Water, Salt and ADH: A School Hydration Study
Three healthy volunteers in a Class 10 practical agree to a hydration test after an overnight fast. Volunteer P drinks 500 mL of plain water. Volunteer Q drinks 500 mL of 1% salt solution. Volunteer R drinks nothing. Urine is collected for 60 minutes after drinking. Results: P passes 320 mL, pale yellow, specific gravity 1.003; Q passes 110 mL, darker, specific gravity 1.022; R passes 60 mL, dark, specific gravity 1.028. A student concludes that 'drinking more water always makes the kidney work harder and wastes water, so the body should retain it'. Another says the salt drink 'pulled water out of the body' because salt is bad. The teacher asks them to use ADH and negative feedback, not just colour.
1. Explain the urine volumes using ADH, osmoreceptors, and negative feedback.
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- Drinking plain water lowers the osmolality of the blood. Osmoreceptors in the hypothalamus detect this and reduce ADH release from the posterior pituitary.
- Less ADH makes the collecting ducts less permeable to water, so more water stays in the urine. This explains P's large volume of 320 mL and low specific gravity of 1.003.
- In Q and R, blood osmolality is higher. ADH release increases, collecting ducts reabsorb more water, and urine volume falls. The smaller, darker, more concentrated urine in Q and R matches this.
- This is negative feedback: the change in blood osmolality triggers a response that brings it back towards normal. The kidney is not 'wasting' water in P; it is removing an excess.
2. Evaluate the claim that the 1% salt drink 'pulled water out of the body'.
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- The claim is an oversimplification. About 0.9% saline is close to the osmolality of body fluids; 1% is only slightly hypertonic.
- The salt load raises blood osmolality. ADH increases, so the kidney conserves water and produces a small concentrated urine. The person may also feel thirsty.
- This does not mean water is simply 'pulled out' of the body. The body retains water to dilute the extra salt, and the salt is eventually excreted, but the immediate response is water conservation.
- Very salty water or seawater is more dangerous because it can worsen dehydration, but a 1% solution is not equivalent to seawater. The student should distinguish a mild salt load from a strong osmotic challenge.
3. Predict the urine volume and concentration if a person cannot release ADH. Name the condition.
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- Without ADH, the collecting ducts remain relatively impermeable to water. Large volumes of dilute urine would be produced, and the person would pass urine frequently.
- The condition is diabetes insipidus. Blood osmolality would tend to rise, causing intense thirst, but the kidney cannot concentrate the urine properly.
- This is different from diabetes mellitus, where glucose in the filtrate can also cause increased urine volume. The key point here is the failure of ADH action, not a lack of insulin.