What are kidneys?

The kidneys are two bean-shaped organs in the renal system. They help the body pass waste as urine. They also help filter blood before sending it back to the heart.

The kidneys perform many crucial functions, including:

  • maintaining overall fluid balance
  • regulating and filtering minerals from blood
  • filtering waste materials from food, medications, and toxic substances
  • creating hormones that help produce red blood cells, promote bone health, and regulate blood pressure


Nephrons are the most important part of each kidney. They take in blood, metabolize nutrients, and help pass out waste products from filtered blood. Each kidney has about 1 million nephrons. Each has its own internal set of structures.

Renal corpuscle

After blood enters a nephron, it goes into the renal corpuscle, also called a Malpighian body. The renal corpuscle contains two additional structures:

  • The glomerulus. This is a cluster of capillaries that absorb protein from blood traveling through the renal corpuscle.
  • The Bowman capsule. The remaining fluid, called capsular urine, passes through the Bowman capsule into the renal tubules.

Renal tubules

The renal tubules are a series of tubes that begin after the Bowman capsule and end at collecting ducts.

Each tubule has several parts:

  • Proximal convoluted tubule. This section absorbs water, sodium, and glucose back into the blood.
  • Loop of Henle. This section further absorbs potassium, chloride, and sodium into the blood.
  • Distal convoluted tubule. This section absorbs more sodium into the blood and takes in potassium and acid.

By the time fluid reaches the end of the tubule, it’s diluted and filled with urea. Urea is byproduct of protein metabolism that’s released in urine.

Renal cortex

The renal cortex is the outer part of the kidney. It contains the glomerulus and convoluted tubules.

The renal cortex is surrounded on its outer edges by the renal capsule, a layer of fatty tissue. Together, the renal cortex and capsule house and protect the inner structures of the kidney.

Renal medulla

The renal medulla is the smooth, inner tissue of the kidney. It contains the loop of Henle as well as renal pyramids.

Renal pyramids

Renal pyramids are small structures that contain strings of nephrons and tubules. These tubules transport fluid into the kidney. This fluid then moves away from the nephrons toward the inner structures that collect and transport urine out of the kidney.

Collecting ducts

There’s a collecting duct at the end of each nephron in the renal medulla. This is where filtered fluids exit the nephrons.

Once in the collecting duct, the fluid moves on to its final stops in the renal pelvis.

Renal pelvis

The renal pelvis is a funnel-shaped space in the innermost part of the kidney. It functions as a pathway for fluid on its way to the bladder


The first part of the renal pelvis contains the calyces. These are small cup-shaped spaces that collect fluid before it moves into the bladder. This is also where extra fluid and waste become urine.


The hilum is a small opening located on the inner edge of the kidney, where it curves inward to create its distinct beanlike shape. The renal pelvis passes through it, as well as the:

  • Renal artery. This brings oxygenated blood from the heart to the kidney for filtration.
  • Renal vein. This carries filtered blood from the kidneys back to the heart.


The ureter is a tube of muscle that pushes urine into the bladder, where it collects and exits the body.

Kidney diagram

Use this interactive 3-D diagram to explore the kidney.

Kidney conditions

Symptoms of a kidney problem

Kidney conditions can cause a range of symptoms. Some common ones include:

  • trouble sleeping
  • fatigue
  • inability to concentrate
  • dry, itchy skin
  • increased or decreased urination
  • blood in urine
  • foamy urine
  • puffiness around the eyes
  • foot or ankle swelling
  • reduced appetite
  • muscle cramps

If you notice any of these symptoms, contact your doctor. Depending on your symptoms, they may do some kidney function tests to make a diagnosis.

Tips for healthy kidneys

The kidneys are important organs that affect many other body parts, including the heart. Follow these tips to keep them working efficiently:

Avoid extra salt

Eating a lot of salty foods can disrupt the balance of minerals in the blood. This can make it harder for the kidneys to work properly. Try swapping out processed foods — which usually have a lot of added salt — for whole foods, such as:

  • fresh fruits and vegetables
  • lean cuts of meat
  • nuts


High blood pressure is a known risk factor for chronic kidney disease. Regular exercise, even for just 20 minutes a day, can help reduce blood pressure.

Stay hydrated

Drinking plenty of water helps the kidneys perform one of their most important functions: removing toxins. Learn more about how much water you should really be drinking every day.

Use medications with caution

Regularly taking certain over-the-counter medications, such as nonsteroidal anti-inflammatory drugs, can cause kidney damage over time. Occasionally taking them is fine, but work with your doctor to find alternatives if you have a condition that requires managing pain, such as arthritis.

Know the risk factors

Several things can increase your risk of developing a kidney condition. Make sure you regularly have your kidney function tested if you: