Outline the role of the kidney in body water homeostasis.
The main concept required was that the renal excretion of water is basic to the maintenance
of constant body water conditions. This renal water excretion is controlled by multiple factors
that influence glomerular filtration and tubular reabsorption. Also the kidney has mechanisms
that allow it to eliminate excess water by excreting a dilute urine or to conserve water by
excreting a concentrated urine.
Better answers provided details of the large GFR and the renal tubular handling of water.
Also the creation of the hyperosmolar medullary interstitium by the counter current multiplier
system, the special characteristics of the Loop of Henle that cause solutes to be trapped in the
renal medulla and the resulting delivery of a hypoosmolar tubular fluid to the collecting
ducts. Finally the variation in water permeability of the collecting ducts under the influence
Extra marks were awarded for details on ADH including its origin, secretion, regulation and
mechanism of action and the concept of electrolyte free water clearance.
Some candidates confused aquaporins with vasopressin receptors. Other candidates produced
long and irrelevant descriptions of the renin angiotensin system which gained no extra marks.
|Segment||What happens to water||Regulatory mechanisms|
Main mechanism is to influence glomerular blood flow:
|Proximal convoluted tubule||
|Descending thin limb||
|Ascending thin and thick limbs||
|Distal convoluted tubule||
|Connecting tubule and collecting duct||
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