The male urethra is a pelvic urinary organ that functions primarily as a connecting cord which transports urine from the urinary bladder to the exterior. On the superior end of the bladder is a pair of 25-30 cm long muscular tubes, the ureters, which also connect and fills the bladder with urine formed in the kidneys.
The male urethra, on the other hand, is an 18-22 cm long muscular tube that conveys urine from the urinary bladder. Hence it runs from the internal urethral orifice of the bladder to the external urethral orifice located at the tip of the glans penis. The urethra also functions to provide an exit for semen (sperm) and glandular secretions) during ejaculation. Thus, in males, the urethra is a part of the urinary system as well as the reproductive system. While the urethra runs the length of the penis in males, in females, it is very short and it is not part of the reproductive system.
When the penis is in a flaccid (non-erect) state, the urethra presents a double curvature and is divisible into four parts, which are the:
- Preprostatic (intramural) part
- Prostatic urethra
- Membranous (intermediate) urethra
- Spongy (Penile) urethra
Early in the 4th week of embryonic development, the urinary system develops. It consists of the urinary system,kidneys, ureters, urinary bladder and the urethra. The urethra is mainly derived from a solid cord of ectodermal cells that grows inward from the tip of the glans penis and joins the rest of the spongy urethra.
The epithelium of the terminal part (part close to the glans) of the urethra develops from the surface ectoderm, while epithelium lining the remaining parts of the urethra is derived from endoderm of the urogenital sinus. The connective tissue and smooth muscle of the urethra develops from the splanchnic mesenchyme.
The wall of the urethra is made up of mucous, submucous and muscular layers. The prostatic urethra is bounded by prostatic tissue, while the penile urethra (spongy urethra) by erectile tissue of the corpus spongiosum. Its mucous membrane consist of a lining pseudostratified columnar epithelium, mainly transitional epithelium on the short part adjoining the urinary bladder and stratified squamous epithelium on the part near the external urethral orifice.
The submucosa consists of loose connective tissue, while the muscular coat is made up of an inner longitudinal layer and an outer circular layer of smooth muscle. In addition, the membranous part is surrounded by striated muscle that forms the external urethral sphincter.
The initial part, which passes through the bladder musculature (“bladder neck”), just below the internal urethral orifice, is referred to as the pre-prostatic urethra or intramural part of the urethra. Ideally it is about 0.5-1.5 cm in length but varies depending on whether the bladder is filled with urine or empty.
The prostatic urethra is the part of the urethra that passes through the prostate. It is about 3-4 cm long and extends from the base of the bladder, just below the pre-prostatic urethra, to the membranous part of the urethra.
The prostatic urethra is surrounded by the internal urethral sphincter near its middle part; it is widest in the middle. The posterior part of the urethra has an elevation called the urethral crest; the prostatic sinuses open up on either side of this crest, which bring prostatic fluid into the urethral lumen. The ejaculatory ducts also open up in the prostatic urethra, which carry the sperms from the testes and the fluid from the seminal vesicles into the urethra. Thus the urinary and reproductive tracts merge at this point.
This is a common site of obstruction to the outflow of urine in patients with benign hypertrophy of the prostate.
The membranous or intermediate part of the urethra is the second shortest part of the urethra that connects the prostatic urethra to the penile urethra. It is 1-1.5 cm long and it is surrounded by the external urethral sphincter. The external urethral sphincter plays an important role in voluntary control of urinary flow.
Spongy (Penile) Urethra
The spongy urethra is the last and largest part of the urethra. It is approximately 15 cm long and is divisible into two main parts - the pendulous urethra and the bulbar urethra. The pendulous urethra runs along the length of the penis, while the bulbar urethra is located in the bulb of the penis. The spongy urethra opens to the outside through the urethral meatus (external urethral orifice), which is a vertical slit-like opening slightly behind the tip of the penis. The bulbourethral glands and the glands of Littre open into the spongy urethra.
Vasculature & Lymphatics
The arterial supply of the male urethra includes the prostatic branches of the inferior vesical and middle rectal arteries.
Veins of the urethra, such as the dorsal veins of the penis and the pudendal veins, drain into the prostatic venous plexus.
The lymphatic vessels of the urethra drain mainly into the internal iliac lymph nodes. Some others drain into the external iliac lymph nodes.
The urethra is innervated by the prostatic plexus. The prostatic plexus includes nerves with a mixture of fibres derived from the sympathetic and parasympathetic trunks, as well as from visceral afferent fibres. Such nerves are referred to as “mixed nerves” because they contain afferent and efferent fibres.