Complex sentence: One advantageous adaptation they have developed is the ability to bypass the renal system completely when doing hard physical activity; this is done by pushing blood through the large venous valves located at the cross between the renal veins and iliac veins.
Split into simpler sentences: One advantageous adaptation they have developed is the ability to bypass the renal system completely when performing strenuous physical activity. Bypassing occurs by pushing blood through the large venous valves located at the cross between the renal veins and iliac veins.