Physiology of Urination
Filling Phase:
The bladder is able to hold increasing volumes of urine (called accommodation)
without increasing pressure by relaxing the detrusor muscle. This relaxation
is produced by sympathetic stimulation of B-adrenergic receptors and
parasympathetic inhibition. At the same time, sympathetic stimulation of
alpha-adrenergic receptors in the internal urethral sphincter increases sphincter tone. The
external urethral sphincter and levator ani muscles act as backup to the
internal sphincter and are innervated by the pudendal nerve and sacral efferent
nerves. The first urge to urinate usually occurs when the bladder volume reaches
about 150 cc.
Emptying phase:
When the bladder reaches its functional capacity (usually between 300-600cc),
voluntary voiding is usually initiated. The sphincter and levator ani muscles
relax (with release of the somatic inhibition and of the a-adrenergic
sympathetic stimulation). The detrusor contracts with activation of parasympathetic
cholinergic receptors.