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Lecture 57

BSC 215 Lecture Notes - Lecture 57: Internal Sphincter Muscle Of Urethra, Urethral Sphincters, Prostatic Urethra

Biological Sciences
Course Code
BSC 215
Jason Pienaar

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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.
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