VIAGRA enables men with ED to respond to sexual stimulation. When a man is sexually excited, the arteries in his penis relax and widen. This allows more blood to flow into the penis. The increased blood flow causes the penis to become hard and erect. The veins that normally carry blood away from the penis then become compressed. This restricts the blood flow out of the penis. With more blood flowing in and less flowing out, the penis enlarges, resulting in an erection.

Hope this answers your question.

Dave