Heat, Sweat and Concentrated Urine
A 22-year-old labourer works for 6 hours in a factory at 34 °C and 80% relative humidity. The factory has no air conditioning and only one working fan. He drinks only 500 cm³ of water during the shift. At the end of the shift he feels dizzy, has a throbbing headache, and passes only 150 cm³ of dark yellow urine. His body temperature is 38.2 °C, pulse 104 beats per minute, and his urine specific gravity is 1.030 (normal 1.005–1.020). A urine test shows no glucose and no protein. His skin is hot and dry after he stops sweating. A supervisor suggests giving him a strong salt drink and letting him rest in the sun. Another worker says he should drink as much water as possible immediately.
1. Explain the normal homeostatic response to his water loss. Name the hormone, its source, its target and the effect on urine. Use his urine volume and specific gravity to decide whether his ADH secretion is high or low.
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Water loss through sweating, with only 500 cm³ drunk over 6 hours, raises plasma osmolarity. Osmoreceptors in the hypothalamus detect the increase. The hypothalamus sends nerve signals to the posterior pituitary, which releases antidiuretic hormone (ADH) into the blood. ADH increases the permeability of the collecting ducts in the kidneys to water, so more water is reabsorbed into the blood. Urine volume falls and the urine becomes more concentrated.
His urine volume is only 150 cm³, it is dark yellow, and its specific gravity is 1.030, above the normal upper limit of 1.020. These findings are consistent with a high ADH level. The absence of glucose and protein means the concentration is not caused by diabetes mellitus or by protein loss through damaged kidneys. The response is negative feedback: concentrated urine conserves water and helps restore plasma osmolarity towards normal.
2. Explain why his core temperature rose to 38.2 °C and why he stopped sweating. Then evaluate the supervisor’s advice.
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At 34 °C with 80% relative humidity, the air is nearly saturated, so sweat evaporates slowly. Evaporation is the main cooling mechanism. Heat gained from the hot environment and from muscle metabolism exceeds heat lost, so core temperature rises to 38.2 °C. He has also lost a lot of water: he drank only 500 cm³ in 6 hours and his urine is very concentrated. Severe dehydration reduces sweat production; without sweating, cooling fails and temperature can rise further, risking heat stroke.
The supervisor’s advice is dangerous. A strong salt drink adds solute without enough water and can worsen the osmotic imbalance. Resting in the sun adds heat instead of removing it. He needs shade or a cool room, cool water in small frequent amounts, and medical assessment. Any salt replacement should be in a properly diluted oral rehydration solution, not a strong salt drink.
3. Predict the effect on urine volume and specific gravity if he drinks 1.5 dm³ of plain water over the next hour. Explain why drinking seawater would not be a good treatment.
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Drinking 1.5 dm³ of plain water lowers plasma osmolarity. Osmoreceptors detect the fall, ADH secretion decreases, and the collecting ducts become less permeable to water. More water remains in the filtrate, so urine volume increases over the next few hours and specific gravity falls towards about 1.005–1.010; the urine becomes pale.
Seawater is hypertonic to body fluids. To excrete the extra salt, the kidneys need more water than the seawater contains. Drinking seawater therefore increases water loss and worsens dehydration. It would raise plasma osmolarity further and keep ADH secretion high, so it is not a treatment for dehydration.