As you noted, blister fluid is clear, not red.
So, rupturing of blood vessels is an unlikely
explanation for the fluid accumulating in the
blister because then it would be filled with
blood. (On the other hand, "Blood Blisters" are
red and filled with blood). You've already hinted
at what the actual process might be. In your
question you noted that "serum from the blood"
collects in a blister. The serum is the clear
liquid secreted from blood vessels that have been
damaged. The kind of biological mechanism that
would allow the water and molecules that are
present in serum to pass across the blood vessel
walls while leaving the rest of blood components
(such as the red blood cells) behind is called
diffusion. Diffusion across a cell membrane is
an important mechanism that allows water and some
molecules to move across the surface of cells.
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