Your kidneys are two fist-sized organs that sit just below your rib cage on either side of your spine, and they do far more than make urine. They filter your blood, regulate blood pressure, activate vitamins, balance minerals, control the acidity of your body fluids, and send hormonal signals that keep your red blood cell count steady. Every day, they filter roughly 1,800 liters of blood, a volume so large it means your entire blood supply passes through them hundreds of times before you go to sleep.
How Your Kidneys Filter Blood
Each kidney contains about a million tiny filtering units called nephrons. Blood enters a nephron through a cluster of microscopic blood vessels called the glomerulus. The thin walls of these vessels act like a sieve: small molecules, waste products, and water slip through into a narrow tube (the tubule), while larger molecules like proteins and blood cells stay in the bloodstream.
From there, the process becomes remarkably selective. A blood vessel runs alongside the tubule, and as filtered fluid flows through it, the body reclaims almost all of the water, along with minerals and nutrients it still needs. What’s left behind, the stuff your body can’t use, continues down the tubule and eventually becomes urine. The tubule also actively pumps extra acid out of the blood, fine-tuning the chemistry of what gets kept and what gets discarded.
Removing Metabolic Waste
Your body constantly generates waste as it processes food, repairs tissue, and breaks down old cells. The kidneys are responsible for clearing these byproducts before they accumulate to dangerous levels. The two most important ones are urea, which forms when the body breaks down protein, and creatinine, a byproduct of normal muscle activity. Both are dissolved in the blood, filtered out by the nephrons, and flushed away in urine.
When the kidneys can’t keep up, these waste products build in the bloodstream. This is what happens in advanced kidney disease, and it produces a recognizable set of symptoms: persistent nausea, loss of appetite, fatigue, mental fogginess, muscle cramps, and itchy skin. In severe cases, fluid can collect in the lungs and cause shortness of breath. These symptoms reflect what happens when the kidneys’ filtering capacity drops significantly below normal.
Regulating Blood Pressure
Your kidneys play a central role in long-term blood pressure control through a hormonal chain reaction. When blood pressure drops, the kidneys release an enzyme called renin into the bloodstream. Renin triggers a cascade: it breaks apart a protein made by the liver, producing a hormone that gets further modified in the lungs and kidneys into its active form. This active hormone narrows the walls of small arteries, which raises blood pressure directly.
It also signals the adrenal glands (which sit on top of the kidneys) to release a hormone that tells the kidneys to hold onto more sodium. And when your blood retains more sodium, it also retains more water. More water means a larger blood volume, which raises pressure further. At the same time, potassium gets released through urine to keep the balance between these two minerals in check. This entire system is why kidney disease so often leads to high blood pressure that’s difficult to manage with lifestyle changes alone.
Balancing Minerals and Electrolytes
Sodium, potassium, and phosphorus all need to stay within narrow ranges for your muscles, nerves, and bones to function. Healthy kidneys maintain those ranges automatically, absorbing what’s needed and excreting what’s excess.
Sodium controls how much fluid your body holds. When the kidneys retain too much sodium, your blood volume increases and tissues swell, often visibly in the feet and ankles. Potassium is equally critical. Too much or too little can cause dangerous heart rhythm problems. The kidneys adjust potassium excretion on the fly based on how much you consume and what your body needs. Phosphorus, meanwhile, is managed in partnership with hormones that regulate bone health. When kidney function declines, phosphorus levels rise, which can pull calcium out of bones and weaken them over time.
Controlling Blood Acidity
Your blood needs to stay within a very tight pH range to support basic cellular function. The kidneys are one of the body’s two main tools for achieving this (the lungs are the other). They work by managing two molecules: hydrogen ions, which make blood more acidic, and bicarbonate, which neutralizes acid.
In the nephron’s tubule, the kidneys pump hydrogen ions into the urine while returning bicarbonate to the bloodstream. For every hydrogen ion secreted, one bicarbonate molecule enters the blood. When your body becomes too acidic, the kidneys ramp up this process, generating new bicarbonate and dumping extra acid into the urine. When the body tips toward being too alkaline, the kidneys dial back hydrogen ion secretion and allow more bicarbonate to leave through the urine instead. This constant adjustment happens without any conscious effort on your part.
Producing Hormones for Red Blood Cells
Your kidneys are also hormone-producing organs. One of their most important hormonal jobs is making erythropoietin, commonly called EPO. This hormone travels through the bloodstream to your bone marrow, where it signals the marrow to produce more red blood cells. Red blood cells carry oxygen from your lungs to every tissue in your body, so the supply needs to be constant.
This is why people with advanced kidney disease often become anemic. Their kidneys can no longer produce enough EPO, so red blood cell production slows and oxygen delivery suffers. The result is persistent fatigue, weakness, and shortness of breath that doesn’t improve with rest.
Activating Vitamin D
Vitamin D from sunlight or food arrives in your body in an inactive form. Your liver processes it partway, but the kidneys perform the final conversion into the active hormone your body actually uses. This active form is essential for absorbing calcium from your intestines and maintaining bone density. Without it, calcium can’t get into your bones efficiently, no matter how much you consume.
When kidney function declines, this activation step falters. The result is a chain of problems: calcium absorption drops, bones weaken, and the parathyroid glands (in your neck) go into overdrive trying to compensate by pulling calcium from bones into the blood. This is one reason why bone disease is a common complication of chronic kidney disease.
How Kidney Function Is Measured
Doctors assess kidney health using a blood test that estimates how efficiently your kidneys filter, reported as an estimated glomerular filtration rate, or eGFR. In adults, a normal eGFR is above 90. This number naturally declines with age: the average for someone in their 20s is about 116, while someone over 70 typically sits around 75, even without kidney disease.
An eGFR between 60 and 89 may indicate early-stage kidney disease. Between 15 and 59 suggests more significant kidney disease. Below 15 points to kidney failure. Because the decline can happen silently for years, kidney disease is often caught only when it’s already advanced. The kidneys have enough reserve capacity that you can lose a substantial amount of function before symptoms appear, which is why routine blood work matters, especially if you have risk factors like high blood pressure or diabetes.