During stimulation, the muscles around the penis relax to let the blood in and be absorbed by a spongy structure on the top side of the penis called the corpus cavernosum. After the blood rushes in, the veins in the penis clamp down like a dam so the blood stays absorbed in the corpus cavernosum. All the while, a soft structure on the underside of the penis called the corpus spongiosum remains relaxed to allow semen to be released at ejaculation.
Without proper blood flow-meaning that arteries are inflamed or clogged-men can't get an erection because they can't get blood in or keep it there. And that's one of the most common causes of erectile dysfunction-a condition that affects some 30 million American men.