The equine heart is an enormous, muscular pump that takes water and oxygen to the working muscles and carries away carbon dioxide and metabolic waste products. The heart mass in thoroughbred horses is about 1 percent of body weight. In fact, there are definite breed dispositions for heart mass to body weight ratios. Racehorses have a relative heart mass of 0.86 compared to draft horses who have a relative heart mass of 0.62!
Oxygen is carried to working muscles by the hemoglobin in red blood cells. The horse is specially adapted to provide working muscles with extra oxygen during exercise because horses store extra red blood cells in their spleen, which contracts in response to fear or exercise and adds additional red blood cells to the circulation. In effect, this means that horses have a high maximal oxygen consumption rates during exercise, compared to other species.
Heart rate measurements during exercise are used to measure fitness and describe the intensity of a given horses workload. So, what makes heart rates relevant to fitness?
There is a direct relationship between the heart rate of a horse and it’s cardiac output. However, heart rate is not the only factor contributing to cardiac output. Stroke volume also contributes and the relationship is shown in this simple equation, HR x SV = CO. Stroke volume is the volume of blood ejected from the heart with each contraction. Cardiac output is the volume of blood ejected out of the heart each minute. In turn, oxygen uptake by muscles is directly related to cardiac output.