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Understanding Erectile Dysfunction Stages of an Erection Knowing how an erection occurs can help you understand how ED happens. How does the penis change from soft to swollen to erect? This change requires a healthy mind-body "team effort" led by the brain. Upon receiving signals from the brain, the blood vessels, nerves, and hormones work together to cause and maintain an erection. The Soft (Flaccid) Penis If nothing is causing a man to become aroused, then his body's erection "team" of brain, nerves, blood vessels, and hormones won't begin working to cause an erection. The amount of blood flowing into his penis equals the amount flowing out. His penis remains soft. Penile nerves carry no messages of sexual stimulation or arousal. Blood flows through narrow blood vessels in the penis' spongy chambers (corpora cavernosa and corpus spongiosum). The same amount of blood that enters the penis through its arteries leaves it through its veins. The Swollen (Tumescent) Penis A man becomes aroused through his senses (such as sight or touch) or his thoughts (such as memories or fantasies). His brain, blood vessels, nerves, and hormones begin working together to cause an erection. More blood flows into his penis than flows out. His penis starts to swell. During arousal, messages brought by nerves cause the blood vessels and spongy chambers to dilate (open up). This creates room for more blood. Arteries carry blood into the spongy chambers. Beins drain some of the blood from the penis. The Erect (Rigid) Penis As arousal continues, blood keeps moving into the man's penis. Blood-swollen tissues press against the veins. Some of the blood is kept from flowing back out. Engorged (filled) with blood, his penis becomes rigid. The man is able to have intercourse. As stimulation continues, nerves keep carrying messages of arousal between the penis and brain. Blood still flows through arteries into the spongy chambers. But the swollen corpora block the veins. This traps blood within the penis. It becomes and stays rigid.

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