Science Questions

If we bleed in a vacuum would it bleed blue?

Sun, 1st Jun 2008

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Steve Casomere asked:

Do we bleed blue blood out of our veins? Presumably if you bleed blood it’s the oxygen that makes it turn red. If we bleed in a vacuum would it bleed blue?


The answer is definitely no.  In hospital they use things called vacutainers which is a vacuum in a tube.  You put a needle into a patient’s vein, put the tube onto the needle and this exposes the inside of the vein to the vacuum and draws the blood into the tube. Momentarily the blood is exposed to a vacuum and the blood comes out very red.  There’s still too much oxygen in venous blood to make sure it stays a red colour.

So why do veins look blue?

This is because when you have a bit of the tissue which doesn’t have enough bloodflow going through it tissue will remove more oxygen from the blood.  It does mean haemoglobin can get to be a blue colour.  Normally that doesn’t happen but if you have an area of the body that doesn’t have enough bloodflow through it then the tissue gets oxygen hungry and removes enough oxygen to make it go blue.  Veins look blue as an optical illusion.  They’re not really blue and if you cut through the skin you’ll see they’re a whitish pink colour.


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Steve Kazemir asked the Naked Scientists: I was talking with my children the other day, and we were noticing our blood vessels under our skin, and how many of then are blue in colour. I explained that blood changes colour when it has been oxygenated. So even if you cut yourself and blood came out of a “blue vein”, the blood would hit the oxygen in the air and turn more bright red. So, if we were to put our arm into a sealed container, with no oxygen in it (presumably with some other non reacting gas…nitrogen perhaps?), and then bled, would we bleed blue blood? Thanks for the fantastic show! Steve What do you think? Steve Kazemir, Fri, 2nd May 2008

There is usually too much oxygen in venous blood for it to turn blue in-situ. It's actually an optical illusion that veins are blue. And sticking a needle into one results in reddish blood emerging, so it's definitely not the blood that's blue.

However, the idea that bleeding into a vacuum might produce blue blood is intriguing and something I'd like to try. I'm not sure I'd want to do it in a vacuum because that could result in quite a haemorrage, but bleeding into an atmosphere of nitrogen ought to prove the principle.

Under these conditions I would predict that the blood would emerge red and then slowly go blue as the haemoglobin gave up its oxygen to the surroundings.

Chris chris, Sat, 3rd May 2008

I believe it's quite a common experience to bleed into a vacuum. A lot of blood samples are taken into evacuated tubes. They still look reddish but less so than if you just cut a finger and it bleeds into air. I will ask one of my colleagues who specialises in sticking needles in people. Bored chemist, Sat, 3rd May 2008

This thread here
also tackles the subject of blue blood neilep, Sat, 3rd May 2008

This is a very good point and I should have mentioned it - in hospital we often collect blood samples using "vacutainers", which are evacuated (contain a vacuum) test-tubes plugged with a soft rubber bung.

To do this, first a suitable needle is positioned in a vein. Then, the other end of the needle, which is also sharp, is used to penetrate the rubber bung of the vacuum tube. The vacuum inside the tube pulls the correct volume of blood from the vein into the tube.

BUT - the tube stops filling once the vacuum has been replaced by blood, so the blood in the tube ceases to be in a vacuum.

This proves, as bored chemist cleverly highlighted, that you don't bleed blue into a vacuum; but it doesn't really show what blood is like in a vacuum, because the tube quickly fills and hence the blood ceases to be in a vacuum for long.

Chris chris, Sun, 4th May 2008

How good of a vacuum do hospitals use because the oxidation reaction of Iron according to thermodynamics Ellingham diagrams you need a really good vacuum greater than 10^-11Pa to prevent oxidation of blood. Scientifically speaking 10^-8Pa is used for SEM and TEM microscopy measurements. Also lower vacuum pressures are not possible because you literally pump hydrogen through the vacuum chamber walls. The vials in hospitals are no where near this order of magnitude and are probably just to induce blood flow. See the chart Science doesn't lie. At room temperature 25*C it is just not thermodynamic-ally favorable to occur. Well Science, Sun, 13th Sep 2015

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