The kidneys are part of the urinary system. There are 2 kidneys in the body, one on either side of the spine under the lower ribs, deep inside the upper part of the abdomen. The adrenal glands are found just above each kidney and are part of the body's endocrine system.
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.
Structure
The kidneys are bean-shaped organs, 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:
- a layer of fibrous tissue called the renal capsule
- a layer of fatty tissue that holds the kidneys in place against the muscle at the back of the abdomen
- a thin, fibrous tissue on the outside of the fat layer called Gerota's fascia
The cortex is the tissue just under the renal capsule. The medulla is the inner part of the kidney. The renal pelvis is a hollow collecting area in the centre of each kidney.
The renal artery brings blood to the kidney, and the renal vein takes blood back to the body after it has passed through 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 2 parts:
- a tubule
- Tubules are tiny tubes that collect the waste materials and chemicals from the blood moving through the kidney.
- a corpuscle
- The corpuscles contain a clump of tiny blood vessels called glomeruli that filter the blood.
Function
The main function of the kidneys is to filter water, impurities and wastes from the blood.
The blood from the body enters the kidneys through the renal arteries. Once in the kidney, the blood passes through the nephrons where waste products and excess 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 is removed from the bladder through the urethra.
The kidneys also act as endocrine glands and produce certain types of hormones:
- Erythropoietin (EPO) stimulates the bone marrow to make red blood cells.
- Calcitriol, a form of vitamin D, helps the colon absorb calcium from the diet.
- Renin helps control blood pressure.