Micturition

Urine made by the nephrons is carried to the urinary bladder and stored there until the central nervous system (CNS) gives a voluntary signal.

The signal starts when the filling bladder stretches: stretch receptors in its wall send signals to the CNS.

The CNS sends back motor messages that make the smooth muscles of the bladder contract while the urethral sphincter relaxes, so urine is released.

Release of urine is micturition, and the neural mechanism behind it is the micturition reflex.

An adult passes on average 1 to 1.5 litres of urine a day. It is a light yellow, watery fluid, slightly acidic (pH 6.0), with a characteristic odour.

On average 25–30 g of urea leaves the body in urine each day.

Many conditions change the character of urine, so urine analysis helps diagnose metabolic disorders and kidney malfunction. Glucose in urine (glycosuria) and ketone bodies in urine (ketonuria) point to diabetes mellitus.