Understanding Urinary Sphincters: Function and Role in Continence

Two sphincters control urine flow, working together to maintain continence and allow voluntary voiding.

1. Internal Urethral Sphincter

• Location: Bladder neck (junction of bladder and urethra).
• Type: Smooth muscle (involuntary).
• Function: Remains contracted to prevent leakage; relaxes reflexively during micturition.

2. External Urethral Sphincter

• Location: Surrounds the urethra in the urogenital diaphragm.
• Type: Skeletal muscle (voluntary).
• Function: Provides conscious control; contracts to hold urine, relaxes when voiding is desired.

Coordinated Action

As the bladder fills, the internal sphincter stays closed. When the micturition reflex triggers, the internal sphincter relaxes, but the external sphincter remains contracted until the brain signals it to open. Bladder contraction then expels urine.

Clinical Relevance

Damage or dysfunction of either sphincter can cause urinary incontinence (stress, urge, or mixed). Treatment may involve pelvic floor therapy, medications, or surgical repair.