The urethra has a different course in males and females.
Male Urethra
Structure
The male urethra is significantly longer than the female urethra (about 20 cm). It is usually considered in four parts.
Pre-prostatic urethra
The pre-prostatic urethra is the short part between the bladder and the prostate. The length of this part is variable.
Prostatic urethra
The prostatic urethra is that part within the prostate gland, and is the broadest part. It is remarkable for its posterior aspect, which contains the urethral crest. The prostatic utricle, a blind ending process, passes inferiorly and superiorly from the central part of the crest and is an important landmark. The two ejaculatory ducts open into the crest, just inferior and lateral to the utricle.
Membranous urethra
This short segment extends from the apex of the prostate until the urethra enters the bulb of the penis. The deep transverse perineal muscle and sphincter urethrae form a sphincter at this level.
Spongy urethra
The urethra makes a 90o turn in the penile bulb and then passes anteriorly, in the ventral part of the corpus spongiosum. It terminates in the glans of the penis at the external urethral orifice.
Microscopic Appearance
The urethra is initally lined with transitional epithelium. This gives way to stratified squamous epithelium within the spongy urethra.
Neurovascular Supply
Arterial
The preprostatic, prostatic and membranous urethra are supplied by branches of the inferior vesical artery. The spongy urethra receives the urethral artery from the internal pudendal artery and also some supply from the dorsal penile artery[1].
Venous
Venous return accompanies that of the surrounding structures and is mostly to the prostatic plexus and then to the internal iliac veins.
Lymphatics
The prostatic portion drains to internal iliac nodes although some may pass to external iliac nodes. The spongy part accompanies penile drainage and may pass to internal iliac or inguinal nodes. Malignancies of the distal urethra must therefore have inguinal nodal involvement considered.
Nerves
Autonomic supply is from the prostatic plexus which receives parasympathetic input from S2-S4. Sympathetic supply is involved in contraction of the muscular wall during ejaculation[1].
Female Urethra
Structure
The female urethra is typically 4 cm in length and follows a much more direct course than the male urethra. It begins at the internal urethral orifice of the bladder, which is frequently found at the mid point of the symphysis pubis[1]. It descends to the perineum in the anterior wall of the vagina. The external urethral orifice is found just anterior to the introitus and may be difficult to visualise.
The distal part of the urethra contains periurethral glands known as Skene's glands. These are analogous to the prostate in males.
Microscopic Appearance
The most proximal urethra is lined by transitional epithelium but the remainder is lined by non-keratinising stratified squamous epithelium which is continuous with the skin of the vulva. The connective tissue of the lamina propria is surrounded by smooth muscle and then striated muscle.
Neurovascular Supply
Arterial
The female urethra is supplied by the vaginal artery.
Venous
Blood returns to the vesical plexus around the bladder neck and thereby to the internal iliac vein. The distal urethra may drain to the external pudendal artery.
Lymphatics
The urethra principally drains to the internal iliac nodes, but may also drain to external iliac nodes and to inguinal nodes (especially distal urethral tumours).
Nerves
The urethra receives parasympathetic input from the sacral plexus along with somatic input to the surrounding sphincter. Patients with cauda equina syndrome therefore lose urinary continence.
Relations
Anteriorly, to connective tissue and the symphysis pubis. Distally, the clitoris lies anterior to the urethra.
Posterior, to the vagina.
Laterally, connective tissue of the perineum and levator ani.
Superiorly, to the bladder.
Inferiorly, to the vulva.
Contouring
The female urethra is difficult to visualise. The position can be estimated by contouring the anterior wall of the vagina from the point where urine in the bladder disappears to the vulva.
