Your kidneys filter about one liter of blood every minute, roughly 20% of your heart’s total output. These two fist-sized organs sit behind your abdominal cavity, one on each side of your spine, and they do far more than make urine. They regulate blood pressure, activate vitamins, balance your blood chemistry, and signal your bone marrow to produce red blood cells.
Size, Shape, and Location
Each kidney is a bean-shaped organ measuring 10 to 12 centimeters long, 5 to 7 centimeters wide, and 3 to 5 centimeters thick. In men, a single kidney weighs about 160 grams; in women, about 135 grams. They sit just below your rib cage in your lower back, tucked between the T12 and L3 vertebrae, behind the lining of your abdominal cavity in a region called the retroperitoneum. That deep, protected position is why kidney problems often show up as lower back pain rather than belly pain.
How Blood Gets Filtered
Each kidney contains roughly one million tiny filtering units called nephrons, and each nephron works in two basic steps. First, blood enters a cluster of miniature blood vessels called a glomerulus. The walls of these vessels are thin enough to let water, salts, sugars, and waste molecules pass through, while keeping larger things like proteins and blood cells in the bloodstream. The fluid that passes through is called filtrate, and it’s essentially a rough draft of urine that still contains a lot of material your body needs back.
That filtrate then flows into a long, winding tube called a tubule. A blood vessel runs right alongside the tubule, and as the filtrate moves through, the blood vessel reabsorbs almost all the water plus minerals, glucose, and other nutrients your body wants to keep. The tubule also pulls excess acid out of the blood. Whatever is left at the end of the tubule, mostly water, urea (a byproduct of protein breakdown), creatinine (a byproduct of muscle activity), uric acid, and other waste, becomes urine.
Your kidneys filter the body’s entire blood supply dozens of times a day. Doctors measure how well this filtering works using a number called eGFR, or estimated glomerular filtration rate. A normal eGFR is 90 or higher, though it naturally declines with age. The average for someone in their 20s is about 116, dropping to around 85 by age 60 to 69 and about 75 after age 70. A consistently low eGFR can signal kidney disease.
Blood Pressure Control
Your kidneys don’t just respond to blood pressure. They actively regulate it. When blood flow to the kidneys drops, specialized cells near each nephron detect the change and release an enzyme called renin into the bloodstream. Renin kicks off a chain reaction that produces a powerful hormone called angiotensin II, which raises blood pressure in several ways at once: it tightens blood vessel walls, tells the adrenal glands to release aldosterone (a hormone that makes the kidneys hold onto more sodium and water), and stimulates thirst so you take in more fluid.
The net effect is that your blood volume increases and your vessels constrict, pushing pressure back up. Once pressure normalizes, the kidneys dial renin production back down. This feedback loop runs constantly, adjusting to posture changes, hydration levels, and exercise. It’s also why kidney disease so often leads to high blood pressure, and why many blood pressure medications work by interrupting this kidney-driven system.
Balancing Sodium and Potassium
Sodium and potassium are the two electrolytes your kidneys manage most carefully, because even small shifts in their balance affect nerve signaling, muscle contraction, and heart rhythm. The fine-tuning happens in the later sections of each nephron’s tubule. There, aldosterone (the same hormone involved in blood pressure control) increases the number of sodium channels on the tubule wall, pulling more sodium back into the blood. As sodium moves one direction, potassium moves the other, getting secreted into the fluid that will become urine.
This trade-off is why eating a very high-sodium diet forces your kidneys to work harder. More sodium reabsorption means more water retention, which raises blood volume and pressure. It also shifts how potassium is handled. Your kidneys constantly recalibrate the ratio based on what you eat, how hydrated you are, and hormone signals from the adrenal glands.
Keeping Your Blood pH Stable
Your blood needs to stay within a very narrow pH range, roughly 7.35 to 7.45, to keep your cells functioning. Your lungs handle fast adjustments by breathing off carbon dioxide, but your kidneys handle the slower, more precise work. They do this by reabsorbing bicarbonate (a natural buffer) back into the blood and secreting hydrogen ions (acid) into the urine.
If your blood starts to become too acidic, the kidneys ramp up bicarbonate reabsorption and dump more hydrogen ions into the urine, nudging pH back up. If blood becomes too alkaline, they do the opposite: let more bicarbonate pass into the urine and hold onto hydrogen ions. This system is slower than the lungs, taking hours to days to fully compensate, but it’s more powerful over the long term. It’s also why severe kidney disease can cause a condition called metabolic acidosis, where the blood becomes dangerously acidic because the kidneys can no longer excrete enough acid or reclaim enough bicarbonate.
Activating Vitamin D
Vitamin D from sunlight or food isn’t usable by your body right away. It first passes through your liver, which converts it into an intermediate form. That intermediate then travels to the kidneys, where an enzyme called 1-alpha hydroxylase converts it into calcitriol, the fully active form of vitamin D. Calcitriol is what allows your intestines to absorb calcium from food and helps maintain strong bones.
This is why people with advanced kidney disease often develop weak bones and low calcium levels, even if they get plenty of sunlight. Their kidneys can no longer complete that final activation step, so they may need a prescription form of active vitamin D.
Triggering Red Blood Cell Production
Your kidneys contain specialized cells that continuously monitor oxygen levels in the blood flowing through them. When oxygen drops, whether from blood loss, anemia, or high altitude, these cells ramp up production of a hormone called erythropoietin, or EPO. EPO travels through the bloodstream to the bone marrow, where it signals stem cells to produce more red blood cells. Once oxygen levels recover, EPO production tapers off.
This is a direct genetic response. Low oxygen activates a protein called hypoxia-inducible factor, which binds to the EPO gene and increases its activity. The whole system explains why chronic kidney disease frequently causes anemia. Damaged kidneys produce less EPO, bone marrow gets a weaker signal, and red blood cell counts gradually fall. Synthetic versions of EPO are used to treat this type of anemia in people with kidney failure.
What Comes Out as Urine
After all that filtering, reabsorbing, and secreting, the kidneys produce about one to two liters of urine per day, though the exact amount depends on how much you drink, sweat, and eat. That urine contains urea (from protein metabolism), creatinine (from muscle metabolism), uric acid (from the breakdown of DNA-related compounds), excess electrolytes, hydrogen ions, and whatever water the body doesn’t need.
Urine color is a rough indicator of hydration. Pale yellow means your kidneys have plenty of water to work with. Dark amber means they’re conserving water by concentrating the urine. Your kidneys make this adjustment by responding to antidiuretic hormone from the brain, which tells the tubules to pull more water back into the blood when you’re dehydrated. The entire process, from blood entering the glomerulus to urine draining into the bladder, is continuous and automatic, running 24 hours a day without any conscious input from you.