Human Anatomy - Kotsan I. Y. 2009

Urogenital System
Urinary Organs
Urethra

The Urethra is a part of the Urinary Tract designed for the periodic excretion of urine. Its Structure differs between men and women.

The male urethra (urethra masculina) is an unpaired tubular organ with a diameter of 0.5–0.7 cm and a length of 16–22 cm. It begins at the Urinary Bladder with the internal urethral orifice (ostium urethrae internum) and terminates at the tip of the glans Penis with the external urethral orifice (ostium urethrae externum). The male urethra serves to transport both urine and seminal fluid (semen).

Topographically, the male urethra is divided into three parts: prostatic, membranous, and spongy; functionally, in terms of mobility, it is divided into fixed and mobile portions. The boundary between the latter is the attachment site of the suspensory ligament of the penis.

The prostatic part (pars prostatica) begins at the internal urethral orifice, pierces the Prostate Gland, and runs through its thickness from top to bottom and from back to front. This is the widest section of the urethra, measuring about 3 cm in length. On the posterior wall of the prostatic urethra, there is an elongated elevation called the urethral crest (crista urethralis). The most prominent part of this crest is termed the seminal colliculus (colliculus seminalis). At the apex of the seminal colliculus, There is a blind recess—the prostatic utricle (utriculus prostaticus), which is a remnant of the embryonic paramesonephric ducts that develop into the Uterus and Vagina in females. The openings of the ejaculatory ducts lie on either side of the prostatic utricle. The elongated depression flanking the urethral crest is known as the prostatic sinus (sinus prostaticus). The openings of the prostatic ducts (ductuli prostatici) empty into the prostatic sinus, with some opening directly onto the seminal colliculus.

The membranous part (pars membranacea) of the male urethra extends from the apex of the prostate gland to the bulb of the penis. This is the narrowest and shortest segment of the urethra (1.5 cm in length). Where the membranous urethra passes through the urogenital Diaphragm, it is surrounded by concentric bundles of striated Muscle fibers that merge from the Muscles of the urogenital diaphragm to form the voluntary urethral sphincter (musculus sphincter urethrae).

The spongy part (pars spongiosa) is the longest section of the urethra, measuring approximately 15 cm in length, and runs through the corpus spongiosum of the penis. It begins with a dilation located in the bulb of the penis and also ends with a dilation situated within the glans penis. The dilation located in the glans penis is oval-shaped and is called the navicular fossa of the urethra (fossa navicularis urethrae).

Along its course, the male urethra features two bends: the upper (or posterior) bend, concavity directed forward and upward (within the prostatic and membranous parts), and the anterior bend, concavity directed downward and backward (at the transition from the fixed part of the penis to the free part). Furthermore, the male urethra has three narrowings and three dilations along its path. The first narrowing is located in the region of the internal urethral orifice, the second at the passage through the urogenital diaphragm, and the third near the external urethral orifice. Dilations of the male urethral lumen occur in the prostatic part, the bulb of the penis, and its terminal segment—the navicular fossa.

The wall of the urethra consists of an adventitial coat, a muscular coat, a submucosal layer, and a mucous membrane.

The outer adventitial coat (tunica adventitia) of the urethra is poorly developed. The muscular coat (tunica muscularis) consists of two layers of Cytology/cytology/32.html">Smooth Muscle tissue: an outer longitudinal layer and an inner circular layer. The well-developed submucosa (tela submucosa) contains a network of venous vessels. The inner mucous membrane (tunica mucosa) of the urethra is lined with epithelium: transitional epithelium at the beginning of the prostatic part, which transitions closer to the membranous part into columnar epithelium, and finally into stratified squamous epithelium at a distance of 5–6 mm from the external urethral orifice (navicular fossa). Throughout its length, the urethral mucosa forms numerous longitudinal folds that account for its distensibility. Along the upper wall of the spongy urethra, transverse folds run in two rows, bounding small (0.5 mm) anteriorly open recesses known as urethral lacunae (lacunae urethrales). Urethral glands empty into these lacunae. The urethral glands (glandulae urethrales), or Littré glands, are tubulo-alveolar glands. They are located in the lamina propria of the mucous membrane and produce mucus. Near the external urethral orifice, on both sides, opens a paraurethral duct (canal) (ductus (canalis) paraurethralis), each of which serves as a common duct for small glands situated in this area. The length of a paraurethral duct is 1–2 cm. On the upper wall of the navicular fossa, there is a relatively large transverse fold—the valve of the navicular fossa (valvula fossae navicularis)—which proximally bounds this fossa.

Blood supply to the male urethra. Different segments of the urethra are supplied by various sources: the prostatic part receives blood from Branches of the middle rectal and inferior vesical Arteries; the membranous part from the inferior rectal and perineal arteries; and the spongy part from the urethral branches of the internal pudendal artery, as well as the deep and dorsal Arteries of the penis. Venous drainage is directed into the Veins OF THE penis and the urinary bladder. Lymphatic drainage from the prostatic part of the urethra flows into the Lymphatic vessels of the prostate gland, while from the membranous and spongy parts it drains into the deep inguinal nodes.

The male urethra is innervated by branches of the prostatic plexus, perineal nerves, and the dorsal nerve of the penis.

The female urethra (urethra feminina) is an unpaired organ intended solely for The excretion of urine. It begins at the urinary bladder with the internal urethral orifice (ostium urethrae internum) and terminates at the external urethral orifice (ostium urethrae externum), which is located in the vaginal vestibule 2–3 cm inferior to the Clitoris. The female urethra is much shorter but wider than the male urethra. Its length ranges from 3 to 4 cm, and its width from 8 to 12 mm. The urethral lumen varies along its length: it is funnel-shaped and dilated near the bladder, and narrowed near the external urethral orifice. The external orifice of the female urethra is rounded and surrounded by firm, ridge-like margins. The female urethra runs parallel to the vagina and is fused with its anterior wall. It is slightly curved posteriorly as it descends, passing inferior and posterior to the inferior margin of the Pubic Symphysis.

The wall of the female urethra consists of an adventitial coat, a muscular coat, a submucosal layer, and a mucous membrane. The outer adventitial coat (tunica adventitia), much like in the male urethra, is poorly developed. The muscular coat (tunica muscularis) is composed of an inner longitudinal and an outer circular layer of smooth muscle tissue. The circular muscle layer of the proximal urethra is a continuation of the detrusor muscle or the internal urethral sphincter. In the region of the urogenital diaphragm, the muscles of the latter blend with the circular muscle layer of the urethra to form the voluntary urethral sphincter (musculus sphincter urethrae). The inner mucous membrane (tunica mucosa) of the urethra is lined with epithelium: transitional epithelium near the internal orifice, which transitions into stratified squamous epithelium, and in places, tall columnar epithelium. Like the male urethra, the mucous membrane of the female urethra features longitudinal folds on its surface. The largest and highest longitudinal fold on the posterior wall of the urethra is called the urethral crest (crista urethralis), which extends from the anterior angle of the trigone of the urinary bladder to the end of the urethra. Within the thickness of the mucous membrane lie urethral glands (glandulae urethrales) that open into urethral lacunae (lacunae urethrales) and paraurethral ducts (canals) (ductus (canales) paraurethrales).

Blood supply to the female urethra is provided by the inferior vesical and internal pudendal arteries. Venous drainage occurs via the vesical venous plexus into the internal iliac vein. Lymphatic drainage from the upper segments of the urethra goes to the internal iliac Lymph Nodes, and from the lower segments to the superficial inguinal lymph nodes.

The female urethra is innervated by branches of the inferior hypogastric plexus, pelvic splanchnic nerves, and the pudendal nerve.