However, there is a limit to the minute volume that can be achieved during maximal exercise. When the horse is at a maximum exercise level--a heart rate that has reached 200 beats per minute and beyond--the blood leaving the lungs might not be carrying enough oxygen to adequately service the tissues. This condition is referred to as arterial hypoxia. Generally speaking, the respiratory system can deliver the required oxygen--at least for a limited time--when the heart rate is 180 beats per minute or less. When the heart rate is beyond that point, it is only a matter of time before the system will begin to falter