A footballer who drank too much water on a hot tournament day
During a one-day football tournament in July, with the shade temperature at 32 °C and humidity high, a 19-year-old midfielder plays three 40-minute matches. He weighs 70.0 kg at 08:00. Immediately after the first match he weighs 68.6 kg. Across the whole day he drinks 4.5 L of chilled plain water and eats very little. He passes large volumes of very pale urine four times. After the third match he complains of a throbbing headache, nausea and confusion, and is taken to hospital. Blood tests show a sodium concentration of 128 mmol dm⁻³ (normal 135–145). A sample of his sweat taken during the first match contained 40 mmol dm⁻³ sodium, compared with 140 mmol dm⁻³ in blood plasma. The doctor describes him as "water intoxicated" rather than dehydrated, which puzzles his coach, since the player had clearly lost weight.
1. Calculate the percentage of body mass lost during the first match. Then explain how he can have lost mass and yet end up with too little sodium in his blood.
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Mass lost: 70.0 − 68.6 = 1.4 kg. As a percentage of body mass: 1.4 ÷ 70.0 × 100 = 2.0 %.
A 2 % loss is enough to count as mild dehydration, so the coach is right that he lost fluid - mostly as sweat, since urine and breath account for little in 40 minutes.
Why sodium fell: sweat contains 40 mmol dm⁻³ sodium against 140 mmol dm⁻³ in plasma, so sweat is dilute compared with blood. Sweating therefore removes proportionally more water than salt: the body loses water fast and sodium slowly. Drinking plain water replaces the water but none of the sodium, so whatever sodium remains is spread through a larger volume - the concentration falls from about 140 to 128 mmol dm⁻³.
The kidneys can only produce dilute urine so fast (roughly 0.8–1 L per hour). Drinking 4.5 L in a day, faster than that, forces the excess water out - and the large volumes of pale urine he passed carried more sodium away with it. So the two findings are not contradictory: he was down on water after each match, and over-diluted by the end of the day.
2. Using osmosis, explain why his brain cells swelled and why this produced headache, nausea and confusion.
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Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential (more dilute) to a region of lower water potential (more concentrated).
Before the tournament his plasma and his cells were at about 140 mmol dm⁻³ and in balance. After drinking 4.5 L of plain water, his plasma fell to 128 mmol dm⁻³ while the contents of his cells stayed near 140. The plasma is now the more dilute solution - it has the higher water potential - so water moves by osmosis from the blood into the cells, including brain cells, and they swell.
Most tissues can stretch, but the brain is enclosed in the rigid skull. Swelling raises pressure inside the skull, which produces the throbbing headache and the nausea, and disrupts normal nerve function, producing confusion. In severe cases it causes seizures.
Contrast this with ordinary dehydration: there the blood becomes more concentrated, so water leaves cells and they shrink. Here the opposite happened, which is why the doctor's description is not the one the coach expected.
3. Recommend what he should have drunk and how much, using the sweat data to justify your answer.
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What: a drink containing sodium - a sports drink or water with added salt, providing roughly 20–30 mmol dm⁻³ sodium (about 500–700 mg per litre). That is below the 40 mmol dm⁻³ in his sweat, so it will not replace salt faster than he loses it, but it is far better than plain water, which contains none. Salty food at breaks helps too.
How much: match intake to loss. He lost 1.4 kg in one match, so about 1.4 L during and around that match, sipped steadily rather than in large volumes, and no more than about 1 L per hour so the kidneys can keep up. The practical method is to weigh him before and after each match and replace roughly the mass lost.
Justification: replacing water without sodium dilutes the plasma and drives water into cells by osmosis. Including sodium keeps plasma concentration near 140 mmol dm⁻³, so water stays in the right compartments. Matching intake to loss avoids the opposite error - drinking beyond his losses, which is what produced the very pale urine and the fall to 128 mmol dm⁻³.