Your kidneys are two bean-shaped organs that filter your blood, remove waste, and help regulate nearly every fluid balance in your body. Each one is roughly the size of a closed fist, measuring 10 to 12 centimeters long, and they sit deep in your back on either side of your spine. Despite their small size, they filter about 200 liters of blood every day, returning most of it to your bloodstream while sending waste products out as urine.
Where Your Kidneys Sit
The kidneys are tucked behind your other abdominal organs, positioned between the lowest rib and the upper lumbar spine (roughly the T12 to L3 vertebrae). The right kidney sits slightly lower than the left because the liver takes up space above it. In men, each kidney weighs about 160 grams; in women, about 135 grams. They’re each about 5 to 7 centimeters wide and 3 to 5 centimeters thick.
Because of their deep position, you typically can’t feel your kidneys. When people experience kidney pain, it usually shows up as a dull ache in the lower back or flank area, which can be easy to confuse with a muscle strain.
How Kidneys Filter Blood
Each kidney contains roughly one million tiny filtering units called nephrons. A nephron works in two stages: first, a small cluster of blood vessels called the glomerulus acts like a sieve, pushing fluid out of the blood and into a series of tubes. This fluid contains both waste products and useful substances your body still needs. In the second stage, those tubes selectively reabsorb the good stuff (water, glucose, minerals) back into the bloodstream while letting the waste continue on to become urine.
One specialized part of this tubing, a hairpin loop that dips deep into the kidney’s inner tissue, is responsible for concentrating your urine. The walls of this loop are built with water channel proteins that pull water back into the body while blocking salts from following. This is how your kidneys can produce either very dilute or very concentrated urine depending on how hydrated you are.
Of the roughly 200 liters of fluid filtered each day, only about 1 to 2 liters leave your body as urine. The rest is recycled back into your blood.
Waste Removal
The kidneys are your body’s primary route for getting rid of nitrogen-containing waste. When your liver breaks down protein, it produces urea, and your body generates about 10 grams of it each day. Creatinine, a byproduct of normal muscle activity, is also cleared almost entirely by the kidneys. Uric acid, which comes from breaking down certain foods and cellular material, follows the same path. If the kidneys can’t keep up with waste removal, these substances build up in the blood and eventually become toxic.
Blood Pressure Control
Your kidneys play a direct role in setting your blood pressure through a hormone chain reaction. Specialized cells in the kidney’s blood vessels detect changes in blood flow and salt delivery. When blood pressure drops or sodium levels fall, these cells release an enzyme called renin into the bloodstream. Renin triggers a cascade that ultimately produces a powerful blood vessel constrictor, which raises blood pressure. This same cascade signals the adrenal glands to release a hormone called aldosterone, which tells the kidneys to hold onto more sodium and water, further increasing blood volume and pressure.
This system works in both directions. When blood pressure is adequate, renin production slows down, and the kidneys allow more sodium and water to leave in the urine. It’s a continuous feedback loop, adjusting throughout the day based on your hydration, posture, and activity level.
Hormones the Kidneys Produce
Beyond filtering, your kidneys function as hormone-producing organs. The most notable hormone they make is erythropoietin, or EPO. When specialized kidney cells detect that blood oxygen levels are low, they ramp up EPO production. EPO travels to your bone marrow and signals it to produce more red blood cells, which carry oxygen throughout the body. This is why severe kidney disease often leads to anemia: without enough EPO, the bone marrow doesn’t get the message to make red blood cells.
The kidneys also perform the final activation step for vitamin D. The form of vitamin D you get from sunlight or food is inactive. Your liver does an initial conversion, and then your kidneys complete the process, turning it into its fully active form (calcitriol). Active vitamin D is essential for absorbing calcium from food and maintaining strong bones. When kidney function declines, this activation stalls, which is one reason people with chronic kidney disease are at higher risk for bone problems.
Electrolyte Balance
Your body needs sodium, potassium, and calcium in tightly controlled amounts, and your kidneys are the organs that maintain those levels. Potassium is a good example of how precise this regulation is. Your kidneys adjust potassium excretion in real time based on how much you consume. When potassium levels in the blood rise even slightly, cells in the kidney’s filtering tubes shift their activity to push more potassium into the urine. When levels drop, the tubes switch to a salt-retaining mode that reduces potassium loss.
Sodium follows a similar pattern. Your kidneys can reabsorb nearly all filtered sodium when you’re dehydrated, or dump large amounts when intake is high. This sodium regulation is tightly linked to water balance, since water follows sodium. It’s the reason a very salty meal can leave you feeling bloated: your kidneys temporarily retain extra water to dilute the sodium.
How Kidney Health Is Measured
The standard way to assess kidney function is through a blood test that estimates how efficiently your kidneys are filtering, expressed as your estimated glomerular filtration rate, or eGFR. For a healthy young adult, a normal eGFR is around 116. It naturally declines with age: by your 50s, an average reading is about 93, and by age 70 and beyond, around 75 is typical. An eGFR below 90 with other signs of kidney damage is considered stage 1 kidney disease, though at that level kidneys are still working reasonably well.
Doctors also look at creatinine levels in your blood. Since creatinine is cleared almost entirely by the kidneys, a rising creatinine level signals that the kidneys are struggling to keep up.
Chronic Kidney Disease by the Numbers
Kidney disease is far more common than most people realize. Globally, about 788 million adults were living with chronic kidney disease as of 2023, more than double the 378 million estimated in 1990. The global prevalence in adults stands at roughly 14%, meaning about one in seven adults has some degree of kidney damage. Many of them don’t know it, because the kidneys can lose a significant amount of function before symptoms appear. The most common drivers are high blood pressure and diabetes, both of which damage the tiny blood vessels that nephrons depend on to filter blood.