Re: If you are cut in space, what color would the blood be?

Hello Kim,

Thanks for your question:-) Your students are very bright in pointing out 
that the presence or the concentration of oxygen that we breathe in would 
change the colour of the blood slightly. I shall try to explain your 
question in several levels:-

Basic physiology:
A normal 70kg, 1.82m male have 5 litres of ciculating blood to supply our 
vital organs with oxygen, cellular fuel and other essential metabolites. 
There is a (very crude and) general "Rule of Three's" stating that human 
can just about survive 3 minutes without oxygen, 3 days without water, 
and 3 weeks without food. Out of the 5 litres of circulating blood, 2 
litres of which are red blood cells. And it is crucially important to 
realise that 98% of oxygen carried in the blood is bound to the 
haemoglobin molecules inside red blood cells, which in turns contribute 
to the colour of the blood. Each haemoglobin molecule can carry 4 oxygen 
molecules, and when occupied by oxygen (known as saturated), it is called 
oxyhaemoglobin - which is responsible for the bright red colouration of 
human blood. On the other hand, when oxyhaemoglobin is stripped off its 
oxygen (known as desaturated), it becomes deoxyhaemoglobin - which is 
responsible for the dull red colouration of human blood. In simple terms, 
the blood in our arteries (except pulmonary arteries and umbilical 
arteries) has high oxygen content and appears bright red in colour; 
whereas the blood in our veins (except pulmonary veins and umbilical 
veins) has low oxygen content and appears dull red in colour. Whenever 
you are cut, assuming you have not sustained a massively deep injury, the 
blood that leaks out from the wound would mostly be venous blood because 
most arteries are deeper (more protected) compared to veins. So in the 
majority of the time, the blood you see after a superficial cut will be 
those with low oxygen content i.e. dull red venous blood. 


When oxygen is low or absent:
By understanding basic physiology, I hope you can deduce that in an 
environment where oxygen is scarce (or indeed absent as it is in space), 
the concentration of deoxyhaemoglobin would rise and the concentration of 
oxyhaemoglobin would fall. Therefore the "brightness" of the red 
colouration of blood would be replaced by the "dullness" due to the 
rising deoxyhaemoglobin concentration. So if you sustain a superficial 
cut in space, assuming that oxygen is the only parameter you are 
considering in space, the blood that leaks out would look pretty much 
like what you would expect to see on earth, or may be just a little bit 
darker. BUT, and a HUGE BUT...see below!!!


What is colour? 
You have to realise that colour of an object is determined by the 
wavelength of the reflected light and blood only appears to be red 
because it absorbs lights in wavelengths other than red in the visible 
spectrum. This is important because in space (or more properly defined, 
in our solar system), the light source is our sun with planets and moons 
only reflecting the light from the sun. So if you happen to be behind a 
shadow, e.g. behind a planet or a moon, there will be no visible light to 
be reflected into our eyes, and hence, not only that you will not be able 
to see your blood in any degree of redness, you will not be able to see 
anything at all!! i.e. total darkness...(difficult to imagine, right? And 
a bit scary~~~)


Spacecrafts, space-suits, temperature, and pressure...
Space may look stunningly beautiful, but it is not at all hospitable to 
human beings. Without the earth's protective atmosphere, temperature 
could be rather extreme. Take our moon as one of the MILDEST examples, 
temperature on the moon have been recorded in the range of 138 to -149 
degrees Celsius! Therefore, human in space would need to be protected 
either in spacecrafts (which maintain an earth-like environment without 
the gravity) or in protective space-suits if we were not to be boiled or 
frozen alive. When astronauts do their "space-walk" in protective space-
suits, they cannot afford to cut themselves. This is because their space-
suits are closed pressurised systems. If the space-suits are in any ways 
breached, they will lose their pressure inside so rapidly that gases 
dissolved in our blood will start "bubbling out" and our bodies will 
expand and eventually explode!! So, a little advice to any prospective 
astranauts: just don't get cut in space!!:-)


I hope the information above will help your students understand more 
about human physiology with respect to space science; and I certainly do 
hope this has not put your students off their interests in space:-)


Joshua Chai
Student Doctor
University of Cambridge, UK