In old space movies, when someone is left exposed in space, they explode. Well this have some merit, it is not 100% true. In space and its pressure (or lack of) causes the water in the human body will expand into water vapor. This causes severe internal swelling up to twice the size. Due to the enlarged veins from the swelling, a person’s blood pressure will drop since the heart would not be capable of pumping blood.
Another danger is hypoxia. Without pressure, the oxygen in the bloodstream will dissolve and escape the blood. A person will last about 14 seconds before passing out.
While you would think to hold your breath like underwater, holding air in your lungs is deadly. If someone tries to hold their breath during decompression of the change in pressure from a spacecraft to space, the lings will rupture internally.
The cold and radiation may kill you later, but the lack of pressure in space will kill you within 90 seconds.