Yes, your kidneys filter your entire blood supply, and they do it with remarkable speed and precision. Each kidney contains about a million tiny filtering units called nephrons, and together they process roughly 150 liters (about 40 gallons) of blood every day. Only about 1 to 2 liters of that ends up as urine. The rest, including water, nutrients, and essential minerals, gets returned to your bloodstream.
How the Filtering Process Works
Each nephron handles filtration in two steps. First, blood enters a tiny cluster of blood vessels called the glomerulus. The walls of these vessels are thin enough to let water, small molecules, and waste products pass through into a connected tube (the tubule), while keeping larger molecules like proteins and blood cells in the bloodstream. Think of it like a sieve that catches the big stuff and lets everything else drain through.
The second step is selective reabsorption. The fluid that passes through the glomerulus contains a lot of things your body still needs, so the tubule reclaims them before waste reaches the bladder. More than 99% of filtered glucose gets reabsorbed, along with sodium, potassium, chloride, phosphate, bicarbonate, and most of the water. What remains, the actual waste, continues down the tubule and eventually becomes urine.
What the Kidneys Remove
The primary waste product is urea, which forms when your body breaks down protein. Creatinine, a byproduct of normal muscle activity, is another major one. But the list goes well beyond those two. Researchers have identified over 100 substances classified as uremic toxins, compounds that build up in the blood when the kidneys can’t clear them properly. These include uric acid (linked to gout), homocysteine, and various organic acids like lactic acid and sulfuric acid.
When kidney function declines and these toxins accumulate, the condition is called uremia. It can damage multiple organ systems, with the nervous system being especially vulnerable. This is why kidney failure has such wide-ranging effects throughout the body, not just on urination.
Blood Pressure and Fluid Balance
Filtering waste is only part of the job. Your kidneys also play a central role in controlling blood pressure through a hormonal system called the renin-angiotensin-aldosterone system, or RAAS.
Here’s how it works: when your blood pressure drops, your kidneys release an enzyme called renin into the bloodstream. Renin triggers a chain reaction that ultimately produces a hormone called angiotensin II. This hormone causes small arteries to narrow, which raises blood pressure. It also signals your adrenal glands to release aldosterone, a hormone that tells the kidneys to hold onto more sodium. Where sodium goes, water follows, so your blood volume increases and pressure rises further. The system also prompts your kidneys to release excess potassium through urine, keeping sodium and potassium in a careful balance.
This is why kidney disease so often leads to high blood pressure, and why many blood pressure medications work by targeting this exact pathway.
Acid-Base Balance
Your blood needs to stay within a very narrow pH range to function properly, and the kidneys are one of two systems (along with the lungs) that maintain it. They do this by adjusting how much bicarbonate they reabsorb and how much acid they excrete into the urine. When your blood becomes too acidic, the kidneys reclaim more bicarbonate and dump more acid. When blood becomes too alkaline, they do the reverse. This compensation can take hours to days, making it slower than the lungs but more precise over time.
Red Blood Cell Production
Your kidneys also monitor oxygen levels in the blood. When oxygen drops, healthy kidneys produce a hormone called erythropoietin (EPO), which signals bone marrow to make more red blood cells. More red blood cells means more oxygen-carrying capacity. This is why people with advanced kidney disease often develop anemia: their kidneys can no longer produce enough EPO to keep red blood cell counts at normal levels.
How to Know If Your Kidneys Are Filtering Well
The standard measure of kidney filtration is the glomerular filtration rate, or GFR. It estimates how much blood your kidneys filter per minute. In healthy young adults, a normal GFR is approximately 100 to 110 mL/min. A GFR below 60 is generally considered the threshold for chronic kidney disease in adults aged 40 to 65, though some researchers have proposed age-adjusted cutoffs: below 75 for people under 40, and below 45 for those over 65 without protein in their urine.
GFR is typically estimated through a simple blood test that measures creatinine. Because creatinine is a waste product your kidneys should be clearing, higher levels in the blood suggest the kidneys aren’t filtering as efficiently. The test is routine in standard blood panels, so many people have already had it measured without realizing it. If your results show an estimated GFR (often listed as “eGFR”) above 90, your filtration is in the normal range.