Urinary System: Normal Anatomy & Physiology

Normal urination (micturition) occurs in the following stages:
    1. Urine is made in the kidneys
    2. Urine is stored in the bladder
    3. The sphincter muscles relax
    4. The bladder muscle (detrusor) contracts
    5. The bladder is emptied through the urethra and urine is removed from the body.

Of course, beyond those 5 steps there are elaborate layers of control. The picture on the right is a "snapshot" of the bladder during the urine storage phase. The text below explains the micturition process, and there is a more indepth animated graphic at the bottom of the page. We invite you to "interact" with it to explore some of the more complex issues that arise when a person has a spinal cord injury.

Summary: Normally, we are able to control where and when we void. This is largely because the cerebrum is able to suppress the sacral micturition reflex. If the sacral reflex is unrestrained, parasympathetic stimulation via the pelvic nerve causes detrusor contraction. Detrusor contraction is suppressed by alpha and beta sympathetic stimulation via the hypogastric nerve. In response to afferent stimulation, the cerebrum becomes aware of the need to void. If it is appropriate, the cerebrum relaxes the external sphincter, blocks sympathetic inhibition, the bladder contracts and urine is expelled.

Instant Feedback:
Sympathetic stimulation inhibits detrusor contraction, while parasympathetic stimulation causes detrusor contraction.

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Instant Feedback:
We become aware of the need to void when the bladder holds about 50 cc.