An understanding of the anatomy of the male urinary organs, namely the male urethra and penis, is crucial to the diagnosis and treatment of urologic conditions. The kidney anatomy, ureter anatomy, and bladder anatomy are similar in males and females, as discussed in the respective articles. Most sex differences in the urinary tract begin at the bladder neck and continue distally, with the exception of the vas deferens (see the image below), which is shown crossing the distal ureter. This article reviews the basic anatomy of the male urinary organs and common anatomical variants, along with the issues they may present.
This section reviews the gross anatomy of the male urinary tract. Starting with the prostate and moving distally through the lower urinary tract, each male-specific organ is discussed. Links to other relevant articles with more in-depth information are provided.
The urethra is a tubular structure that carries urine distally from the bladder, through the prostate, and out of the penis. It begins just after the bladder neck, where the internal urethral sphincter is located, which is comprised of smooth muscle fibers from the bladder (Detrusor) muscle. The urethra is considerably longer in males than in females, with a length of approximately 17-20 cm and 2.5-4 cm, respectively. The male urethra has 4 sections, which start proximally with the prostatic urethra, the membranous urethra, the bulbar urethra, and the pendulous (or penile) urethra, which are reviewed in the Urethral Anatomy article.
Prostate and Prostatic Urethra
Proximal to the bladder neck, male and female urinary anatomy is grossly similar. However, distal to the bladder neck, where the prostate is located, significant differences in the urinary tract are noted. Briefly, the presence of the prostate, above the pelvic floor and below the bladder, is unique to males and reviewed in the Prostate Anatomy article. At the apex of the prostate is the pelvic floor, which consists of the levator ani muscle complex. This is the border at which the prostatic urethra transitions to the membranous urethra.
Pelvic Floor Muscles and Membranous Urethra
The membranous urethra is the segment of urethra immediately distal to the prostatic urethra. It is contained within the levator ani muscle complex that comprises the pelvic floor. Additionally, the membranous urethra also represents the segment of urethra that is surrounded by the external urethral sphincter, which plays a key role in continence following radical prostatectomy.
This segment of urethra begins immediately distal to the membranous urethra and represents the first urethral segment that is invested by corpus spongiosum. Additionally, the bulbar urethra is surrounded by the bulbocavernosus (or bulbospongiosus) muscles, which contract to compress this segment of urethra and facilitate expulsion during ejaculation. Immediately distal to the bulbar urethra is the pendulous or penile urethra.
Penis and Penile Urethra
The penile urethra runs through the penis, within the corpus spongiosum. The urethra exits distally through the glans penis, within which it becomes the fossa navicularis (or glanular urethra). The opening of the urethra on the tip of the glans penis is the urethral meatus. Further information can be found in the Penile Anatomy article. See the image below.
The internal iliac arteries give rise to the bilateral internal pudendal arteries, which subsequently give rise to the common penile arteries that perfuse the penis and most of the urethra. Accessory arteries may also supply this from the obturator or inferior vesicle arteries, among others. The common penile artery branches into the dorsal, cavernous, and bulbourethral arteries. Circumflex arteries provide communication between the dorsal artery and corpus spongiosum or bulbourethral artery at varying points along the length of the penis. The cavernous artery supplies the corpora cavernosa, the dorsal artery supplies the skin and glans, and the bulbourethral artery supplies the urethra and glans. See the image below.
The corpora cavernosa are made up of smooth muscle septae interweaved within and around vascular cavities. The tunica albuginea, which surround the corpora cavernosa are comprised of 2 layers of tough connective tissue. The deep layer of tunica albuginea is comprised of circularly-oriented fibers, while the superficial layer is comprised of fibers longitudinally oriented along the penis.
The corpus spongiosum also has septae, but is considerably smaller in diameter. Also, the corpus spongiosum has no tunical investment to constrain it and produce rigidity during erection. The urethral mucosa varies along the length of the penis. Proximally, it is lined by urothelium (transitional epithelium), and distally, it is lined by squamous epithelium.
Many variants of the male urinary organs are diagnosed and treated in childhood, likely due to the external nature of the penis and routine pre-natal screening in developed countries. Such congenital anomalies can occur anywhere throughout the urinary tract. This section covers male-specific pathophysiologic variants of the lower urinary tract.
Often detected now on prenatal screening, posterior urethral valves are obstructive “flaps” of tissue in the distal prostatic urethra that impede urine flow. As a result, they can interfere with fetal urination, which alters amniotic fluid levels and also lead to urinary tract obstruction and renal damage due to back pressure on the kidneys.
Another condition that can occur is urethral atresia, which is oftentimes associated with other genetic and congenital conditions. This can result in changes similar to those seen with urethral valves. Urethral atresia can be seen in Prune Belly syndrome, among other conditions.
Urethral strictutres can also occur congenitally; however, they are uncommon. More often, strictures of the urethra result from iatrogenic injury. This may occur as a result of difficult urethral catheter placement or surgical trauma. Otherwise, trauma, and specifically saddle injuries, are common causes of stricture in childhood.
A more common urethral condition is hypospadias. In this condition, the urethra may end anywhere from the ventral aspect of the glans, through the ventral surface of the penis, to the perineum, depending on the severity of the condition. The urethral meatus is located wherever the urethra ends.
Less common than hypospadias is epispadias. In this condition, the urethra may end anywhere from the dorsal aspect of the glans through the dorsal surface of the penis. This may be associated with bladder exstrophy, which also imparts numerous other congenital malformations. Again, the urethral meatus is located where the urethra ends.