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Anatomy and physiology of the kidneys

The kidneys are part of the urinary system. There are 2 kidneys deep inside the upper part of the abdomen, one on either side of the spine under the lower ribs. The left kidney is slightly higher than the right kidney.

There is an adrenal gland just above each kidney. These glands are part of the body’s endocrine system, which is the group of glands and cells in the body that make and release hormones into the blood. These hormones control many functions such as growth, reproduction, sleep, hunger and metabolism.

The ureters are thin tubes about 25–30 cm (10–12 inches) long that connect the kidneys to the bladder. The urethra is a small tube that connects the bladder to the outside of the body.

Diagram of the location of the kidneys


The kidneys are bean-shaped organs, about the size of one’s fist. An adult kidney is about 12 cm (4–5 inches) long, 6 cm (2–3 inches) wide and 3 cm (1–2 inches) thick.

Each kidney is surrounded by the renal capsule, which is a layer of fibrous tissue. A layer of fatty tissue holds the kidneys in place against the muscle at the back of the abdomen. Outside the layer of fat is Gerota’s fascia, or renal fascia. It is a thin, fibrous tissue.

The inside of the kidney is made up of an outer part called the cortex and an inner part called the medulla. The renal pelvis is a hollow, funnel-shaped area in the centre of each kidney where urine collects.

Diagram of cross-section of the kidney

The renal artery brings blood to the kidney, and the renal vein takes blood away from the kidney. The area where the renal artery, renal vein and ureter enter the kidney is called the renal hilum.

Inside each kidney is a network of millions of small tubes called nephrons. Each nephron has corpuscles and tubules. The corpuscles contain tiny blood vessels called glomeruli that filter the blood. A glomerulus is surrounded by a layer of cells called Bowman’s capsule. Tubules are tiny tubes that collect the waste materials and chemicals from the blood as it moves through the kidney.


The main function of the kidneys is to filter extra water, impurities and wastes from the blood.

Blood from the body enters the kidneys through the renal arteries. The blood passes through the nephrons, where waste products and extra water are removed. The clean blood is returned to the body through the renal veins.

The waste products filtered from the blood are then concentrated into urine. The urine is collected in the renal pelvis. The ureters move the urine to the bladder, where it is stored. Urine travels from the bladder and out of the body through the urethra.

The kidneys also make certain hormones, which are substances that control certain body functions. Hormones made by the kidneys are:

  • Erythropoietin (EPO) stimulates the bone marrow to make red blood cells.
  • Calcitriol, a form of vitamin D, helps the intestines absorb calcium from the diet.

Renin helps control blood pressure.


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