Your heart rate increases when you begin to exercise, then plateaus off and remains elevated for a prolonged period as long as you maintain the same pace. If you increase your effort, it will go even higher until you reach your maximal capacity. The quick responsiveness of your heart to exercise is due to the demand for oxygen in your muscles.
In order to continually produce energy for contraction, your muscles need oxygen. Oxygen is carried on your red blood cells from your lungs and travels to your heart to be pumped to your working muscles. Inside the muscle cells, tiny organelles called mitochondria combine oxygen with glucose and fat to make ATP, the basic energy molecule. When you increase your muscle action, your heart beats faster and harder to send oxygen to your cells, ejecting a greater volume of blood with each stroke.
Beating the Heat
High environmental temperatures during exercise can increases your heart rate above normal levels because your heart has to send blood to your skin to cool you down while continuing to supply blood to your working muscles. These two demands force your heart to beat faster. The more you train in a hot environment, the more efficient you become at cooling your body while satisfying the energy demands of your muscles.
During long-duration exercise, your heart rate may gradually increase, even if you maintain a set pace. This "cardiovascular drift" occurs as you lose water through sweating and as your heart directs more blood to your skin in order to cool you down. Your heart rate will increase because blood is being diverted from your working muscles, and therefore, it has to pump more often to keep your muscles supplied with oxygen and energy. Cardiovascular drift occurs regardless of whether you are staying hydrated. But according to researchers at the University of Texas, your heart rate will increase even more if you are getting dehydrated. A study of cyclists showed that a rising body temperature will also contribute to an increased heart rate during prolonged exercise.
All Systems Go
All your body's systems depend on oxygen, and your heart must continue to feed all your tissues while you exercise. According to the book "Essentials of Exercise Physiology," many other things affect heart rate during exercise, including your emotional state, how much food you've eaten prior to exercise, your body position and whether the exercise is continuous or characterized by periodic bursts. With regular training, your heart becomes stronger and pumps a greater volume of blood with less effort, lowering your heart rate at rest and during exercise. Your muscle cells grow more mitochondria and become more efficient at extracting and using oxygen, decreasing demand during exercise.