Heat, Water Loss and Dark Urine on a Tobacco Estate
During a heatwave in the Lower Shire Valley, a tobacco estate worker spends six hours tying and harvesting in an open field. The air temperature is 38 C and relative humidity is 30%. He drinks only 0.5 dm3 of water during the shift and eats nothing. By mid-afternoon he is dizzy, has a headache, and passes a small volume of dark yellow urine. His skin feels hot and dry, although he has been sweating earlier. He weighs 2.5 kg less than at the start of the day. His supervisor says he should drink a strong salt solution immediately to replace the salt and that dark urine means the kidneys are working well. A nurse at the estate clinic advises oral rehydration salts and rest in the shade.
1. Explain how the body detects the water loss and reduces urine volume. Name the main hormone and its target organ.
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Water loss raises the osmotic pressure of the blood plasma. Osmoreceptors in the hypothalamus detect this increase. The hypothalamus stimulates the posterior pituitary gland to release antidiuretic hormone (ADH) into the blood. ADH acts on the collecting ducts of the kidneys, making their walls more permeable to water, so more water is reabsorbed into the blood. The urine becomes smaller in volume and more concentrated. Thirst is also stimulated, encouraging drinking.
If blood volume falls, the renin-angiotensin-aldosterone system is activated; aldosterone from the adrenal cortex increases sodium reabsorption in the kidney, and water follows. These responses are negative feedback mechanisms that help restore plasma water potential, but they cannot replace water that has been lost unless the worker drinks.
2. Why is dark, scanty urine not evidence that the kidneys are working well in this situation?
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The dark colour is mainly due to a higher concentration of urochrome and other waste products because little water is being excreted. The small volume shows that ADH is conserving water, which is an appropriate short-term response, but it also shows that the body is dehydrated.
If dehydration continues, blood volume and blood pressure fall, and blood flow to the kidneys may decrease. That can reduce glomerular filtration and, in severe cases, cause acute kidney injury. The 2.5 kg weight loss represents about 2.5 litres of water lost, roughly 4-5% of body mass in many adults, which is enough to cause dizziness and headache. Dark urine is therefore a warning sign, not a sign of healthy kidney function.
3. Evaluate the supervisor's advice and the nurse's advice.
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The supervisor is partly right that salt is lost in sweat, but a strong salt solution is dangerous. It is hypertonic and can draw water out of the body's cells, worsen thirst and cause vomiting.
The nurse's advice is safer. Oral rehydration salts contain sodium chloride, potassium chloride, glucose and a buffer in controlled amounts. The glucose stimulates sodium and water absorption in the small intestine, so the fluid replaces both water and salts. Rest in the shade reduces heat gain and sweating. If the worker is confused, very weak or unable to drink, he needs urgent intravenous fluids.
Plain water alone would be better than nothing, but large volumes without salts can dilute the blood sodium, so ORS is preferred. Cool fluids and small frequent drinks are better than a large volume at once.