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.
