Your kidneys produce urine, hormones, and signaling molecules that regulate blood pressure, red blood cell counts, and the balance of water and minerals throughout your body. Most people think of kidneys strictly as filters, but they are also endocrine organs that release critical substances into the bloodstream. Understanding everything your kidneys make helps explain why kidney damage can affect so many seemingly unrelated parts of your health.
Urine: The Primary Output
The most obvious product of your kidneys is urine. A healthy adult produces roughly 800 to 2,000 milliliters per day, depending on fluid intake. Between 91% and 96% of urine is excess water. The remainder is a mix of waste products and dissolved minerals: urea (from protein breakdown), uric acid (from nitrogen metabolism), sodium, potassium, and phosphorus.
Urine is the end result of an elaborate filtration process. Your kidneys filter about 180 liters of blood plasma every day, then reabsorb nearly all of it, sending only the small fraction your body doesn’t need down to the bladder. This selective process keeps electrolyte levels stable, removes toxins your liver has already processed, and prevents fluid from building up in your tissues.
Erythropoietin: The Red Blood Cell Signal
Your kidneys produce erythropoietin, commonly called EPO, a hormone that tells your bone marrow to make red blood cells. EPO binds to receptors on immature red blood cell precursors inside the marrow, triggering them to multiply and mature into functional cells. It also protects those precursor cells from dying off prematurely, keeping production steady.
When your kidneys sense that oxygen levels in the blood are low, they ramp up EPO release. This is why people living at high altitudes naturally have higher red blood cell counts: their kidneys detect thinner air and respond by producing more of the hormone. It’s also why kidney disease so often leads to anemia. Damaged kidneys produce less EPO, which means fewer red blood cells, less oxygen reaching organs, and persistent fatigue. According to the NIDDK, severe anemia from kidney disease increases the risk of heart problems and stroke because the heart has to work harder to compensate for lower oxygen delivery.
Renin: The Blood Pressure Regulator
Renin is an enzyme released by specialized cells in the kidneys whenever blood pressure drops too low or sodium levels fall. It kicks off a chain reaction called the renin-angiotensin-aldosterone system. Here’s how it works in practice: renin converts a protein in your blood into angiotensin, which narrows blood vessels. Angiotensin also signals the adrenal glands (which sit on top of the kidneys) to release aldosterone, a hormone that tells the kidneys to hold onto more sodium and water. The combined effect of tighter blood vessels and greater fluid volume pushes blood pressure back up.
Three specific triggers cause renin release:
- Low blood pressure detected by pressure-sensitive receptors in arteries near the kidneys
- Low sodium levels sensed directly by kidney cells
- Nervous system activity from your sympathetic (“fight or flight”) system signaling the kidneys through nerve receptors
This system is so central to blood pressure control that an entire class of widely prescribed medications works by blocking it. If your kidneys overproduce renin, blood pressure climbs. If they underproduce it, blood pressure can drop dangerously.
Calcitriol: The Active Form of Vitamin D
Your skin makes vitamin D when exposed to sunlight, and your liver processes it into a partially active form. But it’s your kidneys that perform the final conversion into calcitriol, the fully active version of vitamin D your body actually uses. Calcitriol controls how much calcium and phosphorus your intestines absorb from food and how much your bones store or release.
Without enough calcitriol, calcium levels in the blood drop, bones weaken, and the body struggles to maintain the mineral balance that muscles and nerves depend on. This is why people with advanced kidney disease frequently develop a condition called mineral and bone disorder, where bones become fragile and calcium deposits form in blood vessels and soft tissues.
Prostaglandins: Local Blood Flow Protectors
Kidneys also produce prostaglandins, signaling molecules that dilate blood vessels within the kidney itself. These compounds help maintain steady blood flow through the organ even when blood pressure fluctuates elsewhere in the body. Prostaglandins in the E and A families are particularly effective at widening blood vessels and lowering local pressure, and they also influence how much sodium the kidneys reabsorb.
This is one reason nonsteroidal anti-inflammatory drugs (like ibuprofen) can be hard on the kidneys. Those medications block prostaglandin production throughout the body, including inside the kidneys. With fewer prostaglandins available, kidney blood flow can drop, reducing the organ’s ability to filter properly. For most people, occasional use isn’t a problem, but for those with existing kidney issues, the effect can be significant.
What Happens When Production Fails
Because the kidneys produce so many different substances, kidney failure doesn’t just mean waste builds up in the blood. It means the body loses its primary source of EPO, calcitriol, renin regulation, and prostaglandin balance all at once. The downstream effects cascade: anemia develops as red blood cell production falls, bones weaken from poor calcium management, blood pressure becomes harder to control, and fluid retention worsens.
People on dialysis often need synthetic EPO injections, vitamin D supplements, and medications to manage blood pressure and mineral levels, essentially replacing everything their kidneys would normally produce. This is why a functioning kidney transplant can improve so many aspects of health simultaneously. It doesn’t just restore filtration; it restores a chemical factory that the rest of the body depends on.