What is the physiologic process that causes an erection?
When a male becomes sexually excited, the nervous system sends out messages that cause expansion of the arteries leading to the 3 erectile cylinders (cavernous bodies-2, 1 spongy body) in the penis. As a result, the rate of blood flow into these parallel cylinders increases rapidly. Because blood flowing out of the penis through veins cannot keep up with the inflow, it accumulat3es in the spongelike tissues of the erectile cylinders, causing erection. The penis remains erect until the messages from the nervous system stop and the inflow of blood returns to normal.