Sometimes the diagnosis of kidney failure particularly shocks patients and their loved ones. "But, doctor," they ask, "how can that be? I still pee!"

Many patients with kidney failure make urine, and in some forms of kidney disorders, urine output may be increased (what doctors call polyuria). This outcome makes sense once you know a bit about normal kidney physiology.

Normal Kidney Structure & Function

The kidneys sit in the upper back and receive blood directly from the aorta, the large artery that exits the heart. One fifth of each heartbeat goes into the kidneys for filtration and removal of wastes. The only organs that get blood flow before the kidneys are the heart and the brain; these beans are that important!

Approximately one million nephrons make up each kidney (why we call kidney doctors nephrologists). Each nephron starts with a cluster of blood vessels, the glomerulus (circled with teal in photo), that is surrounded by Bowman’s space (indicated with orange arrow). These filtering units make up about 10% of the kidney. Bowman’s space becomes a series of tubules that clump and twist around within the kidney, almost at random. More than 80% of each kidney consists of tubules. Liquid filtrate leaves the blood and enters Bowman’s space, then flows through the tubules that process the filtrate into urine. Tubules from neighboring nephrons join together into bigger and bigger tubes until they exit the kidney as the ureter, the duct that takes urine to the bladder.

When I decide to remove waste from an area, I generally find ways to collect only the waste and dispose of it. The kidneys take a more complex approach by first clearing most everything out of the blood. Tubules then reabsorb the good stuff and leave the bad behind. Normal kidney function depends on both glomeruli and tubules performing their respective tasks well.

Doctors and scientists measure the rate at which the kidneys clear waste with the glomerular filtration rate or GFR, the amount of blood filtered by the kidney every minute. A number of methods can measure or estimate GFR, but for most patients a simple blood test for creatinine is used. Normal GFR runs about 100 mL/minute (just over 3 oz each minute). The tubules of the kidney then reabsorb virtually all of the filtrate to leave behind wastes in a small amount of urine.

A normal person filters 100 mL/minute. With 24 hours in a day and 60 minutes in an hours, that means normal adults produce 144,000 mL (approximately 38 gallons) of filtrate each day. Our kidneys must retain at least 142 liters of fluid each day for a typical urine output of 1.5 – 2.0 liters! Those tubules have to do a lot of work.

Reduced Kidney Function

So what happens if the GFR falls? Most patients do not have problems with fluid retention until their kidney function goes below 10 mL/minute; how can that be?

Even at 10% of normal GFR, the kidneys still generate approximately 14 liters of filtrate each day, more than 7 times the amount of urine most healthy adults make! A number of kidney problems make tubules less efficient at filtrate processing and water reabsorption. In these conditions, urine output may actually increase because the damaged tubules cannot handle the filtrate load.

People at risk of kidney problems need to have their GFR’s monitored, as well as blood tests for other functions of the kidneys. Waiting for urine output to drop means waiting too long!

Images: Pascale Lane

About the Author: Pascale Lane is the Helen Freytag distinguished professor of pediatrics in the section of Pediatric Nephrology at the University of Nebraska Medical Center and the founding editor of ASN Kidney News, the national magazine of the American Society of Nephrology. Dr. Lane is a member of many professional organizations in nephrology and associated scientific disciplines. She performs and publishes both basic and translational research, teaches in the lecture hall and clinic, and cares for patients in her specialty. Other interests include faculty development, particularly writing skills, and the evolving role of social media in medicine and science. She blogs as WhizBANG! and at her Web site, pascalelane.net.

The views expressed are those of the author and are not necessarily those of Scientific American.