The structure of the kidneys

The size of your fist
We are blessed with two bean-shaped kidneys, the size of a fist, on either side of the spine. The kidneys are located in the abdominal cavity below the diaphragm. It can be found at the lower end of the rib cage in the middle of your back. The left kidney lies a few centimeters higher than the right.

Recycling blood
The renal artery is the blood vessel that carries blood to the kidney to be processed. The renal vein carries the filtered and chemically balanced blood from the kidney back to the circulation. Excess water and dissolved waste products are released, and form urine. Urine leaves the kidneys via the ureters and is stored in the bladder.

A million filters
Each kidney has about a million filters called nephrons. These functional units are where the fluid and chemical interchanges take place. In each nephron a ball of blood-carrying capillaries, called glomeruli, meets the urine-carrying tubules. Most of the filtrate is reabsorbed and returned to the blood, the rest forms urine. At least 400 ml of urine a day is needed to excrete the amount of metabolic waste produced by the body. The volume of urine formed is dependent on how much fluid we consume daily. When more water is taken in, the extra water is excreted by producing a larger volume of diluted urine. Urine is being produced continuously.

The outer layer of the kidney is called the cortex, which contains the blood-filtering glomeruli and an inner section, the medulla. The medulla is important for the concentration of the urine. Each kidney has a fibrous protective layer that surrounds it, called the renal capsule.

Part of the urinary system
The kidneys are part of the urinary system. It consists of two kidneys, two ureters, the bladder and urethra. Urine passes to the bladder via the ureters. When the bladder is full, it contracts and empties through the urethra.

The urinary system is the most vital part of your body’s cleansing systems, which includes the lungs, skin and intestines. All these organs excrete waste and water to maintain the chemical and fluid balance in your body. Kidney failure, infections, kidney or bladder stones, prostate enlargement and bladder control dysfunction are some of the urinary tract problems that can be experienced.

Written by Dr K. Coetzee, reviewed by Dr R. Moosa, head of the Renal Unit, Tygerberg Academic Hospital.

Read more:
Dehydration
Kids need bathroom breaks

National Kidney Foundation

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