Male Reproductive System: Structure

From The Merck Manual of Diagnosis and Therapy, Edition 18, pp. 1947-1950, and pp. 2099-2104, edited by Mark H. Beers. Copyright 2006 by Merck & Co., Inc., Whitehouse Station, NJ. Available at: http://www.merck.com/mmpe/index.html.

 

The penis consists of the root (which is attached to the abdominal wall), the body (the middle portion), and the glans penis (the cone-shaped end). The opening of the urethra (the channel that transports semen and urine) is located at the tip of the glans penis. The base of the glans penis is called the corona. In uncircumcised males, the foreskin (prepuce) extends from the corona to cover the glans penis.

The body of the penis primarily consists of three cylindrical spaces (sinuses) of erectile tissue. The two larger ones, the corpora cavernosa, occur side by side. The third sinus, the corpus spongiosum, surrounds the urethra. When these spaces fill with blood, the penis becomes large and rigid (erect).

The scrotum is the thin-skinned sac that surrounds and protects the testes. The scrotum also acts as a climate-control system for the testes, because they need to be slightly cooler than body temperature for normal sperm development. The cremaster muscles in the scrotal wall relax or contract to allow the testes to hang farther from the body to cool or to be pulled closer to the body for warmth or protection.

The testes are oval bodies the size of large olives that lie in the scrotum; usually the left testis hangs slightly lower than the right one. The testes have two functions: producing sperm and testosterone (the primary male sex hormone). The epididymis is a coiled tube almost 20 feet long. It collects sperm from the testis and provides the space and environment for sperm to mature. One epididymis lies against each testis.

The vas deferens is a firm duct that transports sperm from the epididymis. One such duct travels from each epididymis to the back of the prostate and enters the urethra. Other structures, such as blood vessels and nerves, also travel along with each vas deferens and together form an intertwined structure, the spermatic cord.

The urethra serves a dual function in males. This channel is the part of the urinary tract that transports urine from the bladder and the part of the reproductive system through which semen is ejaculated.

The prostate gland lies just under the bladder and surrounds the urethra. Walnut-sized in young men, the prostate gland enlarges with age. When the prostate enlarges too much, it can block urine flow through the urethra. The seminal vesicles, located above the prostate, join with the vas deferens to form the ejaculatory ducts. The prostate and the seminal vesicles produce fluid that nourishes the sperm. This fluid provides most of the volume of semen, the secretion in which the sperm is expelled during ejaculation. Other fluid that makes up the semen comes from the vas deferens and from mucous glands in the head of the penis.

Testosterone Replacement Therapy

Beginning at about age 30, the production of testosterone (the main male sex hormone) in men usually decreases an average of 1 to 2% per year. This decline differs from the usually rapid and nearly universal hormonal changes of menopause in women, but the decline in testosterone is sometimes referred to as male menopause or andropause. The rate of testosterone decline also varies greatly among men; many men in their 70s have testosterone levels that match those of the average man in his 30s.

All men with low testosterone levels develop certain characteristics associated with aging, including decreased libido, decreased muscle mass, increased abdominal fat, thin bones that easily fracture, decreased energy level, slow mathematical and spatial thinking, and a low blood count. Many men are interested in taking testosterone to slow or reverse development of these characteristics, but this is only helpful for men with abnormally low levels of testosterone.

The most worrisome side effect of testosterone replacement therapy is worsening of prostate disease. Without knowing it, many men have small prostate cancers that would likely never produce symptoms. Testosterone can make prostate cancers grow, so testosterone replacement therapy could cause an unnoticed prostate cancer to produce symptoms or become lethal. Testosterone also worsens benign prostatic hyperplasia, a noncancerous enlargement of the prostate.

Testosterone replacement therapy is recommended only for men whose blood tests show low testosterone levels and who have no prostate disease. Men taking testosterone need to be checked frequently for prostate cancer. Such testing may detect cancers early, when they are more often curable.