Urine is 95 per cent water and 5 per cent uric acid, urea, salts (such as sodium, potassium and chloride) and creatinine. The nephron is part of the filtration unit through which the blood passes in the kidneys. Through this filtration system useful substances are reabsorbed into the bloodstream, and waste substances become urine, which is eventually passed from the kidneys by the ureter.

The urinary system is a sophisticated filtration unit which maintains fluid and chemical balance within the body. Many chemical reactions occurring in cells all over the body produce unwanted by-products and the kidneys filter these from the blood and excrete them in the form of urine.

The urinary system is made up of a pair of kidneys, two ureters, a bladder and a urethra. As well as their important excretory function, the kidney’ s also produce hormones that control red blood cell production and help to regulate blood pressure.

The kidneys are the main organs of the urinary system, with many vital functions:

• Removing toxic waste products from the blood and excreting them as urine.

• Returning useful substances to the bod^y‘s circulation.

• Regulating water balance. Your kidneys conserve water at times of relative dehydration and eliminate any surplus.

THE KIDNEY IN DETAIL

Your kidneys arc about 12 cm (5 in) long and 6 cm (2.5 in) wide and are at the back of ^your abdomen, just in front of your spine. The kidne^y has several regions, each with a different function:

• The cortex – This outer layer houses filtering units called nephtons.

• The medulla ^-The next inner layer is full of cone-shaped urine-collecting ducts.

• The renal pelvis -This is at the heart of the kidney and is where urine collects before passing on to the bladder.

FILTERING THE BLOOD

Blood is transported to each kidney through the renal arteries.

These blood vessels stem from the body’s main artery, the aorta, and carry a rich blood supply, that accounts for a quarter of the blood pumped b^y the heart.

Blood passes from these arteries through a sophisticated filtration system. The principal unit of the system is the nephron, which is made up of a glomerulus and a renal tubule.

PASSAGE OF URINE

Once fluid reaches the end of the renal tubule it is urine. Urine from the fleplirofls travels via collecting ducts in the kidney’s medulla to storage areas that feed into the renal pelvis. Each kidney is drained by a ureter, which is a thin muscular tube about 30 cm (12 in) long. Ureters carry urine to the bladder, which stores the urine until it is emptied. The bladder has a highly folded lining that smoothes out as it expands and fills with urine. A healthy adult excretes 0.5-2 liters (1-3 pints) of urine every day.