Kidney Function
Two fist-sized filters clean your entire blood supply many times a day. How the kidneys balance water, salt and waste.
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What Are the Kidneys?
The kidneys are two bean-shaped organs, each roughly the size of a fist, located at the back of the abdominal cavity on either side of the spine. They sit just below the ribcage, partly protected by the lower ribs. Despite their modest size, they are extraordinarily active organs that work continuously — filtering, adjusting and fine-tuning the composition of the blood 24 hours a day.
Together, the kidneys filter about 180 litres of blood fluid every day, though only about 1 to 2 litres ends up as urine. The vast majority of what is filtered is carefully reabsorbed and returned to the bloodstream, keeping the body's internal chemistry precisely balanced.
To understand how the kidneys connect to overall fluid balance in the body, see our guide on water in the human body and use the hydration calculator to explore your personal fluid needs.
Structure of the Kidney
Looking at a cross-section of a kidney, you can see three distinct regions. The outer layer is the cortex — dense with nephrons, the kidney's main working units. Beneath it is the medulla, arranged in cone-shaped segments called renal pyramids that channel processed fluid inward. At the centre is the renal pelvis, a funnel-shaped space that collects urine and drains it into the ureter — the tube carrying urine down to the bladder.
Each kidney has a large blood supply delivered by the renal artery, which branches off the aorta. Blood leaves through the renal vein. The rich blood supply reflects the kidney's intense filtration workload — the kidneys receive about 20 to 25% of the heart's output at rest, even though they account for less than 1% of body weight.
Nephrons: The Filtration Units
The nephron is the functional unit of the kidney — the microscopic structure where blood filtration and urine formation actually take place. Each kidney contains roughly one million nephrons, and together they handle the immense daily filtration workload.
Each nephron begins with a cup-shaped structure called the Bowman's capsule, which surrounds a tiny knot of capillaries called the glomerulus. Blood pressure pushes water, salts, glucose, urea and other small molecules out of the glomerular capillaries and into the Bowman's capsule — a process called filtration. Large proteins and blood cells are too big to pass through and remain in the blood.
The filtered fluid (called the filtrate) then passes through a series of tubules where the real fine-tuning happens.
Reabsorption Along the Tubules
As the filtrate travels through the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, the kidney reclaims almost everything the body still needs. Glucose is fully reabsorbed under normal conditions. Water, sodium, potassium, calcium and many other substances are recovered in precisely controlled amounts.
The loop of Henle — a long, hairpin-shaped section dipping deep into the medulla — creates a concentration gradient that is essential for producing concentrated urine when the body needs to conserve water.
By the time the fluid reaches the collecting duct, it contains mostly what the body genuinely needs to get rid of: urea, creatinine, excess ions and water. Hormones fine-tune the final composition at this stage.
| Process | Where it happens | What occurs |
|---|---|---|
| Filtration | Glomerulus / Bowman's capsule | Small molecules forced out of blood into nephron |
| Reabsorption | Proximal tubule, loop of Henle, distal tubule | Useful substances reclaimed back into blood |
| Secretion | Distal tubule, collecting duct | Additional waste actively moved from blood into tubule |
Waste Removal
The kidney's primary job in most people's minds is removing waste — and it does this superbly. The main waste products in urine are urea (produced when the liver breaks down amino acids), creatinine (a breakdown product of muscle metabolism) and uric acid (from breaking down certain genetic materials).
These substances are filtered at the glomerulus and then not reabsorbed — so they accumulate in the tubule fluid and end up in urine. Some, like creatinine, are also actively secreted into the tubules to ensure complete removal.
Creatinine is produced at a fairly constant rate by muscles, so the amount in blood and urine is a reliable indicator of how well the kidneys are filtering. Doctors use a measure called the glomerular filtration rate (GFR), partly based on creatinine levels, to assess kidney function.
The liver and kidneys work as partners in waste disposal. The liver converts toxic ammonia into urea, which the kidneys then excrete. For more on the liver's role in this partnership, see our liver function guide.
Fluid and Electrolyte Balance
Beyond waste removal, the kidneys are the master regulators of fluid and electrolyte balance. Electrolytes are minerals with an electric charge — sodium, potassium, calcium, phosphate and others — that are essential for nerve signalling, muscle contraction and keeping fluid in the right compartments.
The kidneys adjust how much of each electrolyte is excreted moment to moment. If sodium is too high, more is excreted; if potassium drops, less leaves in urine. These adjustments happen under the influence of hormones, particularly aldosterone, which signals the kidney to retain sodium (and with it, water).
Water balance is regulated partly by antidiuretic hormone (ADH), released from the brain when blood becomes too concentrated. ADH makes the collecting ducts more permeable to water, allowing more to be reabsorbed and producing more concentrated, darker urine. When you are well hydrated, ADH levels fall, the collecting ducts become less permeable, and you produce more dilute urine. Using the water intake calculator can help you think about daily hydration in relation to your body's needs.
Blood Pressure Regulation
The kidneys play a crucial role in controlling blood pressure, primarily by adjusting the volume of fluid in the blood. More fluid means higher pressure; less fluid means lower pressure. The kidneys can increase or decrease urine output to shift fluid volume.
They also regulate blood pressure through the renin-angiotensin-aldosterone system (RAAS). When blood pressure drops, specialised kidney cells release an enzyme called renin. Renin triggers a cascade that produces angiotensin II, which causes blood vessels to constrict (raising pressure) and stimulates the release of aldosterone (causing the kidney to retain sodium and water, raising volume and pressure).
This feedback system is so important that many blood pressure medications work by blocking parts of it — for example, ACE inhibitors block the conversion of angiotensin I to angiotensin II. This is educational context only; medications should always be discussed with a healthcare professional.
To understand how blood pressure connects to the broader circulatory system, visit our blood circulation guide.
Red Blood Cell Production and Vitamin D
The kidneys are not just filtration organs — they also produce important hormones. When blood oxygen levels fall, the kidneys release a hormone called erythropoietin (EPO), which travels to the bone marrow and stimulates the production of new red blood cells. This is why people with chronic kidney disease often develop anaemia — their kidneys produce insufficient EPO.
The kidneys also perform the final activation step for vitamin D. Vitamin D from food or sunlight is converted first in the liver, then in the kidneys into its active form, calcitriol. Calcitriol stimulates the intestine to absorb calcium, helping to maintain strong bones and proper nerve function.
This dual hormonal role makes the kidneys far more than a filtration system — they are active endocrine organs coordinating with the bone marrow, skin, intestines and parathyroid glands.
From Filtrate to Urine
By the time all the reabsorption and secretion is complete, the remaining fluid drains from the collecting ducts into the renal pelvis, then flows down the ureters (muscular tubes) into the bladder. The bladder is an expandable muscular sac that stores urine until it is convenient to void it.
When the bladder holds about 200 to 400 millilitres of urine, stretch receptors in its wall send signals to the brain, creating the urge to urinate. The brain can voluntarily delay this — up to a point — by keeping the external urethral sphincter closed. When urination occurs, the bladder wall contracts and both sphincters relax, allowing urine to flow out through the urethra.
Urine is typically pale yellow due to a pigment called urochrome, derived from the breakdown of haemoglobin. Darker urine often means the body is more concentrated — a common sign of mild dehydration.
| Component | Approximate daily amount |
|---|---|
| Water | 1,000–2,000 mL |
| Urea | 25–30 g |
| Creatinine | 1–2 g |
| Sodium chloride | 5–9 g |
| Potassium | 2–4 g |
| Phosphate | 0.8–1.5 g |
Hormones That Control the Kidneys
The kidneys do not work alone — they respond to a range of hormones that fine-tune their output to match the body's moment-to-moment needs. Understanding these hormonal signals helps explain how kidney function is woven into the body's broader regulatory systems.
Antidiuretic hormone (ADH), also called vasopressin, is released from the pituitary gland at the base of the brain when blood becomes too concentrated (for example, when you are dehydrated). ADH signals the collecting ducts to become more permeable to water, allowing more water to be reabsorbed into the blood rather than lost in urine. The result is smaller volumes of darker, more concentrated urine. When you are well hydrated, ADH levels fall and the opposite occurs — more dilute, larger-volume urine.
Aldosterone, released by the adrenal glands sitting atop the kidneys, tells the distal tubules and collecting ducts to reabsorb more sodium from the filtrate. Because water follows sodium osmotically, this also increases water reabsorption, expanding blood volume and raising blood pressure. Aldosterone is a central player in the renin-angiotensin-aldosterone (RAAS) blood pressure regulation system.
Atrial natriuretic peptide (ANP) works in the opposite direction. Released by the heart when its chambers are overstretched by too much blood volume, ANP signals the kidneys to excrete more sodium and water, reducing volume and pressure. This elegant feedback mechanism shows how the heart and kidneys collaborate to maintain cardiovascular stability.
Parathyroid hormone (PTH), released by the parathyroid glands when blood calcium falls, stimulates the kidneys to reabsorb more calcium from the filtrate and also triggers them to produce more active vitamin D (calcitriol) — which in turn increases calcium absorption from the gut. The kidneys thus act as a key part of the body's calcium regulation network.
Chronic Kidney Disease: Context for Learners
Chronic kidney disease (CKD) is a condition in which kidney function declines gradually over months or years. Because the kidneys have significant reserve capacity and early CKD often causes no obvious symptoms, it can go undetected for a long time. By the time symptoms appear — fatigue, swelling, changes in urine output — a substantial portion of kidney function may already be reduced.
CKD is classified in stages based on the glomerular filtration rate (GFR). In early stages, lifestyle adjustments and medical management can often slow progression significantly. In advanced stages, the kidneys can no longer maintain the body's internal balance without assistance, and treatment such as dialysis or kidney transplantation becomes necessary.
The most common underlying causes are long-term high blood pressure and diabetes — both of which damage the delicate glomerular capillaries over time. This is why managing both conditions is so important for kidney health in the long term. These are educational facts; personal health decisions always require guidance from a qualified medical professional.
In kidney failure, the consequences extend beyond waste accumulation. Because the kidneys also regulate fluid and electrolyte balance, produce EPO and activate vitamin D, their failure affects blood pressure, red blood cell production, bone health and acid-base balance simultaneously. This illustrates just how deeply integrated kidney function is with the rest of the body.
Supporting Kidney Health
The kidneys function best when the body is well hydrated, blood pressure is in a healthy range and blood sugar is well controlled. Consistently high blood pressure and long-term poorly controlled diabetes are among the most common causes of kidney damage worldwide — both educational context, not personal medical advice.
Adequate fluid intake helps the kidneys flush waste effectively and reduces the risk of kidney stones — crystallised accumulations of minerals in the kidney or ureter. Diet plays a role too: very high intake of salt raises blood pressure, which stresses the glomeruli; and very high protein intake increases the kidney's urea load over time.
If you notice symptoms such as persistent lower back pain, changes in urine colour or frequency, or swelling in the legs, it is always wise to speak to a healthcare professional rather than self-diagnosing.
Explore the anatomy glossary at anatomy glossary for definitions of kidney-related terms, and test your knowledge with the anatomy quiz.