Osmoregulation

In the syllabus and the exam

Osmoregulation closes the excretion topic by explaining how the body keeps the water potential of the blood constant despite constantly varying water intake and water loss through sweat, breath and urine. Candidates should describe the feedback loop centred on the kidney: a rise in blood water potential, for example after drinking a large volume of fluid, is detected, less antidiuretic hormone is released, the collecting duct and distal convoluted tubule become less permeable to water, and more dilute urine in a larger volume is produced to remove the excess water; a fall in blood water potential, for example after heavy sweating, triggers the opposite chain of events, producing concentrated urine in a smaller volume. A common exam format gives a graph of urine concentration or volume against a described condition, such as exercise in hot weather or drinking a large amount of water, and asks candidates to explain the trend using antidiuretic hormone and tubule permeability rather than describing kidney function in general terms. A frequent weakness is failing to name antidiuretic hormone specifically or reversing the direction of its effect on water reabsorption.

Example

On a hot day when a person sweats heavily, osmoregulation reduces urine output so the blood does not lose too much water.

Related terms

NephronUltrafiltrationHomeostasis

Source:SRC-DSKP-EN