Here are some running in the heat tips. Running in the summer can make you feel as though you need gills rather than lungs. Good luck with your heart rate training. Your body has to work harder to keep you cool when the temperature rises. Breathing will be more challenging, and your heart rate will be higher. Your body is directing blood to your skin to help you cool off via sweating. Less blood is therefore available to carry oxygen to your muscles. What normally feels like a leisurely run actually feels like a peak all-out effort.