The kidneys are a pair of bean-shaped organs located below the rib cage, one on each side of the spine. The kidneys help remove waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure.

The kidneys are some of the most important organs in the body. The ancient Egyptians left only the brain and kidneys in position before embalming a body, inferring that they held a higher value than other organs.

In this article, we look at the structure and function of the kidneys, the diseases that affect them, and how to keep them healthy.

The positioning of the kidneys is just below the rib cage, with one on each side of the spine. The right kidney is generally slightly lower than the left kidney to make space for the liver.

Each kidney is approximately 3 centimeters (cm) thick, 6 cm wide, and 12 cm long. In males, the average weight of the kidneys is roughly 129 grams (g) for the right one and 137 g for the left. In females, the average weight of these organs is 108 g for the right kidney and 116 g for the left kidney.

Click on the BodyMap above to interact with a 3D model of the kidneys.

The kidneys are two bean-shaped organs that are roughly the size of a fist. A tough, fibrous renal capsule surrounds each kidney and provides support for the soft tissue inside. Beyond that, two layers of fat serve as further protection. The adrenal glands lie on top of the kidneys.

Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Nephrons flow between these sections. Each nephron includes a filter, called the glomerulus, and a tubule. The glomerulus filters blood, which enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs, but they receive 20–25% of the heart’s output.

The tubule returns necessary substances to the blood and removes waste that then becomes urine. The kidneys excrete urine through the ureter, a tube that leads to the bladder.

What does a kidney look like?

The main role of the kidneys is maintaining homeostasis. They manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.

These organs carry out a wide range of bodily functions.

Waste excretion

The kidneys remove various waste products and get rid of them in the urine. Some major compounds that the kidneys remove are:

  • urea, which results from the breakdown of proteins
  • uric acid from the breakdown of nucleic acids
  • drugs and their metabolites

Reabsorption of nutrients

The kidneys reabsorb nutrients from the blood using tubules and transport them to where they will best support health. They also reabsorb other products to help maintain homeostasis. Reabsorbed products include:

  • glucose
  • amino acids
  • bicarbonate
  • water
  • phosphate
  • chloride, sodium, magnesium, and potassium ions

Maintaining pH

In humans, the range of acceptable pH levels is 7.35–7.45. At levels below or above this range, the body enters a state of acidemia or alkalemia, respectively. In these states, proteins and enzymes break down and can no longer function. In extreme cases, this can be fatal.

The kidneys and lungs help keep the body’s pH stable. The lungs achieve this by moderating the concentration of carbon dioxide in the blood. The kidneys manage the pH by reabsorbing and producing bicarbonate from urine, which helps neutralize acids.

The kidneys can retain bicarbonate if the pH is tolerable and release it if acid levels rise. They can produce new bicarbonate by excreting acid.

Osmolality regulation

Osmolality is a measure of the body’s electrolyte-water balance, which is the ratio between fluids and minerals in the body. Dehydration is a primary cause of electrolyte imbalance.

If osmolality rises in the blood plasma, the hypothalamus in the brain responds by passing a message to the pituitary gland. This gland releases antidiuretic hormone (ADH). In response to ADH, the kidney makes several changes, including:

  • increasing urine concentration
  • increasing water reabsorption
  • reopening portions of the collecting duct that water cannot normally enter, allowing water back into the body
  • retaining urea in the medulla of the kidney rather than excreting it, as this compound draws in water

Regulating blood pressure

The kidneys regulate blood pressure when necessary, but they are responsible for slower adjustments.

They adjust long-term pressure in the arteries by causing changes in the fluid outside of cells. The medical term for this fluid is extracellular fluid. These fluid changes occur after the release of a vasoconstrictor called angiotensin II. Vasoconstrictors are hormones that cause blood vessels to narrow.

These hormones play a role in increasing the kidneys’ absorption of sodium chloride, or salt. This absorption effectively increases the size of the extracellular fluid compartment and raises blood pressure. Anything that alters blood pressure, including excessive alcohol consumption, smoking, and obesity, can damage the kidneys over time.

Secretion of active compounds

The kidneys release several important compounds, including:

  • Erythropoietin: This controls erythropoiesis, which is the production of red blood cells. The liver also produces erythropoietin, but the kidneys are its main producers in adults.
  • Renin: This enzyme helps manage the expansion of arteries and the volumes of blood plasma, lymph, and interstitial fluid. Lymph is a fluid that contains white blood cells, which support immune activity, and interstitial fluid is the main component of extracellular fluid.
  • Calcitriol: This is the hormonally active metabolite of vitamin D. It increases both the amount of calcium that the intestines can absorb and the reabsorption of phosphate in the kidney.

A range of diseases can affect the kidneys. Environmental or medical factors may lead to kidney disease, and they can cause functional and structural problems from birth in some people.

Diabetic nephropathy

In people with diabetic nephropathy, damage occurs to the capillaries of the kidney as a result of long-term diabetes. The symptoms may not become apparent until years after the damage starts to develop. They can include:

  • fluid buildup
  • sleep difficulty
  • poor appetite
  • upset stomach
  • weakness
  • difficulty concentrating

Kidney stones

Stones can form as a solid buildup of minerals in the kidneys.

They can cause intense pain and might affect kidney function if they block the ureter.

Kidney infections

Kidney infections tend to result from bacteria in the bladder that transfer to the kidneys.

The symptoms can include lower back pain, painful urination, and, sometimes, fever. Changes in the urine may include the presence of blood, cloudiness, and an unusual odor.

Kidney infections are more common in females than in males and more likely to affect those who are pregnant. The infection often responds well to antibiotics.

Renal failure

In people with renal failure, the kidneys become unable to filter out waste products from the blood effectively.

If an injury or another factor, such as the overuse of medication, causes kidney failure, the condition may be reversible with treatment.

If the cause is a disease, however, kidney failure often does not have a full cure.

Kidney hydronephrosis

Hydronephrosis means “water on the kidney.”

It usually occurs when an obstruction prevents urine from leaving the kidney, causing intense pain.

In time, untreated hydronephrosis can put pressure on a person’s kidneys and may result in kidney damage.

Interstitial nephritis

A reaction to medications or infection can cause inflammation of the nephrons.

The treatment usually involves addressing the cause of inflammation or changing a course of medication.

Kidney tumor

These tumors can be benign or malignant. Benign cancers do not spread or attack tissue, but malignant cancers can be aggressive.

The most common malignant kidney cancer is renal cell carcinoma.

Nephrotic syndrome

When damage to the kidney affects its function, this causes protein levels in the urine to increase. This effect leads to a protein shortage throughout the body, which draws water into the tissues. The symptoms of nephrotic syndrome can include:

  • puffy eyes
  • increased cholesterol levels
  • weight gain
  • fatigue
  • loss of appetite

Lower back pain and changes in urination, especially on one side, may be signs of kidney problems.

Some common causes of kidney damage may include:

  • Analgesics: Using pain medication over a long period might result in chronic analgesic nephritis. Examples include aspirin, acetaminophen, and nonsteroidal anti-inflammatory drugs (NSAIDs).
  • IgA nephropathy: Also known as Berger disease, this occurs when immunoglobin A (IgA) antibodies build up in the kidney. IgA forms a vital part of the immune system, but a buildup can be harmful. The disease progresses slowly, sometimes taking as long as 20 years to develop. The symptoms include abdominal pain, a rash, and arthritis. It can result in kidney failure.
  • Lithium: Doctors prescribe lithium to treat schizophrenia and bipolar disorder. However, lithium might cause nephropathy with long-term use. Close medical supervision can help a person avoid the negative effects of lithium.
  • Chemotherapy agents: The most common type of kidney issue in people with cancer is acute kidney injury. This might be due to the intense vomiting and diarrhea that are common side effects of chemotherapy.
  • Alcohol: Alcohol alters the kidneys’ ability to filter the blood. It also dehydrates the body, making it harder for the kidneys to redress internal balances, and increases blood pressure, which can also hinder the kidneys.

In the case of severe kidney damage, dialysis might be an option. Doctors only use this treatment for end stage kidney failure involving the loss of 85–90% of kidney function. Kidney dialysis aims to complete some of the functions of a healthy kidney. These include:

  • removing waste, excess salt, and water
  • maintaining the correct levels of chemicals in the blood, including sodium, bicarbonate, and potassium
  • maintaining blood pressure

The two most common types of kidney dialysis are:


An artificial kidney, or hemodialyzer, removes waste, additional fluids, and chemicals. The treating doctor makes an entry point in the body by connecting an artery and a vein under the skin to create a larger blood vessel.

Blood travels into the hemodialyzer, receives treatment, and then returns to the body. This process usually takes place three or four times a week.

Peritoneal dialysis

The doctor inserts a sterile cleansing solution into the abdominal cavity around the intestine. This is the peritoneum, and a protective membrane surrounds it.

In continuous peritoneal dialysis, the fluid drains through a catheter. The individual discards these fluids four or five times a day. In automated peritoneal dialysis, the fluid also drains through a catheter, and the exchanges usually occur throughout the night while the person sleeps.

The following are suggestions for keeping the kidneys healthy and helping avoid kidney disease:

  • Eating a balanced diet: Many kidney problems result from high blood pressure and diabetes. As a result, maintaining a nutritious, well-balanced diet can prevent several common causes of kidney disease. The National Heart, Lung, and Blood Institute recommends the DASH diet for maintaining healthy blood pressure.
  • Getting enough exercise: Exercising for 30 minutes every day can reduce the risk of high blood pressure and obesity, both of which put pressure on kidney health.
  • Drinking plenty of water: Fluid intake, especially of water, is important. Many factors can affect how much water people should drink, but about 4–6 glasses per day can help improve and maintain kidney health.
  • Taking supplements: Be careful when taking supplements, as not all dietary supplements and vitamins are beneficial. Some can harm the kidneys if a person takes too many.
  • Limiting salt: Limit sodium intake to a maximum of 2,300 milligrams (mg) of sodium, and ideally 1,500 mg, each day.
  • Moderating alcohol: Consuming more than one drink per day for females and two per day for males can harm the kidneys and impair renal function.
  • Avoiding smoking: Tobacco smoke restricts blood vessels. Without an adequate blood supply, the kidneys will not be able to complete their normal work.
  • Taking over-the-counter (OTC) medications: A drug is not harmless simply because a person does not need a prescription to get it. Overusing OTC drugs, such as ibuprofen and naproxen, can damage the kidneys.
  • Screening for health conditions: Anyone with high blood pressure or diabetes should consider discussing regular kidney screening with a doctor to help spot any possible health issues.
  • Managing diabetes and heart disease: Following a doctor’s recommendations for managing these conditions can help protect the kidneys in the long term.
  • Taking steps to get quality sleep and control stress: The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommends getting 7–8 hours of sleep each night and seeking out activities to reduce stress.

Keeping the kidneys in full working order is essential for overall health.