Complex sentence: These seals also compensate for prolonged lack of available oxygen by increasing their oxygen carrying capacity, which done by having more red blood cells per unit volume of blood, as well as having more blood relative to other mammals.
Split into simpler sentences: These seals also compensate for prolonged lack of available oxygen by increasing their oxygen carrying capacity. This is done by having more red blood cells per unit volume of blood, as well as having more blood relative to other mammals.