Anatomy of the urethra

The urethra (urethra) is the terminal part of the urinary system. It excretes urine from the urinary bladder.

The male and female urethras differ in structure. Let’s consider them separately.

Male urethra

The male urethra (urethra masculina) is a tubular canal, 16-22 cm long.

It has three parts:

1. The prostatic part (pars prostatica), which passes through the thickness of the prostate.

On the posterior wall of this part, there is an eminence called the urethral crest (crista urethrialis).

The seminal colliculus (colliculus seminalis) is its apex, on which there is a small depression,the so-called prostatic utricle (utriculus prostaticus).

Prostatic utricle (utriculus prostaticus)

Bilaterally to the seminal colliculus, the ductus ejaculatorii (ductus ejaculatorius) and prostatic ducts (ductuli prostatici) open.

2. The membranous part (pars membranacea), which is a small part of the urethra between the apex of the prostate and the bulb of the penis, which passes through the urogenital diaphragm.

This is the narrowest part of the urethra. In this regard, it has an important clinical significance in urinary stone disease, since the stones most often get stuck in the membranous part.

3. The spongy urethra (pars spongiosa), which is the longest part of the urethra.

It passes in the corpus spongiosum of the penis. In the region of the bulb and the glans penis, the urethra expands and forms a navicular fossa of the urethra (fossa navicularis urethrae).

Navicular fossa of the urethra (fossa navicularis urethrae)

The urethra ends at the glans penis and forms a fibrous elastic ring here in the region of the external urethral orifice (ostium urethrae externum).

External urethral orifice (ostium urethrae externum)

External urethral orifice<br />
(ostium urethrae externum) — External urethral orifice
(ostium urethrae externum)

According to mobility, there are two parts in the male urethra: fixed and mobile. The border between the parts is the place of insertion of the fundiform ligament of the penis (lig. fundiforme penis).

The male urethra has two curvatures along its course:

The superior (or infrapubic) curvature (curvatura infrapubica), which faces anteriorly and superiorly with its concavity. It is located in the prostatic and membranous parts.
The anterior (or prepubic) curvature (curvatura prepubica), which faces inferiorly and posteriorly with its concavity at the place where the fixed part of the penis turns into a mobile one.

The male urethra has two orifices:

The internal orifice (ostium urethrae internum), which is located in the neck of the urinary bladder

And the external orifice (ostium urethrae externum), which is located at the apex of the head of the penis.

The urethra slightly narrows in these spots.

Another narrowing is located in the membranous part at the level of the urogenital diaphragm.

Also, the urethra has three regions of expansion: in the prostatic part, in the bulb of the penis, and in the navicular fossa in the region of the glans penis.

The wall of the urethra consists of several layers.

The mucous membrane (tunica mucosa), which is lined with transitional epithelium in the prostatic part. Then it gradually turns into the stratified columnar epithelium, and at the external orifice, it is replaced by the non-keratinized stratified epithelium.

The mucous membrane contains a large number of glands of the urethra or Littre glands (glandulae urethrae), which open into small depressions called lacunae or crypts (lacunae urethrales).

Glands of the urethra / Littre glands (glandulae urethrae)

The muscle coat consists of two layers:

The internal longitudinal layer (stratum longitudinale).

And the external circular layer (stratum circulare).

The circular layer, together with the muscular membrane of the urinary bladder, participates in the formation of the internal urethral sphincter (sphincter urethrae internus).

Internal urethral sphincter (sphincter urethrae internus)

In the area of the membranous part of the urethra, there is the external urethral sphincter (sphincter urethrae externus)

It consists of striated muscle tissue and is a perineal muscle.

There is no muscular membrane in the spongy part.

The adventitia is the outer sheath of the urethra.

Topography

The prostatic and membranous parts are located in the lesser pelvis, the spongy part is located in the penis.

Skeletotopy

The prostatic and membranous parts are located under the pubic symphysis.

Syntopy

The prostatic part consists of the urinary bladder, which is located superiorly, surrounded by the prostate; the membranous part consists of the arched public ligament anteriorly and superiorly, the bulbourethral glands posteriorly, which perforate the urogenital diaphragm; the spongy part is surrounded by the corpus spongiosum of the penis.

In addition to excreting urine, the male urethra also conducts seminal fluid.

Blood supply to the male urethra

Blood supply and innervation of the male urethra are carried out by the following structures.

The male urethra is supplied with blood by the internal pudendal artery (a. pudenda interna).

Venous drainage from the male urethra

Venous blood flows through the pudendal plexus (plexus pudendus) into the internal iliac vein (v. iliaca interna).

Lymph drainage from the male urethra

Lymph drains into the deep inguinal lymph nodes.

Innervation of the male urethra

Innervation is carried out by the pudendal plexus.

Next, we will discuss some of the features that distinguish the female urethra from the male one.

Female urethra

The female urethra (urethra feminina) is much shorter, its length varies from 3 to 6 cm, so it is not divided into parts.

Topography

It is located in the lesser pelvis, and its external orifice opens in the urogenital triangle into the vestibule of the vagina.

Syntopy

It is located in the anterior wall of the vagina, passes inferiorly and posteriorly to the pubic symphysis from the urinary bladder to the external urethral orifice above the vaginal orifice. It penetrates the urogenital diaphragm (diaphragma urogenitale) and is surrounded by sphincter fibers.

In relation to the peritoneum, it is located extraperitoneally (retroperitoneally)

Blood supply and innervation of the female urethra are carried out by the following structures.

Blood supply to the female urethra

The female urethra is supplied with blood by the inferior vesical artery (a. vesicalis inferior) and the internal pudendal artery (a. pudenda interna).

Venous drainage from the female urethra

Venous blood drains through the vesical venous plexus (plexus venosus vesicalis) into the internal iliac vein (v. iliaca interna).

Lymph drainage from the female urethra

Lymph drains into the iliac and inguinal lymph nodes.

Innervation of the female urethra

Innervation is carried by the pudendal plexus and pelvic nerves.

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