When men become sexually aroused, hormones, muscles, nerves, and blood vessels all work with one another to create an erection. Nerve signals, sent from the brain to the penis, stimulate muscles to relax. This, in turn, allows blood to flow to the tissue in the penis. Once the blood fills the penis and an erection is achieved, the blood vessels to the penis close off so that the erection is maintained. Following sexual arousal, the veins to the penis again open up, allowing the blood to leave.
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