How Erections Occur
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:
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