The kidneys receive roughly a fifth to a quarter of the blood the heart pumps at rest, and between them they filter around 180 litres of fluid out of that blood every day — something like forty times the entire blood plasma volume of an adult body. And yet a healthy adult typically produces only one to two litres of actual urine in a day. Somewhere between the filtering and the finished product, the body takes back well over ninety-nine percent of what it just filtered out.
Filter everything first, ask questions later
Each kidney contains roughly a million tiny filtering units called nephrons. At the start of each one, blood pressure forces plasma — water, salts, glucose, amino acids, waste products, all of it — through a filtering membrane, leaving behind only blood cells and large proteins that are too big to pass. This first step isn't selective in any meaningful sense: it filters almost everything small enough to fit through, useful molecules and waste alike, producing a filtrate that starts out looking almost exactly like blood plasma with the cells and big proteins removed.
Reclaiming the useful 99% along a single long tube
What makes the filtrate become urine is everything that happens next, as it travels the length of the nephron's tubule. Along that path, the body actively reabsorbs nearly all the filtered glucose, most of the salt, and most of the water back into the bloodstream, while letting genuine waste — urea, excess ions, drugs and other unwanted substances — continue on to be concentrated and excreted. Hormonal signals, particularly antidiuretic hormone and aldosterone, fine-tune exactly how much water and sodium get reclaimed at the very end of that path, which is the mechanism behind urine becoming more concentrated when the body is dehydrated and more dilute when it's had plenty to drink.
What we're still unsure about
The glomerular filtration rate doctors use to assess kidney health isn't measured directly in routine clinical practice — it's estimated, using blood markers like creatinine combined with demographic variables such as age and sex as a proxy for the true filtration rate. Nephrology has an active, ongoing debate about exactly which variables belong in that estimate and how much the commonly used equations can misjudge true kidney function for different groups of patients, including a fairly recent and still-contested shift away from including race as a variable in the standard formulas. It's a live, evolving area of clinical practice, not a fixed piece of arithmetic every doctor applies the same way.
This sits inside Renal Physiology & Fluid Balance, one of seven topics in Physiology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.