ADH, Sweat Loss and Rehydration
A 22-year-old runner completes a marathon on a warm day. She drinks 400 cm3 of water during the race but loses about 2.5 dm3 of sweat. After the race she feels dizzy and her urine is dark and concentrated. A sports scientist measures her blood: plasma osmolarity 305 mOsm/kg (normal 275–295), sodium 150 mmol/dm3 (normal 135–145), and ADH 8.2 pmol/dm3 (normal 1–5). She is given an oral rehydration drink containing water, glucose and sodium. Two hours later her urine output increases and plasma osmolarity falls to 290 mOsm/kg. The scientist says her kidneys responded to ADH, but a friend says ADH makes you produce more urine. Her body temperature is 38.1 °C; she has lost 1.8 kg since the start. The drink contains 60 mmol/dm3 sodium and 2% glucose.
1. Explain why the runner's ADH concentration is high and how this helps restore her blood.
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The runner has lost about 2.5 dm3 of sweat but replaced only 400 cm3 of water, so her blood volume falls and her blood becomes more concentrated. Plasma osmolarity is 305 mOsm/kg, above the normal 275–295, and sodium is 150 mmol/dm3, above normal. Osmoreceptors in the hypothalamus detect the increased osmolarity. The posterior pituitary releases more ADH, which is why her ADH is 8.2 pmol/dm3. ADH makes the collecting ducts of the kidneys more permeable to water, so more water is reabsorbed from the filtrate into the blood. This produces a small volume of concentrated, dark urine and helps conserve water. Thirst also increases water intake. Together these responses lower plasma osmolarity back towards normal.
2. Evaluate the friend's statement that ADH makes you produce more urine.
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The friend has it the wrong way round. ADH reduces urine volume: it increases water reabsorption in the collecting ducts, so less water is lost in urine. A person who cannot secrete ADH, as in diabetes insipidus, produces large volumes of dilute urine. In this runner, the high ADH is consistent with dark, concentrated urine and a small urine volume. Two hours after drinking the rehydration fluid, her plasma osmolarity falls to 290 mOsm/kg. The osmoreceptors detect this fall, ADH secretion decreases by negative feedback, less water is reabsorbed, and urine output increases. That later increase is not evidence that ADH makes more urine; it shows that ADH has been switched off as the blood returns to normal.
3. Explain why the rehydration drink contains glucose and sodium rather than plain water, and why her urine output increased after two hours.
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Plain water would replace volume but not the sodium lost in sweat. If she drinks only water, plasma sodium can become diluted, and in severe cases this can cause hyponatraemia. The drink contains sodium to replace the salt lost and glucose to provide energy. In the small intestine, glucose and sodium are absorbed together by co-transport, and water follows osmotically, so the drink is absorbed faster than plain water. The sodium also helps restore plasma osmolarity without diluting it too much. After two hours, the absorbed water and sodium lower plasma osmolarity from 305 to 290 mOsm/kg. ADH secretion then falls, so the kidneys reabsorb less water and urine output increases. This is a normal negative-feedback response, not a failure of ADH.