Your kidneys filter about 200 liters of blood every day, removing waste, balancing fluids, regulating blood pressure, and producing hormones that keep the rest of your body functioning. Each kidney is roughly the size of a fist, sitting just below your rib cage on either side of your spine. Despite their small size, they perform at least half a dozen critical jobs simultaneously.
Filtering Blood and Removing Waste
The kidneys’ most fundamental job is cleaning your blood. Every minute, a large volume of blood flows through them, passing into tiny filtering units called nephrons. Each kidney contains about a million of these. Inside each nephron, a cluster of tiny blood vessels acts as a filter, pushing fluid, waste products, and excess substances out of the blood while keeping larger molecules like proteins and blood cells where they belong.
The specific waste products your kidneys remove are byproducts of normal metabolism. Urea comes from the breakdown of protein in your diet. Creatinine is produced when your muscles use energy. Uric acid results from breaking down certain compounds in food. If these substances built up in your blood, they would become toxic. Your kidneys catch them, dissolve them in water, and send them out as urine.
Of the roughly 200 liters of fluid filtered each day, only about 1 to 2 liters actually leave your body as urine. The rest, along with useful nutrients like glucose and amino acids, gets reabsorbed back into your bloodstream as the filtered fluid travels through a long, winding tube attached to each nephron. This reabsorption process is remarkably selective, pulling back what the body needs and letting waste pass through.
Controlling Blood Pressure
Your kidneys play a direct role in setting your blood pressure through a hormonal chain reaction. When blood pressure drops, the kidneys release an enzyme called renin into the bloodstream. Renin triggers a series of chemical conversions that ultimately produce a hormone called angiotensin II, which narrows the walls of small arteries and raises blood pressure.
Angiotensin II also signals the adrenal glands (small glands sitting on top of the kidneys) to release another hormone that tells the kidneys to hold onto sodium. When sodium levels rise in the blood, water follows. More water in the bloodstream means higher blood volume and, in turn, higher blood pressure. This entire feedback loop lets the kidneys fine-tune pressure levels continuously throughout the day. It’s also why kidney disease so often leads to high blood pressure, and why many blood pressure medications work by interrupting this specific chain reaction.
Balancing Water and Hydration
Your kidneys decide how much water your body keeps and how much it lets go. They take instructions from a hormone released by the brain called antidiuretic hormone, or ADH. When you’re dehydrated or have lost blood, your brain detects the change in blood concentration and releases ADH. This hormone travels to the kidneys and tells them to insert water channels into the walls of their collecting tubes, allowing water to flow back into the bloodstream instead of leaving as urine. The result is darker, more concentrated urine and better water conservation.
When you’re well hydrated, ADH levels drop. The water channels get pulled back inside the cells lining the tubes, and the walls become watertight again. More water passes through and out as dilute, pale urine. This system is sensitive enough to respond to changes in blood concentration as small as a fraction of a percent. It’s the reason your urine color shifts throughout the day depending on how much you’ve been drinking.
Managing Electrolytes
Sodium, potassium, calcium, phosphorus, and magnesium all need to stay within tight ranges for your muscles, nerves, and heart to work properly. The kidneys manage this by adjusting how much of each mineral gets reabsorbed or excreted at different points along the nephron’s tube system.
Sodium and potassium have a particularly close relationship in the kidney. When potassium levels are low, the kidneys ramp up sodium reabsorption in a specific segment of the tube. This reduces the amount of sodium that reaches downstream sections where potassium would normally be swapped out and lost in urine, helping the body conserve potassium. The reverse happens when potassium is high. Magnesium handling is closely linked to this same system, which is why low potassium and low magnesium often show up together. A drop in one can directly cause the kidney to lose more of the other.
Keeping Blood pH Stable
Your blood needs to stay in a very narrow pH range, slightly alkaline, for enzymes and cells to function. Normal metabolism constantly produces acids as a byproduct, and the kidneys are responsible for getting rid of that acid and keeping pH in check.
They do this in two ways. First, they reabsorb nearly all the bicarbonate (a natural acid buffer) that gets filtered out of the blood. About 70 to 80 percent of this reabsorption happens in the first stretch of the nephron’s tube, with the rest picked up further along. Second, the kidneys generate brand-new bicarbonate to replace what the body has used up neutralizing acids. Both processes work by pumping hydrogen ions (the acidic component) into the urine. For every hydrogen ion the kidney sends out, a bicarbonate molecule goes back into the blood. This is why urine is typically more acidic than blood.
Producing Key Hormones
Beyond filtering and balancing, the kidneys function as hormone-producing organs. Their most well-known hormonal role is making erythropoietin, often called EPO. This hormone signals the bone marrow to produce red blood cells, which carry oxygen throughout your body. When kidney function declines, EPO production drops, and anemia (a shortage of red blood cells) commonly follows. This is why people with advanced kidney disease often feel fatigued and short of breath.
The kidneys also perform the final, essential step in activating vitamin D. The liver does the first conversion, but the kidney handles the second one, turning vitamin D into its fully active hormonal form. This active form is what allows your intestines to absorb calcium from food and helps regulate calcium and phosphorus levels in your bones. Without functioning kidneys, even people getting plenty of vitamin D from sunlight or supplements can develop weakened bones because the vitamin never gets activated.
How Kidney Function Is Measured
Doctors assess kidney health primarily through a blood test that estimates how well your kidneys are filtering, reported as your estimated glomerular filtration rate, or eGFR. This number represents how many milliliters of blood your kidneys clean per minute. A result of 60 or higher is considered normal. Below 60 suggests kidney disease, and a reading of 15 or lower indicates kidney failure, the point at which dialysis or a transplant may become necessary.
Because the kidneys have so much built-in capacity, you can lose a significant amount of function before noticing any symptoms. Many people with early kidney disease feel perfectly fine. That’s why routine blood work matters, especially if you have risk factors like high blood pressure or diabetes. By the time symptoms like swelling, fatigue, or changes in urination appear, the kidneys may have already lost a substantial portion of their filtering ability.