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• Biology of the Male Reproductive System
4

• The penis and the urethra are part of the urinary and reproductive systems.
• The scrotum, testes (testicles), vas deferens, seminal vesicles, and prostate gland comprise the rest of the reproductive system.

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 consists primarily 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 and ends as the glans penis. When these spaces fill with blood, the penis becomes large and rigid (erect).

The scrotum is the thick-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 wall of the scrotum 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 that average about 1.5 to 3 inches (4 to 7 centimeters) in length and 2 to 3 teaspoons (20 to 25 milliliters) in volume. Usually the left testis hangs slightly lower than the right one. The testes have two primary functions: producing sperm (which carry the man's genes) and producing testosterone (the primary male sex hormone). The epididymis is a coiled tube almost 20 feet (6 meters) 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 joins with the seminal vesicle. The seminal vesicles store sperm until it is released during ejaculation. In the scrotum, 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 lies just under the bladder and surrounds the urethra. Walnut-sized in young men, the prostate 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 a very small amount of the semen comes from the vas deferens and from mucous glands.

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 is sometimes referred to as male menopause or andropause. However, the hormone decline in men differs greatly from what women experience in menopause, during which female hormones almost always decline rapidly over just a few years. The rate of testosterone decline varies greatly among men. Some men in their 70s have testosterone levels that match those of the average man in his 30s. Whether young or old, men with low testosterone levels may develop certain characteristics associated with aging, including decreased libido, decreased muscle mass, increased abdominal fat, thin bones that easily fracture (osteoporosis), decreased energy level, slowed mathematical and spatial thinking, and a low blood count (anemia). It is not clear whether low testosterone levels increase the risk of coronary artery disease. Many men with normal testosterone levels are interested in taking testosterone to slow or reverse development of these characteristics, but currently testosterone replacement therapy is recommended only for men with low levels of testosterone. The most worrisome side effect of testosterone replacement therapy is worsening of prostate disorders. Without knowing it, many men have small prostate cancers that would likely never cause symptoms or be lethal. The body's own testosterone can make prostate cancers grow, so testosterone replacement therapy, at least theoretically, could cause an unnoticed prostate cancer to cause symptoms or become lethal. Testosterone replacement also can worsen benign prostatic hyperplasia, a noncancerous enlargement of the prostate. Thus, testosterone replacement therapy is recommended only for men whose blood tests show low testosterone levels and who have no prostate disorders. Men taking testosterone need to be checked frequently for prostate cancer. Such testing may detect cancers early, when they are more likely to be curable.

Male Reproductive Organs

Last full review/revision August 2007 by Paul D. Lui, MD