Reproductive System

The penis provides the route for transmitting sperm to an egg for reproduction. However, in its relaxed state, it can not effectively deliver sperm. In order for the sperm to have the best chance of fertilizing an egg, the penis must become erect and ejaculate semen close to an egg in the female reproductive tract.

The penis is part of the male's external reproductive system which becomes longer, thicker, and stiff during erection. It comprises a shaft region which is the cylindrical body of the penis and the glans, or head region. The glans and the shaft are separated at the coronal ridge which is a rim of tissue that is very sensitive to touch. The skin covering the penis is loose and allows for expansion during erection. Some males have a prepuce or foreskin, which is a movable skin that covers the penile glans. Circumcised males have had this foreskin removed. Uncircumcised males must carefully clean the foreskin daily to prevent bacteria and foul-smelling secretions (called smegma) from accumulating.

Three cylinders of spongy erectile tissue make up the internal portion of the penis. Two cylinders run along the inner roof of the penis and are called the corpora cavernosa. The third cylinder runs along the lower side of the penis; it contains the urethra and is called the corpus spongiosum, or spongy body. The spongy body includes the penile tip and is more sensitive to touch than the rest of the penis. Several nerves and blood vessels run through the spongy body. An erection occurs when blood flow to the spongy tissue vessels increases. An average erect penis is 6.25 in (15.9 cm) long and 1.5 in (3.8 cm) wide at its base.