Example 1
Water reabsorption rises while filtered water stays constant. Predict urine volume.
Less water remains in the tubule, so urine volume tends to fall, all else equal.
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Year 12 · SACE Stage 2
Water balance, ADH and nephrons
Practise kidney processes and the direction of ADH responses.
Method and worked examples
Kidneys regulate water and solute balance while removing wastes. In a nephron, glomerular filtration transfers water and small solutes from blood into the capsule while normally retaining blood cells and most large proteins. Tubules selectively reabsorb useful substances and water into the blood; secretion moves some substances from blood into tubule fluid. Urine contains material remaining after these processes.
ADH is made in the hypothalamus and released from the posterior pituitary. When body fluids become more concentrated, increased ADH increases water permeability in parts of the nephron, including collecting ducts, through aquaporin-related mechanisms. More water is reabsorbed, usually producing a smaller volume of more concentrated urine. Lower ADH favours more dilute urine. Water retention also influences blood volume and pressure.
Example 1
Example 2
Urine output falls from 120 to 75mL/hour. Calculate the reduction.
120−75=45mL/hour. The decrease is consistent with increased water reabsorption but does not alone identify the cause.
Watch out: ADH usually increases water reabsorption in responsive nephron regions.