A lot of things happens to me when I am running on my treadmill. My breathing becomes faster. I am running at 6-8 miles per hour.Usually I run for about 45 minutes at 15% incline. My body gets very tired. It gets hard to breath. Some things happen to other part's of my body too. My legs get tired from running so much. It is really hard for me to keep running really fast for such a long time. I sweat a lot since my body is really hot. Our body produces sweat through our sweat glands when we are hot. The sweat cools us off a little bit. When I am running on my treadmill, my heartbeat gets really fast. My cardiovascular system is being tested when I am running so fast. My heartbeat gets faster. Your heartbeat is supposed to be 60. My heartbeat one time went to 180 when I was running on the treadmill. I like running on the treadmill. I usually burn 400 calories.