I only faint when my blood vessels are involved. And apparently its fairly common.
Vasodepressor syncope is caused by a reflex of the involuntary nervous system called the vasovagal reaction. The vasovagal reaction leads the heart to slow down (bradycardia) and, at the same time, it leads the nerves to the blood vessels in the legs to permit those vessels to dilate (widen). The result is that the heart puts out less blood, the blood pressure drops, and what blood is circulating tends to go into the legs rather than to the head. The brain is deprived of oxygen and the fainting episode occurs.
So its a nervous reflex from having my veins fiddled with. My little vessels don't like being prodded! The solution is to give blood lying down with your legs elevated. How dignified. Come to think of it, when I have had a blood test lying down its been okay. Still felt a bit woozy. In rare cases you can DIE! Your blood pressure gets so low and your heart stops! Death ensues! But that's pretty rare. apparently.