Micturition

Micturition, or urination, is the process that finally releases the urine that the nephrons have spent hours producing. Urine drains continuously from the nephrons into the urinary bladder, where it is held until the bladder is deliberately emptied. As the bladder fills, stretch receptors in its wall are increasingly stimulated; this stretching signal travels to the central nervous system along sensory neurons of the parasympathetic nervous system and triggers contraction of the bladder's smooth muscle (the detrusor muscle), while somatic motor neurons simultaneously keep the external sphincter closed. As filling continues, the balance between these excitatory and inhibitory signals shifts until the stimulatory drive exceeds the inhibitory threshold, at which point the internal sphincter opens passively and the external sphincter relaxes, allowing urine to be expelled. A healthy adult passes about 1-1.5 litres of urine a day; it is normally a yellow, watery fluid, mildly acidic (around pH 6.0, though diet can shift it anywhere from 4.5 to 8.0), with a characteristic odour and a yellow colour from the pigment urochrome. About 25-30 g of urea is excreted daily, and because urine compo …