Inside the penis there are three parallel cylinders of spongy tissue. Two big ones lying side by side on top (1), are called the ‘cavernous bodies.’ And a small one at the bottom (2), which surrounds the urethra (3), is called the ‘spongy body.’
When you jerk-off, blood flows into your penis through the arteries (5). This is absorbed by the spongy tissue in the three cylinders, making it swell. The result is a hard on.
As soon as the penis becomes erect, the blood flux system closes. The veins that drain the penis (4) are compressed by the erection. The penis remains stiff for a while because the blood is trapped inside the penis. If this mechanism doesn’t work well, the blood leaks away and the guy looses his erection.