19B01: Exam Report

Describe the physiological consequences of the oral ingestion of 1 litre of water in a young adult.

28% of candidates passed this question.

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.

I1i / 19B01: Describe the physiological consequences of the oral ingestion of 1 litre water in a young adult


  • Adult GI tract handles 9L water per day
  • 2L/day through food & drink, remainder from GI secretions
  • Water output in faeces is 100ml/day
  • Remainder is absorbed, majority from Small bowel
  • Rate of absorption will be dependent on rate of gastric emptying and extent of osmotic gradient across intestinal mucosa.

Absorptive Mechanisms


  • Via aquaporins that penetrate and span the enterocyte membrane


  • Through tight junctions between the enterocytes
    • Perijunctional actomycin ring in intestinal tight junction can be
      activated to increase or decrease permeability of intercellular junction
    • Initiation of any of the active transporters causes tight junction to
      become more permeable to water


  • Net water movement across the intestinal wall is passive and dependent on
    osmotic and hydrostatic pressures
  • Water absorption across the intestine is considered to be a passive
    consequence of solute absorption that establishes a local osmotic gradient
  • Na is absorbed by co-transport with glucose and amino acids, which move Na

into the enterocyte from the lumen

  • Water and Na diffuses into capillary blood within the intestinal villus


    • Consider 70kg patient: TBW = 42L, ICF = 28L, ECF = 14L, ISF = 10.5L, PV = 3.5L
    • 1L Water is distributed throughout all body compartments (TBW): intracellular,
      interstitial fluid and plasma volume
    • 2/3 ICF -> 667ml distributed into ICF
    • 1/3 ECF -> 333ml distributed into ECF
      • 84ml distributed into PV
      • 250ml distributed into ISF

Physiological Effect

Volume Homeostasis

    • Normal plasma volume = 3500ml
    • After oral ingestion of 1L, PV can be expected to increase to 3584ml
    • Thus, 2.4% increase in plasma volume
    • Change not detected by volume receptors (threshold 7-10%)

Osmotic Homeostasis

    • Before ingestion of 1L fluid, plasma osmolality = 290mOsml/Kg
    • Total body solute = 42 x 290 = 12180mOsm
    • After ingestion of 1L fluid, TBW = 42 +1 = 43L
    • Total body solute unchanged
    • New plasma osmolality = 12180/43 = 283mOsm/Kg
    • Thus, 2.4% decrease in plasma osmolality
    • This will be detected by osmoreceptors in hypothalamus (threshold 2%)


  • Drop in plasma osmolality without much effect on plasma volume
  • No circulatory reflex response
  • Main effect will be via hypothalamus-mediated inhibition of ADH release
    from the posterior pituitary
  • ADH is nonapeptide which acts on the V2 receptors on the basolateral
    membrane of the principal cells in the collecting duct
  • This causes aquaporin 2 molecules to be inserted into the luminal membrane,
    allowing water to be reabsorbed down its osmotic gradient
  • However, when ADH concentration decreases, the luminal membrane
    remains impermeable to water
  • This leads to a larger volume of more dilute urine being produced, until
    homeostasis is achieved through excretion of water

Author: Craig Johnston