A leisurely walk would require you to carry out aerobic respiration. You might notice the respiration rate increasing as your need for oxygen increases. The cells contain mitochondria which carry out respiration. Oxygen combines with glucose to produce ATP, the energy currency of the cell. Every glucose molecule that undergoes aerobic respiration, yields 36 molecules of ATP, along with the wastes carbon dioxide and water vapor, which are exhaled. However, if an emergency arises, like sprinting after your dog, you may need additional ATP molecules. They are produced by lactic acid fermentation. In the muscles, there is stored energy in the form of glycogen. If it is needed, it is converted to glucose and anaerobic respiration can occur. This produces 2 molecules of additional ATP, along with the waste product lactic acid. If you do a cool down exercise, it will eventually break down. But, if you stop suddenly from your activity, the lactic acid may cause a cramping sensation in the muscles. You are panting during the run due to the body attempting to maintain homeostasis, even during emergency situations. You would have had an increased respiratory and heart rate to help you get the oxygen and glucose you needed to carry out strenuous activity.