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What causes you to faint


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I've just had to give some blood samples and fainted! Apparently I was out for about 10 secs or so.

I used to faint quite a bit as a kid usually with things about blood/hospitals after an incident of no hands on a bike aged 6.

Is it the lack of blood going to the brain that causes you to spark out?

Still feeling a tad odd now even after 1.5 hours.

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Pretty much, actually it tends to be a lack of oxygen to the brain, but seeing as the blood carries the ovygen the two go hand in hand.

When the brain detects it has a lack of oxygen it'll start to prioritise it's own needs and start shutting down other systems. So it'll shut down blood flow in other areas of the body to keep supply to itself. It does this by vasoconstriction to other area, it'll often increase the pulse rate aswell. It'll often cause hyperventilation to increase the oxygen in the blood.

There are other reasons for fainting aswell, such as low fluid intake, low blood pressure, low sugar levels and too much physical excercise that uses more than the bodys energy reserves.

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I've just had to give some blood samples and fainted! Apparently I was out for about 10 secs or so.

I used to faint quite a bit as a kid usually with things about blood/hospitals after an incident of no hands on a bike aged 6.

Is it the lack of blood going to the brain that causes you to spark out?

Still feeling a tad odd now even after 1.5 hours.

:grin: when giving blood some people can go into a mild state of shock (feeling/seeing the needle and/or seeing/feeling the blood leaving them) which leads to a drop in blood pressure and you can pass out/faint. Its the body's way of levelling you out so the blood can get where it needs to +++

:grin: I've seen a grown man have his ear pierced with one of those piercing guns - about 2 mins later he went white as a sheet, really sweaty and promptly passed out :roflmao:

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What's weird about taking blood is that it also works the other way around. I've had my blood taken a lot unfortunately over the years and sometimes the nurses struggle. I often find it's easier to do it myself - this seems to really freak people out. Somebody drawing their own blood.

I don't have a problem with it, it's often less painful, but it's not unusual to see the look of 'going to vomit now' on somebody even though they're quite happy to take the blood in the first place. Never really understood that :confused:

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  • 2 years later...
I've just had to give some blood samples and fainted! Apparently I was out for about 10 secs or so.I used to faint quite a bit as a kid usually with things about blood/hospitals after an incident of no hands on a bike aged 6.Is it the lack of blood going to the brain that causes you to spark out?Still feeling a tad odd now even after 1.5 hours.

Vasovagal syncope

Thread resurrection....Dentist by day so I see this more than I would like.....and a little alarming when a patient complexion turns grey and their eyes roll back so that you can only see the whites of their eye balls.....and to make matters worse they may even start to shake....like a mini fit...and to make them feel even more embarrassed when they do come round the patient may also discover they`v pee`d themselves.....and if they are middle aged and and taking a cocktail of drugs for high blood pressure and or heart disease you`ll feel like fainting yourself.....Simple vasovagal faints can be quite alarming for anyone who witnesses one.Vasovagal syncope occurs when something triggers a temporary malfunction in your autonomic nervous system.

The autonomic nervous system is the part of the nervous system responsible for regulating many of the body’s automatic functions, such as heartbeat and blood pressure.

Vasovagal syncope may be caused by:

  • sudden exposure to an unpleasant sight or experience, such as the sight of blood/sudden pain
  • standing for long periods of time
  • spending a long time in hot or stuffy environments
  • a sudden intense episode of stress, emotional upset, fear or anxiety

In other words...pychosomatic....the mind plays tricks on the autonomic nervous system

Edited by Mr Man
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Interesting to see the medical explanation of it.

I've been known to go at the sight of my own blood, once when giving blood which was a tad embarrassing!

Oddly enough I've no problem with seeing other people's injuries. Obviously tend more towards sadism than masochism. :P

If your prone to fainting...the trick is not to look...either close your eyes or look the other way....however;... patients who are very anxious are the ones who will not/never/refuse to close their eyes.....For the nurse/practitioner it can be a little scary because in many cases it will take several minutes....and in some cases 30-60 minutes before the patients blood pressure returns to a level when they can sit up without fainting again

And just to show how debilitating a vasovagal faint can be....on one occasion a 60yr guy...no medication...turned up with a tooth ache...he sat in the chair....said he felt faint...I reclined the chair...measured BP..low....30minutes later he was still lying their on Oxygen...he used £100 worth!!!....In the end we had to call an ambulance.....he spent the night in the local hospital before his BP was high enough for him to get up.....and his last words as he was being carried out of the surgery...What about my tooth ache....weirdly he returned a day or two later and was fine:confused:

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I took my 6 year old son to blood doning. He wanted to come but I was a little concerned although I had no reason to be. They seem to accept anything at that age!

He watched it come out and the Nurse let him hold the bag. He wasn't at all phased. All he said was "It's hot!"

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Blood doning for me is fine. I don't mind watching my blood flowing out (in this situation). The only 'painful' bit is when they test for Iron!

My youngest seems to love watching the medical emergency type programs. Where the helicopter crews/ambulance crews/paramedics race around and take people to hospital. She is not phased at all by their injuries! ;)

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