An erection occurs when your imagination or one or more of the senses (vision, hearing, touch, smell, taste) are stimulated and you become aroused. Your central nervous system sends nerve impulses that increase blood flow to your penis. The blood fills the spongy chambers (corpora cavernosa) in the penis, causing them to expand and become rigid. The veins that would normally drain blood from the penis are constricted, trapping the blood inside and maintaining the erection.

An erection goes down because the arteries narrow, the veins expand, and the blood drains out of the penis. The penis returns to its normal size and is no longer rigid.

See an illustration of the blood supply to the penis.

Four requirements for a normal erection are:

• A properly functioning nervous system that sends the necessary signals to the penis.
• An intact system of blood vessels (vascular system) to allow blood to flow into and out of the penis.
• Normal smooth muscle in the penis, which must relax so the penis can fill with blood and enlarge.
• The ability to trap the blood in the penis so that it stays firm.