Thermoregulation During Exertion in Heat
A 24-year-old runner completes a 10 km race on a 34 °C day with 70% humidity. At 5 km his skin is flushed and sweating heavily; his core temperature is estimated at 39.2 °C. He drinks little. At 8 km he stops sweating despite continued heat exposure, becomes confused, and his skin is hot and dry. His heart rate is 168 beats per minute and his blood pressure has fallen from 120/80 to 95/60 mmHg. His plasma sodium is 141 mmol/L. Bystanders move him to shade and apply cool wet towels.
1. Compare the thermoregulatory state at 5 km with that at 8 km, and explain why the loss of sweating is a warning sign rather than an improvement.
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At 5 km he is in a compensated state: sweating is active, skin is flushed (vasodilation), and heat is being lost by evaporation. Core temperature is elevated but the effector response is working.
At 8 km the effectors have failed. Stopping sweating is not a sign that he has cooled - his skin is hot and dry and his core temperature is presumably higher. Sweating is the main avenue of heat loss in a hot, humid environment; without it, heat gain exceeds heat loss and core temperature climbs rapidly.
The failure is likely driven by a combination of dehydration reducing plasma volume, cardiovascular strain (heart rate 168, blood pressure falling to 95/60), and central dysfunction - confusion indicates the brain is affected. Confusion plus hot dry skin plus collapse of blood pressure is a medical emergency, not fatigue.
The judgement: the transition from 'sweating heavily' to 'not sweating' during continued heat exposure means the cooling system has been overwhelmed. That is the opposite of improvement.
2. Explain how the falling blood pressure connects to the failure of sweating, and why giving a large volume of plain water rapidly would be unwise.
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Sweating loses water from the plasma, reducing blood volume. To maintain blood pressure, the body vasoconstricts skin and splanchnic vessels and raises heart rate. But skin vasoconstriction reduces heat loss - it competes directly with thermoregulation. When volume loss is severe enough, the cardiovascular need to defend blood pressure overrides the need to dissipate heat, and the sweating response can fail. So the low blood pressure and the absent sweating share a cause: volume depletion.
Why not rapid plain water: his plasma sodium is 141 mmol/L, near normal, but he has lost both water and salt through sweat. Rapid large-volume plain water intake in a person with altered consciousness risks dilutional hyponatraemia and, more immediately, aspiration if he is confused. In an emergency setting, the priority is active cooling and controlled fluid replacement, not oral water. The judgement is that the immediate threat is hyperthermia and circulatory collapse, and fluid strategy must be measured, not a rapid bolus by mouth.