Heat Loss and Dehydration in a 10 km Race
On a hot day (32 °C), a trained runner completes a 10 km race. Before the race her body temperature is 37.1 °C, heart rate 68 beats/min, breathing rate 14/min, and urine output over the previous 2 hours was 400 cm³. Immediately after the race her body temperature is 38.6 °C, heart rate 156 beats/min, breathing rate 32/min, and she has lost 1.2 kg in mass. She drank 300 cm³ of water during the race. Her skin is flushed and sweaty. Over the next hour her heart rate falls to 80 beats/min and she produces only 60 cm³ of dark yellow urine. The race starts at 11:00 and lasts 52 minutes. She wears light clothing. The coach notes that she sweats heavily, her skin stays warm, and she does not pass urine during the race.
1. Explain how sweating and vasodilation help the runner lose heat. Why does her body temperature still rise during the race?
Read the worked answer
Sweat glands release water onto the skin. As sweat evaporates, it takes latent heat from the skin, cooling the blood. Vasodilation widens arterioles near the skin surface, so more warm blood flows to the skin and more heat is lost by radiation and convection. However, exercise produces heat faster than it is lost: muscle respiration releases heat, and the hot day reduces the temperature gradient between skin and air. Evaporation can also be limited by humidity. So heat loss increases, but not enough to prevent the rise from 37.1 °C to 38.6 °C.
2. Explain why, one hour after the race, her urine is small in volume and dark yellow. Refer to ADH and the kidney.
Read the worked answer
Heavy sweating and a 1.2 kg mass loss leave her dehydrated, so blood water potential falls. Osmoreceptors in the hypothalamus detect this and stimulate the pituitary to release more ADH. ADH makes the collecting ducts in the kidney more permeable to water, so more water is reabsorbed into the blood. Less water remains in the urine, so volume falls to 60 cm³. The urine is dark because waste such as urea is more concentrated in a smaller volume of water.
3. Her heart rate one hour after the race is 80 beats/min, above her resting 68 beats/min. Suggest two reasons and explain why drinking water is important after the race.
Read the worked answer
First, she may still be repaying an oxygen debt: lactic acid from anaerobic respiration must be removed and oxidised, which needs oxygen transport, so the heart remains faster. Second, her body temperature is still above normal and dehydration reduces blood plasma volume, so the heart must beat faster to maintain blood pressure and supply. Drinking water restores plasma volume, supports sweating and heat loss, allows the kidneys to excrete waste, and reduces the risk of heat exhaustion. Plain water is suitable after a 10 km race if no extreme salt loss is mentioned.