Human’s can survive only for a few minutes without oxygen. This is because of the susceptibility of the brain cells. Even though the brain is only about 2 percent of the body’s weight. It consumes about 20 percent of the oxygen we breath in. Hence, if O2 supplied (by the blood) is stopped for a few minutes, the brain ceases its function resulting in death. We will lose consciousness in 8-10 seconds after the blood supply to the brain is cut.
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Brain cells can survive for long periods without oxygen at very low temperatures, as O2 consumption is reduced. This technique induced Hypothermia is made use of during major surgeries. Other tissues can survive for long periods without oxygen and that is why it is possible to remove organs, from the body of dead persons, for transplantation.