If an athlete restricts water intake before prolonged exercise, why does plasma osmolality increase despite vasopressin release affecting the collecting ducts?

💡 Explanation

The medullary osmotic gradient, established by the loop of Henle, drives water reabsorption in the collecting ducts. Dehydration reduces plasma volume and therefore the body's ability to effectively create a strong gradient, limiting vasopressin's effectiveness, because the osmotic gradient is insufficient to overcome the reduced water availability. Therefore, plasma osmolality increases despite hormonal action, rather than sodium excretion, filtration, or GFR being primary limiting factors.

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