Function: Urine Formation and Excretion

Urine is composed of unwanted substances that have been filtered from the blood by nephrons, the functional units of the kidneys. The urine formed in the kidneys passes through the ureters and is temporarily stored in the bladder. From here it is emptied, normally under voluntary control, through the urethra. A healthy adult excretes 0.5–2 litres (1–4 pints) of urine each day.

How a nephron makes urine

Blood entering the nephron is filtered through a cluster of capillaries called the glomerulus. The filtrate then enters the renal tubule, along which a complex process of secretion and reabsorption occurs. Useful substances such as glucose are reabsorbed; the acidity of the blood is regulated; and water levels are adjusted. The resulting fluid is called urine.

The path through a nephron

Filtrate from the glomerulus flows through the renal tubule, which has three sections: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

What is urine made of?

Urine consists of a mixture of waste products and other substances. The mixture is balanced so that the body’s internal environment remains constant. The water content of urine depends on whether there is too much or too little water in the body.

Mainly water

Urine is about 95 per cent water. The remainder includes wastes and other substances not needed by the body.

How urination is controlled

When the bladder is full, nerves in the bladder wall send signals to the spinal cord. Signals are then sent back to the bladder, making it contract and expel urine. In older children and adults, the timing of urination can be regulated because this process is controlled by the brain. Infants lack this control, and the bladder empties when full.

Emptying the bladder

To empty the bladder, muscles in the bladder wall contract and the sphincter muscles relax, forcing urine out of the bladder and down the urethra.

Inside the bladder

This highly magnified view shows the folds in the lining of the bladder wall. The folds in the wall stretch out as the bladder fills.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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