After ejaculation, the blood vessels that enter the penis and arteries are contracted by the same nerves that trigger ejaculation. By decreasing the inflow of blood, the pressure is decreased. When the pressure drops below a critical point, the walls of the erectile bodies, corpora cavernosa, separate. This separation opens up the exiting vessels and veins and allows blood to exit the penis rapidly, and the penis becomes soft again.

When using our sink analogy, ejaculation turns off the faucet and the water level begins to drop. Once the sink is only partially full, the drain snaps open, and the water drains out of the sink at a rapid rate.

I hope this gives you a little better understanding of the complex nature of erectile function. By having better knowledge of normal erectile function we will be able to better understand the causes of erectile dysfunction.