Why would the blood boil? Is it because of the temperature or pressure? Because I really can't figure it out. I thought space didn't have a temperature above freezing unless close to a star or the sun. But if it is the pressure how would that work?
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There are lots of related questions on this site but I couldn't find one that answered your question exactly. If you're interested try searching the site for boiling vacuum or something similar.
The boiling point of a fluid depends on the external pressure. Specifically a fluid will boil when its vapour pressure is greater than or equal to the external pressure. The vapour pressure of water at body temperature is about 0.06 atmospheres, so when the external pressure has fallen below 0.06 atmospheres the water in your body will start to boil.
However as water boils it cools, because heat is carried off by the water vapour. So the boiling will cool your blood/saliva/whatever. The vapour pressure of water is temperature dependant so the cooling will reduce the rate of boiling, and of course at some point it will be cooled enough to freeze. There is some debate about how fast you would freeze because we've never done the experiment. There is some discussion of this in Direct exposure to the vacuum of space and How does space affect the human body (no space suit, no space craft).
Note that ice sublimes in a vacuum so even after freezing you would slowly lose water and dry out. However the rate of water loss gets very low at low temperatures, which is why it's possible ice can be found on the Moon.
John Rennie
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In space there is no air pressure at all, and pressure is the continuous physical force exerted on or against an object by something in contact with it. Also the temperature in space can be as low as 0 Kelvin, but also as high as 2.7 Kelvin because any object in space will just radiate heat until it cools, and it will stay cool. However this is for objects that are not exposed to sunlight
Because there is no pressure in space to keep the liquids in our bodies from being a liquid state, because the boiling process would cause them to lose heat very rapidly, then the fluids would freeze before they were totally evaporated.
Your tissues i.e the skin, heart etc would expand due to the boiling fluids, but they wouldn't explode. You would also be exposed to a number of different types of radiation and would experience the absolute 0 of space
Joe Hilton
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What you are looking for is called the "Armstrong limit". The lower the pressure then the easier a fluid can boil... so beyond a certain pressure, and without a pressurized suit, your blood could be capable of boiling at your normal body temperature.
Ian
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