, low molecular weight substances and water from the small intestine.
- Describe absorption/secretion in the colon, how cholera changes it, and how flow rate/transit time affects colonic absorption.
- Describe motility patterns in the small and large intestine, and how increased luminal fluid volume affects motility.
- Describe and explain the expected changes in plasma Na, K, Cl, urea, haematocrit, arterial HCO3, pH and PCO2 in this person.
- Summarise the mechanism by which cholera toxin causes diarrhoea.
- Describe how this patient should be managed, and the physiological basis of oral rehydration composition.
- Describe the overall consequences for ECF volume and circulation as the illness progresses, including CVS compensations, and compare this to someone who is simply not drinking water.
Self-test
- Define diarrhoea and state the cut-off used to distinguish acute from chronic diarrhoea.
- What is the fundamental problem underlying diarrhoea, and what four consequences follow from it?
- List the four mechanisms of diarrhoea, with one causative example of each.
- Describe, in order, the steps by which cholera toxin produces secretory diarrhoea.
- In the secretory epithelial cell model, name the basolateral transporter that brings Cl⁻ into the cell and the apical channel that releases it into the lumen, and state how Na⁺ and H2O then follow.
- Why does oral rehydration therapy remain effective in cholera despite ongoing Cl⁻ secretion?
- Explain how cholera-induced secretory diarrhoea produces hypokalaemia and metabolic acidosis via the colon’s normal ion-exchange activity.
- Distinguish osmotic diarrhoea from secretory diarrhoea in terms of the underlying transport defect.
- This patient lost 4 kg over 8 hours and had a pulse of 136/min and BP of 84/46 mmHg. What do these findings indicate about the volume and tonicity of the fluid lost, and what is the first-line treatment?
- Integrative: trace the physiological sequence from cholera toxin binding an intestinal secretory cell through to this patient’s tachycardia and hypotension.
Answers
Reveal answers
- Diarrhoea is decreased stool consistency with or without increased frequency; it is classed as chronic once it has lasted more than 3 weeks.
- The fundamental problem is an increase in water and electrolyte volume in the lumen. This causes increased stretch and motility (cramps/pain), increased water/electrolyte loss leading to volume depletion and CVS compromise, consequent biochemical changes, and problems from the underlying cause itself (e.g. fever).
- Osmotic (secretions not reabsorbed, e.g. fibre/MgSO4), secretory (increased secretion, e.g. cholera), inflammatory (mucosal inflammation/disruption, e.g. Campylobacter/colitis), motility (high motility, no time to absorb).
- Cholera adheres to cells and its toxin enters the secretory cell -> triggers cAMP pathways -> increases apical Cl⁻ channel activity -> Cl⁻ moves into the lumen -> Na⁺ follows down its electrochemical gradient -> H2O follows the osmotic gradient -> increased luminal volume increases stretch and motility -> excess secretions are not reabsorbed, giving isosmotic volume loss with a marked CVS effect.
- The basolateral Na⁺/K⁺/2Cl⁻ cotransporter brings Cl⁻ into the cell; the apical Cl⁻ channel releases it into the lumen. Na⁺ and H2O then follow paracellularly into the lumen, driven by the Cl⁻ gradient.
- The Na⁺/glucose cotransporter on the apical membrane of small intestinal enterocytes is a separate transporter from the Cl⁻-secretion pathway hijacked by cholera toxin, so it still works; giving glucose with Na⁺ orally drives Na⁺- and water-absorption despite ongoing secretion.
- Excess Na⁺ and Cl⁻ delivered to the colon increases the colon’s normal Na⁺/K⁺ and Cl⁻/HCO3⁻ exchange activity, increasing loss of K⁺ and HCO3⁻ in the stool; this produces hypokalaemia and a metabolic acidosis (from base/HCO3⁻ loss).
- Osmotic diarrhoea results from luminal solutes retaining water that is not reabsorbed; secretory diarrhoea results from active secretion of fluid into the lumen (e.g. via toxin-driven Cl⁻ channel activity), independent of unabsorbed luminal solute.
- The 4 kg weight loss over 8 hours reflects roughly 4 litres of isosmotic fluid loss; the tachycardia (136/min) and hypotension (84/46 mmHg) reflect the resulting intravascular volume depletion and CVS compromise. First-line treatment is IV 0.9% saline in large volume, matching ongoing losses.
- Cholera toxin enters the secretory cell and raises cAMP, increasing apical Cl⁻ secretion; Na⁺ and H2O follow into the lumen; the resulting isosmotic fluid volume is not reabsorbed and is lost from the body, depleting intravascular volume; this drives the compensatory tachycardia and the hypotension seen in this patient.