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 about 150 quarts of blood every day, removing waste, balancing fluids, regulating blood pressure, activating vitamins, and producing hormones that keep your blood healthy. Only 1 to 2 quarts of that filtered blood actually become urine. The rest, along with the nutrients and minerals your body still needs, gets returned to your bloodstream.
Filtering Waste From Your Blood
The kidneys’ most well-known job is removing metabolic waste, the chemical byproducts your body generates just by being alive. Two of the most important waste products are urea and creatinine. Urea comes from the breakdown of protein, whether that’s protein from food or from your own tissues turning over naturally. Your kidneys excrete roughly 10 grams of urea per day. Creatinine, on the other hand, is a byproduct of normal muscle activity. Both substances are filtered out of the blood and flushed away in urine.
Each kidney contains about a million tiny filtering units called nephrons. Inside each nephron, a cluster of small blood vessels acts as the initial filter, letting water, waste, and small molecules pass through while keeping larger proteins and blood cells in the bloodstream. A series of small tubes then reabsorbs the water, sugars, and minerals your body needs and sends the remaining waste toward your bladder as urine. This is why, out of 150 quarts of fluid filtered daily, your kidneys produce only a tiny fraction as actual urine.
Regulating Blood Pressure
Your kidneys are one of the body’s primary blood pressure controls. When blood pressure drops, the kidneys release an enzyme called renin into the bloodstream. Renin triggers a chain reaction: it converts an inactive protein from the liver into a hormone that narrows the walls of small arteries, which immediately raises blood pressure. That same hormone also signals the adrenal glands to release aldosterone, which tells the kidneys to hold onto more sodium. Since water follows sodium, more fluid stays in the bloodstream, increasing blood volume and pushing pressure back up.
This system works in reverse, too. When blood pressure is adequate, the kidneys dial back renin production, allow more sodium and water to leave through urine, and blood pressure decreases. Many common blood pressure medications work by interrupting this exact pathway, which is why kidney health and blood pressure are so tightly linked.
Balancing Electrolytes and Fluids
Sodium, potassium, and calcium all need to stay within narrow ranges for your nerves, muscles, and heart to function properly. The kidneys adjust the levels of each one continuously.
- Sodium: Aldosterone triggers increased sodium reabsorption in the kidneys. A simple way to remember this: aldosterone saves salt, and water follows salt. When your body needs more fluid volume, it holds onto sodium, and water comes along for the ride.
- Potassium: Aldosterone works in the opposite direction for potassium. When potassium levels rise or sodium levels fall, the adrenal glands release aldosterone, prompting the kidneys to excrete potassium while reabsorbing sodium. This trade-off keeps both minerals in balance.
- Calcium: Parathyroid hormone, released by small glands near the thyroid, tells the kidneys to reabsorb more calcium when blood levels drop. The kidneys also work with the intestines and bones to restore calcium to its normal range.
Activating Vitamin D for Bone Health
The vitamin D you get from sunlight or food isn’t ready to use right away. It has to be converted into its active form, and the kidneys handle the final step of that conversion. This active form of vitamin D can double the rate at which your intestines absorb calcium, boosting absorption from about 10 to 15 percent up to 30 to 40 percent. It also helps the kidneys themselves reabsorb calcium and phosphorus rather than losing them in urine.
This is one reason kidney disease often leads to bone problems. When the kidneys can’t complete that activation step, calcium absorption drops, bones weaken, and the risk of fractures increases over time.
Producing a Key Hormone for Red Blood Cells
Healthy kidneys produce a hormone called erythropoietin, or EPO, which signals the bone marrow to make red blood cells. Red blood cells carry oxygen to every tissue in your body. When kidney function declines, EPO production falls, and the body can’t make enough red blood cells. This is why anemia, a shortage of red blood cells that causes fatigue, weakness, and shortness of breath, is so common in people with chronic kidney disease.
Maintaining Blood pH
Your blood needs to stay within a very tight pH range, slightly alkaline, to support normal cell function. The kidneys help maintain this balance by adjusting how much bicarbonate (a natural buffer) they reabsorb and how much acid they excrete into urine. When blood becomes too acidic, the kidneys hold onto more bicarbonate and push more hydrogen ions out. When it’s too alkaline, they do the opposite. This process is slower than the lungs’ ability to adjust pH through breathing, but it’s more precise and handles longer-term corrections.
What Happens When Kidneys Start to Fail
Kidney disease is often called a “silent” condition because it causes few symptoms in its early stages. You can lose a significant amount of kidney function before noticing anything wrong. Doctors measure kidney function using a blood test called estimated glomerular filtration rate, or eGFR. A score of 90 or above is normal. Between 60 and 89 may indicate early-stage disease. Below 15 is considered kidney failure.
When kidney disease progresses, waste products and fluid build up in the body. Advanced symptoms include nausea, loss of appetite, fatigue, trouble sleeping, decreased mental sharpness, and blood pressure that becomes difficult to control. Fluid retention is common in later stages, sometimes causing swelling in the legs, shortness of breath from fluid in the lungs, or a sudden unexplained increase in body weight.
Because the kidneys handle so many different jobs, losing kidney function doesn’t just mean waste builds up. It also means blood pressure becomes harder to regulate, electrolytes drift out of balance, bones weaken from poor vitamin D activation, and anemia develops from reduced EPO production. This cascade of effects is why chronic kidney disease affects the entire body, not just the urinary system.