answer:Learn to relax your diaphragm so that when the cardio becomes strenuous, your lungs will continue to be able to fully expand, making the oxygen exchange most efficient. My cross-country running coach taught me how to accomplish this: inhaling through your nose and exhaling through your mouth. There’s actually something very zen about it as well. Practice this type of breathing while you’re out walking, biking, etc. and as it becomes more natural you will be able to maintain that balance when you exercise as well. And don’t forget to stretch!