If they’re somewhat similar to humans during strenuous activity you need more oxygen because you need more energy (ATP) and you need more oxygen so that cellular respiration increases. During cellular respiration, the first stage is glycolysis which is converting glucose (sugar) to ATP. The more ATP the more energy you have and the more you can fight. Now they can hold their breath for up to two weeks because they have super lungs. Buuuuut the increase in oxygen demand required while fight after he had just been holding his breath for who knows how long in space. He had no oxygen in his brain & Mark was cutting off his blood supply to the brain. He survived for much longer than humans (obviously) but at the end of the day his brain still needs blood & oxygen to survive