Have you ever donated blood? I just did today, and nearly passed out near the end-- everything went black for about 30 seconds.
It was my first time, for the blood drive at school. I wasn't the only one to have complications though, a couple people literally passed out and had to go home, they missed the vein in one of my friend's arms, and my cousin clotted up when he was donating.
Either way, I'm satisfied that I'm helping someone when the time comes.