Filters blood, removes waste and balances water, salts and acidity.
Evidence & sources
Each day the kidneys filter about 150 quarts (about 140 L) of blood to make about 1–2 quarts of urine. Evidence: established. Known: Basic physiology. Unknown: Varies with body size and hydration. Source: NIH NIDDK, Your Kidneys & How They Work (Current revision).