During sustained high-intensity exercise in a hot environment, a marathon runner develops hyperthermia (core temperature 40.5°C) and begins to sweat profusely. Which integrative response best describes the cardiovascular–thermoregulatory conflict that limits heat dissipation?

• A High core temperature directly impairs hypothalamic thermosensory neurons, reducing sweat rate despite continued hyperthermia
• B Prolonged sweating causes hyperchloraemia that inhibits Na-K-2Cl cotransporters in sweat glands, reducing sweat secretion
• C Competitive demand for cardiac output between active skeletal muscles and cutaneous vasodilation means that as central venous pressure falls, the hypothalamus must compromise between thermoregulation and maintaining muscle perfusion, resulting in reduced cutaneous blood flow and impaired heat loss ✓
• D Splanchnic vasoconstriction during exercise selectively redirects blood to the hypothalamus, cooling it and falsely signalling adequate thermoregulation

Explanation

During exercise-heat stress, two major vascular beds compete for limited cardiac output: exercising skeletal muscle (requiring high flow to sustain ATP production) and cutaneous vasodilation (required for heat loss by convection and radiation). As dehydration and exercise duration reduce venous return and stroke volume, baroreceptor reflexes restrain cutaneous vasodilation to maintain blood pressure and muscle perfusion—this cardiovascular constraint limits further heat dissipation even as core temperature rises. This is the cardiovascular–thermoregulatory conflict. Hypothalamic thermoreceptors are actually more sensitive at higher temperatures (not impaired). Sweat gland Na-K-2Cl transport is not inhibited by hyperchlorinaemia in this setting.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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