Let's suppose we're casually walking in the park and all of a sudden we have a dog that begins to chase us. Now, if we choose to run away from that dog or if we choose to fight that dog off, in either case it's the sympathetic nervous system that actually kicks in. So let's suppose we decide to run away. Now as we begin to run, what begins to happen is more blood begins to pump to our skeletal muscles so that we can run away. And as more blood is being pumped to our muscle, those muscles are basically using more energy creating more ATP molecules and they need more oxygen. And so the rate of our respiration increases as well as the rate of our heart.