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Chapter 17. Hypothalamic Regulation of Hormonal Functions

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An athlete competing in an obstacle course event on a hot day sustains a serious cut as he clears the last hurdle on the course. He reports an intense feeling of thirst to responding paramedics. His thirst is most likely is stimulated by

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A. increases in plasma osmolality and volume.

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B. an increase in plasma osmolality and a decrease in plasma volume.

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C. a decrease in plasma osmolality and an increase in plasma volume.

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D. decreases in plasma osmolality and volume.

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E. a decrease in plasma osmolality with no change in plasma volume.

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The correct answer is B. Because he has been engaged in vigorous exercise, the athlete likely has lost water in excess of solute by sweating, with an associated increase in plasma osmolality. In addition, plasma volume will be reduced by hemorrhage. As shown in Figure 17–2, hypertonicity activates chemoreceptors in the hypothalamus and hypovolemia activates peripheral baroreceptors that in turn induce renin secretion and an increase in circulating angiotensin II. Angiotensin II then acts on circumventricular organs to signal to the hypothalamus. The hypothalamus integrates these various inputs and triggers brain areas concerned with thirst. Plasma tonicity and volume are independent regulators of the thirst response but the explanation above rules out options A, C, D, and E. In particular, option C would never trigger thirst mechanisms, nor would plasma volume ever be increased in the circumstances described.

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Figure 17-2: Diagrammatic representation of the way in which changes in plasma osmolality and changes in ECF volume affect thirst by separate pathways.

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In an experiment, a student is asked to sit quietly, unclothed, in a room in which the air temperature is 21°C (69.8°F) and the humidity 80%. The greatest amount of heat lost from their body will be by

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A. elevated metabolism.

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B. respiration.

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C. urination.

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D. vaporization of sweat.

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E. radiation and conduction.

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The correct answer is E. A is ruled out because, while metabolism contributes to heat production, it is not responsible for heat loss from the body. All of the remaining mechanisms contribute to heat loss, but the magnitude of heat loss by each route differs considerably. For example, respiration and urination (assuming the latter occurred during the study) would each be responsible for no more than 2% of heat loss (rules out B and C). ...

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