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 a picture 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
- 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.
Erection Problems (Erectile Dysfunction)