How the kidneys balance water and salts — Health, 14–17 years
Your kidneys do more than make urine. They constantly adjust water, salts and waste in your blood, helping cells work properly even when you drink, sweat or eat different foods.
A careful water manager
Blood carries useful substances and waste through your body, but its water and salt levels cannot swing wildly. Tiny filtering units in the kidneys remove some water and dissolved substances, then return what the body still needs. The rest becomes urine.
Why this control is needed
Cells need the right mixture around them to exchange substances and send signals. Heavy sweating, drinking very little or taking in lots of salt can disturb that mixture. Kidney control prevents a temporary change in your habits from becoming a dangerous change in the blood.
After a hot football match
Imagine losing 1 litre of sweat during a match and then drinking only 300 millilitres. Less water reaches the blood, so the kidneys save more water by making a smaller amount of darker urine. Once you drink normally again, they can release more water and the urine becomes paler.
Urine is not just waste
It is reasonable to think urine is simply everything the body does not want, because it leaves the body. In fact, the first fluid filtered by a kidney contains many useful substances, including water and glucose. Most are reclaimed before the final urine is made.
A clue about hydration
Urine colour can give a rough clue about how concentrated it is: darker urine often means the kidneys are conserving water. It is not a perfect medical test, because foods, medicines and illness can change colour. Use it as one clue, not as a diagnosis.
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