Howdy everyone, Severus here. I was listening to the latest podcast from The Overlords when the subject of a tyranid being exposed to space came up. A physicist by the name of Ricard explained the principles behind heat gain and loss in space for a tyranid (sorry if I botched the spelling there, not a name that I am familiar with as an american). The question was brought up what would happen to the tissues (aka eyes exploding and vessels popping). Jon asked for a “medic” to write in to answer the question. In real life I am a veterinarian, and my main area of focus is exotic animal medicine. So, I feel qualified to answer this subject. The following is probably not for the squeamish.
First off, explosive decompression of a body would not be possible. The vacuum does not tear you apart. The internal pressure within a body would be the only force acting on it to make it “explode”. Fortunately, even in us squishy humans, our skin and connective tissues are strong enough to prevent those internal pressures from causes an explosion. Now, one would assume the armored tyranids would also have as strong skin and connective tissue as a lowly guardsman. Any biomorph intended to be exposed to space would probably have much harder outer skin than the normal biomorph.
Now, this pressure within our circulatory system would be strong enough to make small blood vessels, called capillaries, burst. These are located through out the body. Likely the surface vessels would be the only vessels exposed to enough of a pressure differential to burst. These small vessels connect the arteries and veins and are the main way oxygen is delivered to tissues. So, yes, the vessels in your eyes would burst. But these vessels are small enough that there would not be an major bleeding due to clotting mechanisms and there very small diameter (usually only wide enough to fit a single blood cell or two through at a time). The main damage would be the lack of oxygen to tissue.
The last major affect of exposure to vacuum would be escape of gas from a body. In the case of a human, this would mean all the air in your respiratory system (lungs, trachea, mouth, sinus) and all the gas in your gastrointestinal system (stomach, intestines, colon). Essential the would be a massive fart/burp along with a giant cough, leaving your body devoid of any gasses, mainly oxygen. This would cause you to black out and suffocate.
Now, I don’t have a tyranid in front of me to examine it’s respiratory system and circulatory system. I can extrapolate that the respiratory system of most of the ground based organisms we see are similar to ours. Whether breathing through a mouth or nose into some sort of lung like organ. They would have some connection to the outside environment and would therefore experience the gas robbing effects of the vacuum. AKA, a tyranid biomorph not designed for void exposure would likely suffocate unless they had some sort of emergency valve to seal off there respiratory system.
The circulatory system on the other hand is a whole new story. The closest analog to a tyranid in the animal kingdom is insects or crustaceans. They have a different circulatory system. Humans have a closed system (blood contained in veins, arteries, and capillaries). Insects and crustaceans have an open system. In an open system, the heart pumps the hemolymph (the bug equivalent of blood) into a cavity that surround all the organs. The hemolymph bathes the organs in nutrients and oxygen. So, in exposure to vacuum, there are now small fragile vessels to burst. The tyranid is likely one big squishy balloon of organs and hemolymph. So, tyranids of any sort would not likely have vessels that burst in the vacuum of space.
Now for the final question, would the eyes explode? There is a small amount of pressure contained with in the globe of each eye in most animals. This pressure would not be enough to break through the connective tissues that make up an eye. So they eyes would not burst. They would not pop out of the sockets either since there is no significant sources of pressure behind the eye. Any tyranid biomorph’s eyes would be safe from explosion in the void. A squishy human would likely have the vessels on the surface of the eye burst, but not the eye itself.
So, more science than and explanation than was likely necessary. But now you know. Suffocation and a few burst small vessels are what a squishy guardsman could expect. Tyranids would likely only suffer suffocation unless they had some biomorphs to prevent the loss of gasses (aka chitin oxygen tanks in there body). More of a reason to fear the creepy space bugs. Until next time, this is Severus saying have a good one and take it easy.