Two fist-sized organs that filter roughly 180 liters of blood plasma a day and return almost all of it.
How it works, step by step
Blood enters about a million nephrons per kidney.
The glomerulus filters water and small molecules out of blood.
Tubules reabsorb what the body needs: water, glucose, sodium, potassium.
What's left becomes urine.
Balance
Kidneys fine-tune potassium, sodium, water and acid levels, and activate vitamin D.
Practical
People with reduced kidney function may need to limit potassium, phosphorus or protein โ this is individual and set with a clinician.
Evidence & sources
The kidneys filter about 180 liters of fluid from blood each day. Evidence: established. Known: Established renal physiology. Unknown: โ Source: NIH NIDDK, Your Kidneys & How They Work (Current revision).