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.

First aid for fainting Lying down with elevated feet usually helps recovery. It can also be a good idea to loosen any tight clothing.

Lying down with elevated feet usually helps recovery. It can also be a good idea to loosen any tight clothing. Usually, a fainting episode will only last a few seconds, although it will make the person feel unwell and recovery may take several minutes.

Usually, a fainting episode will only last a few seconds, although it will make the person feel unwell and recovery may take several minutes. If a person does not recover quickly after lying down, or there are injuries from the fall, always seek urgent medical attention.

If a person does not recover quickly after lying down, or there are injuries from the fall, always seek urgent medical attention. Fainting due to serious causes is less common, but Dr Corcoran said everyone who has a faint should see their doctor to discuss the episode and see if further investigation is required.

Some other fainting triggers, some of which are frankly, bizarre, include:

needles (having an injection or just seeing a needle)

needles (having an injection or just seeing a needle) coughing or sneezing

coughing or sneezing urinating or defaecating (doing a poo)

urinating or defaecating (doing a poo) swallowing

swallowing trumpet playing

trumpet playing experiencing pain

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.

Some other fainting types though are purely emotional: "You see it in the movies, people told about a relative dying and then swooning away."

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."

: 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.

: 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."People don't want to collapse, so they look around for a chair and there isn't one or by the time they've found one, it's too late. You just pretend you've dropped something or your shoelace is undone [and squat]. It's something you can do really quickly and hopefully it will stop it.

The brain senses reduced blood flow to the upper body, and a message is sent via nerves to blood vessels in the legs and pelvis to make them tighten and so force blood back up your body to restore the flow.

But if you are prone to fainting, the chemical signal that makes blood vessels tighten is not strong enough.

This means blood can start pooling in the lower body. You will start to feel unwell and your blood pressure will slowly drop. Then something happens to make blood pressure drop dramatically, and you quickly fall to the ground.

"We've worked out quite a lot about what happens during the slow drift down [in blood pressure]. But the final point where it plummets - it just goes 'bang' - we don't yet understand," Dr Corcoran said.

While fainting can be scary, it serves to fix the problem because the effect of gravity is removed and blood flow to the brain is restored.

Blood phobia

Fainting at the sight of blood, which affects 2 to 4 per cent of people, is classed as part of a response to a specific phobia of blood.

It comes under the broader category of "blood-injection-injury phobia" in the Diagnostic and Statistical Manual of Mental Disorders published by the American Psychiatric Association. But Dr Corcoran questions this classification.

"We find that propensity to any kind of fainting does tend to cluster with anxiety but we're not sure if people faint because they're anxious or if they're anxious because they faint," she said.

"It's called phobic fainting but I personally think people are phobic of the response; it's not the blood, but how they feel when they see blood."

Research has shown blood fainters do not experience the emotion of disgust any more readily than people who do not faint at sight blood, she said.

Also, in every other phobia, the heart rate and blood pressure go up - the opposite of what triggers the tell-tale wooziness people feel when they are about to faint from seeing blood.

But there does seem to be an emotional component to many blood fainting episodes.

"We do clinically find that when people faint at the sight of blood, usually it's their own blood or blood from someone they care about. Often when people describe it, it's the child rushing in gushing blood or they're shocked that they've cut themselves with a knife and there's that whole emotional response," she said.

Fainting can run in families, but the triggers for an episode may be different from one family member to the next.

Be aware that anxiety can make fainting more likely because it releases the hormone adrenaline, which promotes blood flow to the legs. But knowing strategies that can stop you from fainting can increase your confidence and therefore reduce both anxiety and fainting.