Cells make wastes, and the amount of water and salt in the blood has to stay in a narrow range. The kidneys handle both. They depend on osmosis and are part of the team of body systems, working closely with the circulatory system.
Key idea
Each kidney has about a million filtering units called nephrons. A nephron filters water and small molecules out of the blood, reabsorbs what the body needs (most of the water, glucose, many salts), and leaves the wastes, such as urea, in the urine. The hormone ADH makes the kidneys keep more water when the body is short of it.
Why it works
Why filter almost everything and then take most of it back? Filtering by size is simple and catches every small waste, even ones the body has never met. Then the tubule can be choosy, returning only what is useful. The numbers are striking: the kidneys filter about 150 quarts of blood a day, and only 1 to 2 quarts become urine (NIDDK).
Worked example: How much is reabsorbed?
Suppose a person's kidneys filter 180 liters of fluid in a day and 1.8 liters leave as urine. What percent of the filtered fluid was reabsorbed?
- Reabsorbed: 180 − 1.8 = 178.2 liters.
- 178.2 ÷ 180 = 0.99.
Answer: 99%.
Worked example: A hot day without water
After hours outdoors with nothing to drink, what happens to ADH and to the urine?
- The blood is short of water, so the brain releases more ADH.
- ADH makes the nephrons reabsorb more water.
Answer: ADH rises, and the urine is smaller in amount and darker (more concentrated).
Glucose is filtered too, but healthy kidneys reabsorb all of it, so normally none is in the urine. The kidneys also help control salt levels, blood pressure and the blood's pH.
Common mistake
It is natural to think the kidneys pick out only the harmful things. They filter almost everything small out of the blood, useful or not, and then take back what the body needs. Check with glucose: it leaves the blood in the filter and returns in the tubule.