Micturition

Micturition is the act of passing urine out of the body. It is not continuous: urine produced by the nephrons is first carried down to the urinary bladder, where it collects and is held until the body chooses a suitable moment to void it. Because the final step is under voluntary control by the central nervous system, an adult can decide when to release the stored urine.

How the reflex is triggered

As urine keeps accumulating, the wall of the bladder stretches. This stretching is what sets the whole reflex in motion.

• Stretch receptors sitting in the bladder wall detect the rising fullness and send nerve signals up to the central nervous system.
• The central nervous system responds by sending motor messages back to the bladder.
• These messages make the smooth muscle of the bladder contract while, at the same time, the urethral sphincter relaxes.
• With the muscular wall squeezing and the sphincter open, urine is pushed out through the urethra.

The release of urine itself is called micturition, and the co-ordinated set of nerve signals that brings it about is called the micturition reflex.

Amount and nature of urine

On an average day an adult human passes out about one to one and a half litres of urine. This urine is a light yellow, watery fluid. It is slightly acidic, with a pH of about six, and carries a characteristic odour. Of the wastes it removes, roughly twenty-five to thirty grams of urea leaves the body in the urine each day.

Urine as a diagnostic clue …