Aerobic exercise requires that you train at 70-80% of your max HR in MOST CASES. This is the preferred zone if you are training for an endurance event. In this zone, your functional capacity will greatly improve and you can expect to increase the number and size of blood vessels, increase vital capacity and respiratory rate and achieve increases in pulmonary ventilation, as well as increases in arterial venous oxygen. Moreover, stroke volume (amount of blood pumped per heart beat) will increase, and your resting heart rate will decrease. What does all this mean? It means that your cardiovascular and respiratory system will improve and you will increase the size and strength of your heart. In this zone, 50% of calories burned are from carbohydrates, 50% are from fat and less than 1% is from protein. And, because there is an increase in intensity, there is also an increase in the total number of calories burned.