The brain is the most nutritionally demanding organ The number of "branches" that make up the blood vessels is incredibly huge☝🏻 The brain can't exist without oxygen or glucose. Nerve cells need these two substances to function and survive. If the brain does not get enough oxygen or glucose within 3-5 minutes, irreversible damage occurs. As long as the brain cells still have oxygen and glucose in them, they won't die for another few minutes.