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18/02/2023 - Physiology
1) On a hot summer day, John, a high school student, went for a run and felt extremely thirsty. He stopped to drink a bottle of water and wondered how his body regulates water balance. Which of the following statements about the effect of an increase in plasma osmolality on the circulating levels of antidiuretic hormone (ADH) is true?
A) An increase in plasma osmolality leads to a decrease in the circulating levels of ADH.
B) An increase in plasma osmolality leads to an increase in the circulating levels of ADH.
C) An increase in plasma osmolality has no effect on the circulating levels of ADH.
D) An increase in plasma osmolality initially leads to a decrease, followed by an increase in the circulating levels of ADH.
Answer
The correct answer is B) An increase in plasma osmolality leads to an increase in the circulating levels of ADH.
Antidiuretic hormone (ADH), also known as vasopressin, is a hormone produced by the hypothalamus and released by the pituitary gland in response to changes in plasma osmolality. Its primary function is to regulate water balance in the body by promoting water reabsorption in the kidneys.
When the plasma osmolality increases, it means that there is a higher concentration of solutes (such as salt and urea) in the blood, which signals the hypothalamus to increase the release of ADH. This, in turn, stimulates the kidneys to reabsorb more water from the urine, leading to decreased urine output and increased water retention in the body.
Therefore, an increase in plasma osmolality leads to an increase in the circulating levels of ADH, not a decrease, as stated in option A. Option C is incorrect because an increase in plasma osmolality does have an effect on the circulating levels of ADH. Option D is incorrect because there is no evidence that an increase in plasma osmolality leads to an initial decrease in ADH levels before an increase.