An erection is the result of increased blood flow into the penis. During arousal, muscles in the penis relax, allowing for increased blood flow through the penile arteries. This blood fills two chambers inside the penis called the corpora cavernosa, causing the penis to go rigid. In erectile dysfunction, the penis cannot sustain an erection. There are numerous causes of erectile dysfunction, including cardiovascular disease, in which the arteries are too damaged to allow blood to properly flow into the penis. Other causes may include stress, anxiety, diabetes, and drug or alcohol use.

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