Kidneys
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If you had lots to drink all the time, you might just let the remaining filtrate go out in your urine. After all, you've reabsorbed the foods in the proximal tubule, and reabsorbed Na+ and water to maintain blood volume. You've also secreted excess K+. Why mess with the urine any more? There's one big reason -- you still have a lot of water in the urine as it leaves the distal tubule and enters the collecting duct. And as a land-dwelling animal, you lose water all the time through evaporation from your skin and lungs. You don't want to lose more in your urine. |
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