Reactions to Blood Donation Cuts: Fear, Low Blood Pressure & Fainting

[Mentallic]

I went for my 2nd ever blood donation today. Last time I managed to only give 1/4 of what was expected, apparently my (right) arm was tensed too much and wouldn't allow blood to flow freely. This time we went to try the left arm.
Well, the guy that inserted the needle did a poor job of it and missed the vein. The blood spurted out of my arm. Another more experienced assistant came to try my right arm and successfully inserted the needle. However, she also said that the blood wasn't flowing fast enough so we had to abort the procedure.

I've never been good with cuts (while the sight of blood doesn't bother me). I regularly get that sinking feeling inside my stomache that makes me feel like I'm about to faint. I researched a bit on it and found that this is due to a lowered blood pressure in my body not being able to send enough blood to my brain. But why would I feel like fainting?
Well, apparently I have a subconscious thought of fear and this lowers my blood pressure, causing me to nearly faint.

What a pathetic natural reaction! When you're scared (possibly being preyed in the wild), there should be a pump of adrenaline running through the body to allow for survival, not a faint feeling. What good is it if the predator can simply scare its prey to death? Why do so many people feel faint after giving blood or taking a needle?

 

[hypatia]

Predators rarely look like giant needles. If the sight of your own blood doesn't bother you, more then likely its the needle.

 

[Monique]

What a pathetic natural reaction! When you're scared (possibly being preyed in the wild), there should be a pump of adrenaline running through the body to allow for survival, not a faint feeling. What good is it if the predator can simply scare its prey to death?

Many animals play dead in order to survive a predator attack.

Why do so many people feel faint after giving blood or taking a needle?

It is called a vasovagal response: a reflex of the involuntary nervous system that can lead to loss of consciousness.

This is an interesting publication: http://www.springerlink.com/content/r60g85m5v2p6p21m/"

Vasovagal syncope, although often seen as a disease, is the result of a neurophysiological reflex which can be induced in most healthy people after a 30 % reduction in blood volume either by venous pooling or by hemorrhage. Studies in mammals showed that the activation of certain brainstem nuclei including the caudal midline medulla is responsible for hypotension and bradycardia following central hypovolemia. The hypothesis is presented that vasovagal fainting developed during the evolution in order to support hemostasis. Bleeding animals with a central mechanism for the initiation of hypotension had presumably a better chance for blood clot formation and hemostasis than animals with normal blood pressure. In the context of this hypothesis, vasavagal fainting with blood or injury displaying stimuli can be understood as an early attempt to support hemostasis before the development of larger blood losses.

 

[berkeman]

This is an interesting publication: http://www.springerlink.com/content/r60g85m5v2p6p21m/"

Interesting -- kind of like a human "Reset".

BTW, here's a thread from mid-08 in this forum where we talked more about this reaction:

https://www.physicsforums.com/showthread.php?t=252438

.

 

[Mentallic]

Predators rarely look like giant needles. If the sight of your own blood doesn't bother you, more then likely its the needle.

It has happened to me from certain cuts too.

Many animals play dead in order to survive a predator attack. It is called a vasovagal response: a reflex of the involuntary nervous system that can lead to loss of consciousness.

If I had a choice to either be conscious or unconscious when playing dead to try avoid predators, I would rather stay conscious to at least have the chance of trying to run if the predator decides to play with its food.

Bleeding animals with a central mechanism for the initiation of hypotension had presumably a better chance for blood clot formation and hemostasis than animals with normal blood pressure. In the context of this hypothesis, vasavagal fainting with blood or injury displaying stimuli can be understood as an early attempt to support hemostasis before the development of larger blood losses.

wow this is truly interesting! Perfect sense to explain an imperfect reaction.