The process of
micturition (or
voiding) is the process of elimination of
urine from the
bladder.
The
bladder is elastic and stretches from its original volume of ~100 mL to accommodate new
urine from the
kidneys. The
bladder stretches superiorly into the abdominal cavity.
Stretch receptors in the
bladder are activated once it fills to ~200 to 300 mL and send the information that the
bladder is full to the CNS. The
pelvic splanchnic nerves send
motor signals to the
smooth muscles of the
bladder to cause
contraction. The
nerves also cause the
internal urethral sphincter to relax, allowing
urine to pass through and press on the
external urethral sphincter. This increases the
urge to void.
Once the decision has been made to void, the
external urethral sphincter is relaxed under conscious control. The control of the
external urethral sphincter must be learned, since it is controlled with
skeletal muscles.
Voiding can be voluntarily delayed and the
bladder fills even more. However, eventually the
nervous system will override control of the
external urethral sphincter causing unvoluntary
voiding.
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1 - Anatomy and Physiology Basics