Kidneys: filtering blood and balancing water — Biology, 14–17
Kidneys do not merely remove ‘waste’. They filter blood, recover useful substances and adjust water and ions, producing urine that changes with what your body needs.
The idea
Each kidney contains many nephrons, tiny filtering units. A nephron lets water and small molecules leave the blood, then selectively takes back glucose, useful ions and much of the water; what remains becomes urine.
Why it matters
Your cells work only within narrow limits of water, salts, acidity and waste products. The kidneys solve the problem of a changing diet and water supply by adjusting what returns to the blood instead of throwing everything away.
A worked example
After drinking little water, your blood becomes more concentrated. The brain signals release of ADH; kidney tubules then become more permeable to water, so more water returns to the blood and a smaller volume of darker urine leaves the body.
The common trap
It sounds sensible to say that urine is simply filtered blood, because it comes from blood in the kidneys. But useful substances are filtered first and then reclaimed; healthy urine normally contains waste and excess ions, not large amounts of glucose or proteins.
Where it is used
Doctors use blood and urine tests to assess kidney function, for example by checking creatinine or protein in urine. When kidneys fail, dialysis partly replaces their filtering role by moving wastes and extra water across a special membrane.
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