Describe how a penis becomes erect during sexual intercourse.

For males, intercourse requires an erection. The penis consists mostly of long cylinders of spongy tissue. When a male is not sexually aroused, his penis is limp, because the arteries that transport blood to its spongy tissue remain constricted. When he becomes aroused, increased activity of sympathetic nerves causes smooth muscle in the walls of these arteries to relax so that the arteries dilate. The resulting inflow of blood expands the spongy tissue and compresses veins that carry blood out of the penis. This pressure enlarges and stiffens the penis so it can be inserted into a female's vagina.