Describe the regulation of body water.
Better answers for this question used the “sensor, integrator/controller, effector” structure. They also included appropriate detail relating to the site and mechanism of angiotensin II and the subsequent stimulation of ADH and aldosterone release. A detailed description of ADH was necessary to score well. Lengthy descriptions of body water distribution or renal handling of water did not attract additional marks. Answers that scored less well were often disorganised, with limited structure and incorrect facts.
The stem of this question is identical to Question 1 from the second paper of 2021, but the college comments are sufficiently different that one might think the marking criteria must have changed from paper to paper. In 2021, there was no mention of "site and mechanism of angiotensin II and the subsequent stimulation of ADH and aldosterone release". The pass rate has increased from 28% to 42%, suggesting that some of the candidates have been looking at the past papers.
Extracellular tonicity is the sensed variable.
Hypothalamic osmoreceptors are the sensors
Afferent signal is via OVLT and subfornical organ nerves fibres to the hypothalamus
Another mechanism of stimulating the same system is via hypotension
Central controller/integrator is the hypothalamus
Efferent signal is vasopressin
Effector organ is the renal collecting duct
Effect on urine concentration
Another effector mechanism is thirst
Indirectly, total body water is also affected through sodium regulation by:
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