Sunday, August 17, 2014

Male Urinary Organ Anatomy • Author: Patrick Joseph Fox Jr, MD; Chief Editor: Thomas R Gest, PhD

Overview


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. While it is true that the longer male urethra confers some protection against urinary tract infections, it can also pose other problems more common to men than women, including strictures and stenosis. This article provides some basic anatomy of the urinary organs specific to the male. There is also a brief discussion of anatomical variations and the complications arising therein. The anatomy of the kidneys, ureters, and bladder are similar for males and females. See image below.
Male  urinary organs, anterior view.

Gross Anatomy

Penis

The penis is the external genital organ of the male. The spongy or penile urethra travels through the penis and opens at the urethral meatus of the glans penis. The urethra is contained within the corpus spongiosum, one of three corpora, or erectile bodies of the penis.[1] The paired corpora cavernosa comprise the other two. Each corpora is contained within a fibrous tissue layer called the tunica albuginea.[2] More superficially, deep penile (Buck) fascia encircles the three corpora, and then with superficial perineal (Colles) fascia, an extension of the membranous layer of superficial fascia (Scarpa fascia) of the abdominal wall.[2]The penis is contained within a layer of epidermis.[2] See the images below.

Corporal bodies of the penis.


Cross-sectional  anatomy of the penis.

Tunica coverage of the penis.

Urethra

The urethra is the tubular structure that carries urine from the bladder to the exterior. It is considerably longer in males than in females, with a length of approximately 17-20 cm and 2.5-4 cm, respectively.[1] The male urethra has 3 sections, including the prostatic urethra, the membranous urethra and the penile, or penile (spongy) urethra.
The prostatic urethra is the most proximal section of urethra exiting the bladder and is so named as it is surrounded by the prostate gland.[3]
Distal to the prostatic urethra, the membranous urethra begins at the lower end of the prostate and extends to the perineal membrane. This section is encompassed by the external urethral sphincter.[2, 3]
Finally, the penile, or spongy, urethra runs through the corpus spongiosum of the penis and is the longest portion of the male urethra. The penile urethra begins at the perineal membrane and continues to the urethral meatus. Just proximal to the meatus, the penile urethra contains the fossa navicularis, a widened portion of the urethra located in the glans.[2] The penile urethra can be further subdivided into the bulbar and pendulous urethra. The bulbar urethra is the more proximal portion of the penile urethra at the widened proximal end (or bulb) of the corpus spongiosum as it makes the curve from the pelvic floor to the join the corpus cavernosa. Once the three cavernous bodies have joined, the more distal portion of the penile urethra is termed the pendulous urethra.
Additionally, the urethra is divided into both anterior and posterior segments.[2] The anterior segment includes the urethral meatus to the bulbar penile urethra. The membranous and prostatic urethra are considered elements of the posterior segment. See the image below.


Divisions of the urethra.

The internal urethral sphincter, located at the junction of the urethra and the bladder, is made up of smooth muscle fibers from the bladder’s detrusor muscle and is involuntarily controlled.[1] The external urethral sphincter is made up of the skeletal muscle comprising the pelvic floor and is under voluntary control.
Branches of the internal pudendal arteries serve the penis and urethra.[2] These branches include the deep penile arteries, a dorsal artery of the penis, and the artery of the bulb.[2] The deep penile arteries supply the corpora cavernosa, while the dorsal artery and artery of the bulb supply the glans, urethra, and corpus spongiosum.[2] See the image below.
Arterial supply to the penis

Venous return is via the deep dorsal vein, which lies beneath the deep penile (Buck) fascia between the dorsal arteries of the penis.[2] The superficial dorsal vein, located outside of the deep penile fascia, drains to the femoral vein via the superficial external pudendal vein, while the deep dorsal vein drains into the prostatic plexus, which in turn drains to the internal pudendal vein.[2] See the image below.
Venous drainage of the penis.

Microscopic Anatomy

The corpora are made up of smooth muscle septae around vascular cavities.[2] The urethral mucosa is made up of both squamous epithelium as well as transitional epithelium. As the urethra transverses the glans penis, it is lined with squamous, and more proximally, by transitional epithelium.[2] The submucosa of the urethra contains connective tissue, elastic tissue as well as smooth muscle.[2]
As mentioned above, the internal urinary sphincter is made up of smooth muscle cells from the detrusor muscle of the bladder. Conversely, the external urethral sphincter is made up of voluntarily controlled skeletal or striated muscles.


Natural Variants

Overall, the length of the male urethra may vary from person to person and depends on numerous factors. The penile urethra has the most variation in length compared with the other segments. Congenitally short penis is termedmicrophallus.


Pathophysiological Variants

Many variants of the male urinary organs are diagnosed and corrected in childhood.
Meatal stenosis is a narrowing of the urethral meatus as it opens on the glans penis. The stenosis is most often thought to be caused by friction and inflammation following circumcision, leading to scarring of the meatus.[2]
Another anomaly, urethral stricture can be either congenital or acquired. Congenital strictures of the urethra, while uncommon, occur most often in the fossa navicularis or membranous urethra.[2] Acquired strictures of the urethra are more common than congenital ones and most often result from pelvic or perineal trauma or infection. Straddle injuries can cause bulbar urethral strictures, whilepelvic fractures and trauma can lead to disruption of the membranous urethra, leading to strictures.[2] The collagenous tissue that forms the stricture, either congenital or acquired, causes outflow obstruction.[2] This obstruction may lead to damage of the kidneys and bladder if not corrected.
Posterior urethral valves are the most common obstructive urethral lesions in children and infants.[2] They are found at the distal prostatic urethra and are formed by mucosal folds resembling membranes and can obstruct urine outflow, causing damage to the kidneys.
Hypospadias is a condition in which the urethral meatus opens on the ventral aspect of the penis. There are varying types of hypospadias, with type 1 being a glandular hypospadias where the orifice opens on the glans, but more proximal than the orthotopic meatus.[2] Type 2 occurs when the meatus opens on the coronal sulcus of the glans. Type 3 involves the shaft of the penis.[2] Type 4 is a penoscrotal opening, and type 5 is a perineal opening.[2] Approximately 70% of hypospadias cases are of type 1 or 2.[2] Epispadias, conversely, is a meatus opening on the dorsal aspect of the penis and is much less common than hypospadias.
These are just a few of the numerous urethral anomalies that can lead to problems with urination, as well as damage to the bladder and kidneys.

Other Considerations

As men age, the prostate gland, located just distal to the bladder, and surrounding a portion of the urethra, can continue to grow, or hypertrophy, leading to a condition known as benign prostatic hypertrophy. As men age, there is a proliferation of epithelial and stromal cells in the periurethral area of the prostate.[4]As the prostate grows, it can compress the urethra as it passes through the prostate, thus causing signs of obstruction, as mentioned above.

References
1.      Scanlon VC. Essentials of Anatomy and Physiology. 6th ed. F.A. Davis Company: 2011.
2.      Tanagho EA. Smith's General Urology. 17th ed. McGraw-Hill: 2008.
3.      Brooks JD. Anatomy of the lower urinary tract and male genitalia. In: Wein AJ, Kavoussi LR, Novick AC, Partin AW, Peters CA. Campbell Walsh-Urology. Vol 1. 9th ed. Philadelphia: WB Saunders Elsevier; 2007:Sect 1, Chap 2, pp 61-63.
4.      Reynard J. Oxford Handbook of Urology. 2nd ed. New York: Oxford University Press: 2009.

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