The knife slips into your finger when you are preparing dinner. Your child appears with a nose bleed. Or perhaps you just see blood on a TV show.
You start to feel light-headed, nauseous and sweaty. Your face goes white and you drop to the ground.
Fainting at the sight of blood is just one of a number of fainting scenarios that have scientists somewhat mystified, researcher and cardiologist Dr Susan Corcoran said.
Some other fainting triggers, some of which are frankly, bizarre, include:
- needles (having an injection or just seeing a needle)
- coughing or sneezing
- urinating or defaecating (doing a poo)
- trumpet playing
- experiencing pain
- having your hair cut or cut or brushed
But in most cases, we faint from more readily understood triggers, like standing for long periods, hot weather and dehydration (including being hungover), said Dr Corcoran, from Melbourne’s Baker IDI Heart and Diabetes Institute.
Nonetheless the exact mechanism behind almost all fainting episodes is not known.
“For something that’s very common and has been in the literature for centuries, we still have an incredible lack of understanding as to what’s going on,” she said.
So what do we know?
About 40 per cent of us will faint at some point in our lives, Dr Corcoran said.
In about a sixth of cases, an underlying heart condition is to blame. But in everyone else, including those who faint when they see blood, the temporary loss of consciousness is due to a problem controlling blood pressure.
It means the brain does not get enough blood and you collapse to the ground.
This does not necessarily mean that people who faint have a problem with their blood pressure at other times. It is just that in response to certain stressors, their bodies do not respond as well as normal to changes in the distribution of blood when they stand up.
One thing that is known is that those who faint at the sight of blood will often also faint in response to needles and/or certain types of pain.
"Some people who whack their funny bone or get a sudden injury will faint. It's usually an unexpected severe pain, like say, hitting your thumb with a hammer," Dr Corcoran said.
But emotions will only trigger fainting in a person with an underlying susceptibility.
"People do tend to faint more in emotional circumstances so we see people faint more at weddings and funerals, those kind of events," she said.
A blood pooling problem
Normally, our bodies have a highly efficient system to compensate for changes that happen when we stand up.
Dr Corcoran's tips to prevent fainting.
Drink up: If you know ahead of time you'll face a situation where you may faint, drinking half a litre of water 20-30 minutes before. This will help tighten up your blood vessels. "But it does wear off, so drinking it four hours before won't help."
Squeeze your butt: If you're stuck in a situation you can't escape and start to feel faint, cross your legs and tighten the muscles in your legs and buttocks. This will help push blood back from your legs to your head.
Squat: If you don't improve, you could try squatting down, which more forcefully pushes blood out of your legs and stomach and up to your brain.