Ankit's 400 m race on a humid afternoon
Ankit, a Class 10 student, runs the 400 m race at the annual sports meet on a hot, humid afternoon - 38 °C with relative humidity around 85%, and a light breeze only near the finish. He finishes in 62 s, faster than any of his practice runs. For several minutes afterwards he is bent double, breathing rapidly and deeply, about 40 breaths per minute against his resting rate of 16. His calf muscles cramp and he feels a burning sensation in the legs. Sweat pours off him, but his skin feels less cool than he expects on such a hot day. He is extremely thirsty, drinks a large bottle of plain water, and still feels weak and slightly giddy. Later his urine is dark yellow and scanty, and he notices his heart pounding for several minutes after he stops. His coach wants a written explanation before the next meet.
1. Explain why Ankit's muscles burn and cramp during the race and why his breathing rate stays high afterwards. Use the idea of aerobic and anaerobic respiration.
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Sprinting 400 m demands far more energy per second than the lungs and blood can deliver oxygen for. Muscle cells therefore respire anaerobically for part of the race: glucose is broken down without oxygen, releasing a smaller amount of energy and producing lactic acid in the muscle cells.
Accumulation of lactic acid and the associated changes in the muscle lower the pH and interfere with muscle contraction, producing the burning sensation and cramp. This is not a sign of disease; it is the direct consequence of oxygen supply lagging behind demand.
After the race, the fast, deep breathing is the body repaying the oxygen debt. Extra oxygen is carried to the muscles and to the liver, where lactic acid is oxidised back to carbon dioxide and water, or converted back to glucose. That takes time, which is why the breathing rate and depth stay high for several minutes, and why the heart continues to beat fast - it must keep circulating oxygen and clearing the lactic acid.
So the sequence is: high energy demand → oxygen shortage → anaerobic respiration → lactic acid → cramps and burning → heavy breathing and fast heart rate to clear it.
2. Why did sweating fail to cool Ankit on that particular day, and why was plain water alone not the right drink? Suggest what the coach should have given him.
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Sweating cools the body only when the sweat evaporates, because evaporation absorbs latent heat from the skin. On a day with 85% relative humidity, the air already holds a great deal of water vapour, so the rate of evaporation from the skin falls sharply. Sweat dribbles off instead of evaporating, and very little heat is removed. This is why his skin feels less cool than expected and why the same temperature with dry air would have felt far more tolerable. A light breeze near the finish temporarily helped by sweeping vapour away and increasing evaporation.
Plain water alone is inadequate because sweat is not pure water - it contains sodium chloride and small amounts of other salts. Heavy sweating causes loss of water and salts. Drinking large volumes of plain water restores the volume but dilutes the remaining body salts, which can cause weakness, giddiness and cramps. The dark, scanty urine shows he is dehydrated and the kidneys are conserving water.
The coach should give an oral rehydration solution - water with a pinch of salt and a little sugar - or a sports drink, sipped steadily rather than gulped. Sugar also provides glucose to replenish muscle fuel.
3. Ankit's breathing and heartbeat continued automatically while he was concentrating on the race. Name the control system responsible, and explain the role of adrenaline in what happened to him.
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Breathing and heartbeat are involuntary actions controlled by the medulla oblongata in the brain stem. It continuously monitors the carbon dioxide concentration of the blood and sends nerve impulses to the diaphragm and rib muscles to set the rate and depth of breathing, and to the heart via the autonomic nerves. Because control is involuntary, Ankit did not have to think about breathing during the race, and he cannot hold his breath indefinitely even if he wants to.
Adrenaline, secreted by the adrenal glands on top of the kidneys, reinforced the response: it increases heart rate and the force of contraction, widens the airways to speed oxygen intake, raises blood glucose by promoting the breakdown of glycogen in the liver, and diverts blood flow from the digestive system to the skeletal muscles. This is why his heart was pounding for minutes after he stopped and why he had extra fuel available.
Comparing the two systems is the key point: the nervous system gives rapid, short-lived control through electrical impulses, while hormones such as adrenaline give slower but longer-lasting effects carried in the blood. Both were acting on him at once - nervous control of breathing, hormonal reinforcement of heart rate and fuel supply.