To understand why the erectile dysfunction occurs, it is useful to know how an erection arises. When a man is sexually excited, his brain sends signals to nerve endings in the tissues of the penis. This causes dilation of blood vessels in the penis and increases blood flow to it, which, in turn, leads to an erection. At the same time, those vessels, through which usually occurs outflow of blood from the penis, are dwindling. This combination of increased blood flow to the penis and reducing it outflow, which leads to a full erection. After the sexual act occur the outflow of blood from the penis, and he returns to the relaxed state.