Why did you pass out when you were giving blood?

Answer 1

You shouldn't ! As a long-term blood donor (over 35 years) myself, I've never passed out after donating. The amount you donate (500ml) is approximately 10% of your total blood volume - an amount the body can lose without detrimental effect. It's replaced by the body within a few days of donating. Common causes of feinting after giving blood are either (1) they haven't eaten before donating, or (2) haven't rested a short while afterwards. Feinting after donating is a rare event.

Answer 2

It is a self defense mechanism of the body. There is nothing you can do about it. I have seen a Medal of Honor winner pass out when blood was drawn, it sure wasn't because he was scared! The needle going into the blood vessel triggers the effect.

Answer 3

Not everyone passes out when giving blood. However, giving blood reduces the amount of blood in the body. Our body detects this and can cause the vagus nerve to react.
The vagus nerve controls blood to the gut and can over react on occasions, to a lowering of blood pressure, by diverting blood to the gut and therefore reducing it to the brain.
The body's reaction to the blood loss can make you scared and trigger a faint. Individuals react differently.
This kind of syncope (fainting) is an automatic reaction of the body, possibly a self defense mechanism.

Fainting (passing out)