The Fainting Runner
A 17-year-old athlete runs a 10 km race on a hot day (32 °C) and drinks only 300 ml of water beforehand. During the race he sweats heavily and loses about 1.5 litres of sweat. At the 8 km mark he feels dizzy, his skin is pale and clammy, and he collapses briefly. A bystander wants to give him a strong salty drink and sit him upright in the sun. His pulse is rapid and thready, and his breathing is shallow. He recovers when laid flat with legs raised and given small sips of a weak electrolyte solution.
1. Explain, using the concept of negative feedback and the circulation, why the runner collapsed.
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Heavy sweating caused a large loss of water (about 1.5 L) and some salts, lowering blood volume. Lower blood volume reduces venous return and stroke volume, so cardiac output falls and blood pressure drops (hypotension). Baroreceptors detect the fall and trigger a compensatory response: heart rate rises (hence the rapid pulse) and peripheral vessels constrict, which makes the skin pale and clammy. Blood is diverted away from the skin and gut to the brain and muscles. The brain's blood supply becomes briefly inadequate when he stands or stops moving, causing dizziness and a short loss of consciousness. This is a failure of the negative feedback loop that normally keeps blood pressure stable: the corrective response (vasoconstriction and tachycardia) could not fully compensate for the fluid lost. Lying flat with legs raised restores venous return and cerebral perfusion, which is why he recovered.
2. Evaluate the bystander's plan to give a strong salty drink and sit him upright in the sun. What should be done instead, and why?
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The plan is unsafe for three reasons. First, sitting him upright in the sun keeps blood pooled in the legs and reduces venous return to the heart, so brain perfusion stays low and he may faint again. Second, a strong salty drink is hypertonic: it draws water out of cells into the gut and blood, worsening cellular dehydration and causing vomiting, which loses more fluid. Third, the heat and sun increase sweating and heat gain, adding to the fluid deficit. Correct first aid: lay him flat, raise the legs above heart level to improve venous return, move him to shade and cool him, loosen clothing, and give small frequent sips of a weak electrolyte solution once he is fully conscious. Monitor breathing and pulse. If he does not recover quickly, seek medical help, because prolonged hypotension or heat stroke can damage the brain and kidneys.
3. Suggest two changes to his pre-race preparation and explain how each would reduce the risk of collapse.
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- Drink more fluid before and during the race, for example 500–750 ml in the two hours before and small amounts at water stations. This reduces the sweat deficit, so blood volume and blood pressure are maintained and cardiac output does not fall as sharply. 2) Acclimatise to heat and include electrolytes in his drink. Heat acclimatisation increases sweat rate but lowers the salt concentration of sweat and increases plasma volume, so he loses less salt and tolerates the heat better. Including modest electrolytes replaces the salts lost in sweat and helps maintain osmotic balance so water is retained in the blood rather than lost in urine. Together these keep venous return, stroke volume and blood pressure within the range the negative feedback system can correct.