Diffusion. Oxygen levels in tissue are about 3%, compared to 20% in the atmosphere. In fact, that was quite a finding back in ... the late 80s I think: they cou

Diffusion. Oxygen levels in tissue are about 3%, compared to 20% in the atmosphere. In fact, that was quite a finding back in ... the late 80s I think: they could coax fibroblasts to divide many more generations simply by incubating them in a low oxygen environment. Free radicals are brutal.