"During an erection, the blood enters the penis faster and the veins pinch off (occlude) causing the tunica albuginea to expand. In a way, the penis is like a sink with a faucet and drain. The faucet is always on (not full force) and the drain is open. An erection is in a way like turning up the flow from the faucet and plugging the drain.In simple terms, an erection happens because the brain tells nerves to release a substance in the penis."