Structure

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.

See the sidebar Testosterone Replacement Therapy.