Re: I gave only a vile of blood for testing, why did i faint?

The short answer is that your neurovascular system over-responded with an 
event known as a vaso vagal response. That is what I assume based on what 
you tell me. What did your doctor tell you? 

I have the same thing happen. It usually starts with an event such as 
yours. A person fasts, gets volume depleted, the body is in the survival 
mode - heightened neurovascular tone, etc. A needle is stuck into the 
skin, perhaps not the smoothest stick, It hits the vein, the vein 
collapses or constricts (being in survival mode - can't afford to lose any 
thing, especially blood). The phlebotomist keeps trying to get blood, pain 
is experienced somewhere along the line, a combination of above triggers a 
response from the Vagus nerve or it's offshoots and there is momentary 
vaso dilitation and the pulse drops (effects of the Vagus on the heart and 
vessels). This results in a loss of pressure through the system. I imagine 
you were sitting up in a chair. The brain is above the heart in this 
position and detects the drop - this is where you begin to gray out. Somw 
people actually lose consciousness momentarily - the brain goes 
to 'GENERAL QUARTERS' sending out a flurry of messages all over the place -
 resulting in a massive surge of adrenalin to vasoconstrict the vessels, 
increasing resistance again in the system and accelerate the heart. This 
is usually where most people get sick to their stomachs. 

I would have them draw the blood while you are lying down on a table. 
Afterwards, don't just jump up, wait a few, to let the system 
recallibrate. If you are flat, and the first part happens, you will not 
know it as your head is at the same level as your heart. Thus, if 
you 'vagal out' the system is still perfusing the 'squash' (brain). 
Therefore, the horrible pendulum effect of grayout/blackout followed by 
intense nausea and cold clammy sweat is avoided. I always have my guys 
drink plenty of water when they are fasting. Fasting results in a greater 
consumption of intracelluar water due to needs of the body. It feeds off 
of stores held in the liver, and requires extra water for this plus other 
metabolic processes.

Bottom line - lay flat when they draw your blood. You can pretty much 
count on not having that experience again if you are flat. If you are 
standing or sitting it will probably reoccur. Why? This is anecdotal, 
meaning everyone I have talked to has had this reoccur once it starts - 
varying degrees of wooziness - I have not read a study that looked at it - 
there may be one out there. I think that some kind of neuro vascular pain 
mediated cycle gets set up, this includes the apprehension due to the last 
experience. I know that I was fine until I had a flight physical one day 
in Okinawa. Fasting, went on a 3 mile run just prior, had to really work 
to urinate a sample for them which  was very dark (warning sign of low 
body water) and the corpsman stuck me a number of times - fortuantely the 
msater chief was walking by and saw my skin color go deathly pale and got 
me down on the ground before I got there on my own.

Hope this helps. When I give blood, whether for donating or testing, I lay 
flat and avoid the whole unpleasant experience. Don't let anyone tease you 
or give you grief about this. It is not a reflection on your manhood or 
manliness, it is totally involuntary and involves that part of the nervous 
system that we do not consciously control.