W "...The equine heart is made of muscle tissue, more rounded in shape than a human's, built with the sole purpose of pumping blood throughout the body. It consists of 4 chambers: the left and right atria, and the left and right ventricles. The average adult horse has a 3.6 kilograms (7.9 lb) heart, although it can be more than twice this size. The heart grows until the horse is 4 years of age, although it can increase slightly in size as a response to conditioning. Heart size does not necessarily correlate to the size of the horse.
Circulatory capacity is partially determined by functional mass of the heart and spleen. Once the oxygen has entered the bloodstream it must be transported to working muscle and waste products removed. The equine cardiovascular system is hugely compliant with a heart rate range from 20 - 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. However, studies on Thoroughbreds have shown that the proportion of skeletal muscle exceeds 50% of body weight, and so the energetic capacity of the musclular system far exceeds the capacity of the cardiovascular system to deliver oxygen. ..."