Case 4: Six hours after the marathon
A runner completes a marathon on a hot day (31 °C, low humidity). She weighs 62.0 kg before the race and 59.5 kg afterwards, having drunk 1.2 L of water during the 4-hour run. Over the next six hours she passes only 400 mL of dark yellow urine. She then drinks 1.5 L of plain water quickly, and two hours later passes 300 mL of pale urine. A blood sample taken at the end of the race shows a higher solute concentration than her pre-race sample, and she stopped sweating about an hour after finishing. Her coach says the dark urine proves her kidneys have stopped working, and advises her to drink as much plain water as possible, as fast as possible, to 'flush the system out'.
1. Explain, using ADH, why she produced only 400 mL of dark urine in the six hours after the race, and why the urine became pale and more plentiful after she drank.
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At the end of the race her blood has a higher solute concentration than normal because water was lost in sweat faster than it was replaced, and she also stopped sweating once she cooled down.
- Detection. Osmoreceptors in the hypothalamus detect the raised solute concentration (lower water potential) of the blood.
- Secretion. The posterior pituitary releases more ADH into the blood.
- Action. ADH makes the walls of the distal convoluted tubule and collecting duct more permeable to water, so more water is reabsorbed by osmosis into the blood.
- Result. A small volume of concentrated (dark) urine - 400 mL over six hours - and blood solute concentration pulled back towards normal. This is negative feedback.
After she drinks 1.5 L, blood solute concentration falls, the osmoreceptors are no longer stimulated, ADH secretion drops, the collecting ducts become less permeable to water, less water is reabsorbed and a larger volume of dilute (pale) urine is produced. The pale urine is the expected response, not a sign that the kidney has recovered from a fault.
2. Explain what the change in body mass shows about her water balance, and how sweating links water balance to temperature control.
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She lost 2.5 kg in four hours. Since body mass changes over a race are almost entirely water, this represents about 2.5 L of water lost - roughly 2.5 kg of sweat minus the 1.2 L she drank (so nearer 3.7 L of sweat if no other losses are counted). The key point is that output exceeded input: she was in negative water balance, which is exactly why her plasma solute concentration rose and ADH secretion increased.
Sweating links water balance to temperature control:
- Sweat glands release water onto the skin; as it evaporates it takes latent heat from the skin, cooling the blood flowing near the surface.
- Low humidity on a hot day makes evaporation fast, so cooling is effective but water loss is large - the body cannot cool itself without losing water.
- Blood flow to the skin increases (vasodilation), so more heat is lost by radiation as well.
So thermoregulation and osmoregulation make competing demands on the same water. Losing 2.5 kg (about 4% of body mass) is enough to impair performance and to raise heart rate, because plasma volume falls and blood becomes more viscous; losses beyond about 5% risk heat exhaustion.
3. Evaluate the coach's two claims: that dark urine proves kidney failure, and that she should drink as much plain water as fast as possible.
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'Dark urine means the kidneys have stopped working' - incorrect. Dark, concentrated urine is the normal output of a healthy kidney when ADH is high: the tubules have reabsorbed as much water as possible, leaving little urine that is dark because wastes such as urea are concentrated in a small volume. That is the kidney working properly, conserving water. Kidney failure would more likely produce large volumes of dilute urine, or a failure to excrete urea at all, together with swelling and raised blood solute levels that do not correct with fluid intake. Her urine becoming pale and more plentiful after drinking shows the feedback system is intact.
'Drink as much plain water as fast as possible' - potentially dangerous.
- Rapid intake of large volumes of plain water dilutes plasma sodium (hyponatraemia). Water moves by osmosis into cells, including brain cells, causing headache, nausea, confusion and, rarely, seizures.
- After endurance exercise ADH levels can stay high for some hours, so much of the water is retained rather than excreted - which makes the dilution worse.
- Better advice: drink to thirst, in modest amounts, and include electrolytes to replace sodium lost in sweat; weigh before and after to estimate the deficit and replace it gradually.