The urethra drains urine from the urinary bladder to an exterior opening of the body, the external urethral orifice. In females, the urethra is about 3 to 4 cm (1.5 in) long and opens to the outside of the body between the vagina and the clitoris. In males, the urethra is about 15 to 20 cm (6 to 8 in) long and passes through the prostate gland, the urogenital diaphragm, and the penis. In these regions, the urethra is called the prostatic urethra, membraneous urethra, and spongy (penile) urethra, respectively. In both males and females, a skeletal muscle, the external urethral sphincter, surrounds the urethra as it passes through the urogenital diaphragm.
Micturition, or urination, is the process of releasing urine from the urinary bladder into the urethra. When the urinary bladder fills to about 200 mL to 300 mL, stretch receptors in the urinary bladder trigger a reflex arc. The signal stimulates the spinal cord, which responds with a parasympathetic impulse that relaxes the internal urethral sphincter and contracts the detrusor muscle. Urine does not flow, however, until a voluntary nerve impulse relaxes the skeletal muscle of the external urethral sphincter.
The internal urethral sphincter is involuntary. The external urethral sphincter is voluntary. It is the external urethral sphincter that we learned to control when going through the potty training phase of life.