The urethra is the final passage in the urinary tract. It extends from the apex of the trigone (internal meatus) of the bladder and opens into the vestibule of the valve as the external urethral meatus, in female it passes between the levator and muscle and enclosed in the membranous sphincter of the urethra in the outer layer of the anterior vaginal wall. The urethra is tubular in shape and about 4cm long in female but becomes elongated during labor.
The wall contains small blind ducts which open to the urethra just beside the meatus in the vestibule urethral crypts, 2 longest tubes known as the Skene’s ducts. The urethra forms the junctions between the urinary tract and the external genitalia.
The upper half in human is lined with transitional epithelium while the lower half is lined with squamous epithelium. The human is normally close except when passing urine.
· Sub-mucous Coat: Beneath the epithelium is a bed of vascular connective tissue.
· Muscle layer: The muscle layer is arranged in 2 layers: inner longitudinal fiber which continues with the inner muscle fiber of the bladder. Outer layer of circular muscle fiber. The circular fiber is thickened around internal urethral meatus forming a sphincter to open the sphincter during micturation.
· The outer layer: is continuous with the outer vaginal wall of the connectives tissue.
At the lower end of the urethra, voluntary, striated muscle fiber form the membranous sphincter of the urethra. It is not a sphincter but give some voluntary control over the urge to urinate. The lower levator ani muscle also assist in controlling continence of urine.
Blood Supply: Inferior vesical artery and pudendal artery.
Venous Drainage: Corresponding veins.
Lymphatic Drainage- Internal iliac glands
Nerve Supply: Sympathetic and parasympathetic nerves to the internal urethral
Sphincter. External sphincter is under the control of will via the pudendal nerves.
Anterior vaginal wall and the Pelvic floor muscles Function: Convey urine from the bladder to the external.