Complex sentence: This decline is due to an increasingly poor match between ventilation and perfusion in the lung as we age, making ventilation-perfusion abnormalities the most common reason for a reduced arterial oxygen content.
Split into simpler sentences: This decline is due to an increasingly poor match between ventilation and perfusion in the lung as we age. For this reason alone, we can say that ventilation-perfusion abnormalities are the most common reason for a decline in arterial oxygen content.