A penis is the male sexual and reproductive organ, consisting of a head, called the glans, and the shaft or body. The body or shaft of the penis is made up of three cylinders of soft, spongy tissue, which contain many small blood vessels. The entire penis contains many nerve endings that make it sensitive to touch, pressure, and temperature.

The glans, however, has a higher concentration of nerve endings than the shaft of the penis and is thus particularly sensitive to physical stimulation.

Two other areas that are highly sensitive are the rim that separates the glans from the shaft of the penis, called the coronal ridge, and the small triangular region on the underside of the penis where a thin strip of skin called the frenulum attaches to the glans.

During sexual excitement, the blood vessels of the spongy tissue fill with blood and swell up. The rapid, forceful dilation of these blood vessels throughout the entire penis causes it to become firm and grow in size. This transition from a soft (flaccid) penis to a harder, stiffer penis is called getting an erection. The penis usually returns to a softer (non-flaccid) state shortly after ejaculation or after the penis is no longer being stimulated.

The erect penis is inserted into a woman's vagina during the sexual act called intercourse or coitus.

When a man reaches the pinnacle of sexual excitement, his orgasm is often accompanied by spurts of semen from the opening at the tip of the glans. This is called ejaculation. The semen that is released from the penis during ejaculation contains sperm, the reproductive cell capable of fertilizing an ovum or egg, the female reproductive cell.

When one of the millions of sperm encounters an egg inside a woman's body, their union can produce a pregnancy. The appearance of the penis varies considerably from one male to another. There can be differences in color, shape, size, and the status of the foreskin (circumcised or uncircumcised).