Question 1

College Answer

It was expected candidates would provide details the consequences of water ingestion from its rapid absorption in the small intestine to the resultant impact on plasma osmolarity and the minimal impact of plasma volume of this volume. Some detail on the mechanisms of absorption (transcellular vs osmosis) was expected and the distribution of water across body fluid spaces. Many candidates accurately described the small drop in plasma osmolarity that is sufficient to trigger osmoreceptors with better answers providing details of the locations and mechanisms involved. The physiological consequences of inhibition of ADH, including the renal effects of decreased water permeability in distal renal tubules and collecting ducts. The volume load after distribution would be lower than the plasma volume triggers for the circulatory reflex responses.

Discussion

Absorption of water is near-complete, rapid, and mainly occurs in the proximal small bowel 

• Most of the diffusion is transcellular
• It is driven by osmotic mechanisms (active absorption of other electrolytes, especially sodium)

Distribution  is equal into all body fluid compartments and proportional to their relative sizes:

• 66% into the intracellular fluid
• 25.5% into the interstitial fluid
• 8.5% into the circulating blood volume

Elimination is by renal excretion, where the filtered fraction is reabsorbed by a highly regulated mechanism.

Physiological consequences of oral water ingestion:

• Osmolality decreases 
  • For one litre of pure water, this would be a 2.5% decrease in osmolality
  • This is sensed by the osmosensitive circumventricular organs (OVLT); even 1% would be enough
  • Via the hypothalamus, this causes the posterior pituitary to decrease its secretion of vasopressin.
  • The decreased vasopressin level decreases aquaporin expression in the collecting duct of the kidney
  • Water reabsorption in the nephron decreases, and diuresis ensues
• Plasma sodium concentration decreases
  • For one litre of pure water, this would be a 4 mmol decrease
  • This increases the stimulus for angiotensin and aldosterone release
  • The net effect of these is to increase the reabsorption of sodium in the nephron
• Blood volume increases (very slightly)
  • For one litre of pure water, this would be a 80ml increase in volume, or about 1.6%
  • This change is below the sensitivity threshold of the cardiovascular regulatory reflexes, and should not lead to any change in blood pressure for a normal healthy person

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