> Flight recorder data from the single cosmonaut outfitted with biomedical sensors showed cardiac arrest occurred within 40 seconds of pressure loss. […] The autopsies took place at Burdenko Military Hospital and found that the cause of death proper for the cosmonauts was hemorrhaging of the blood vessels in the brain, with lesser amounts of bleeding under their skin, in the inner ear, and in the nasal cavity, all of which occurred as exposure to a vacuum environment caused the oxygen and nitrogen in their bloodstreams to bubble and rupture vessels. Their blood was also found to contain heavy concentrations of lactic acid, a sign of extreme physiologic stress. Although they could have remained conscious for almost 40 seconds after decompression began, less than 20 seconds would have passed before the effects of oxygen starvation made it impossible for them to function.

https://en.wikipedia.org/wiki/Soyuz_11