The cardiovascular system pumps blood around the body. During exercise, this allows the body to transport oxygen & energy substrates, such as glucose & fatty acids, to the muscle fibres and remove waste products, like lactate, water, and carbon dioxide. The blood pumped around the body by the cardiovascular system also plays an important role in thermoregulation.
The heart is the pump which circulates blood around the body. It is situated within the thoracic cavity, in front of the horse’s elbow. It weighs approximately 4kg. A larger, more muscular heart is more desirable in a sports horse, as it allows a greater volume of blood to be pumped around the body with each beat. Heart size is one comparison that can be made between horses of similar ages and types.
The heart is divided into left and right, each of which is subdivided into an atrium (atria – plural) and a ventricle, that are connected by the atrioventricular openings. The atria collect blood being returned to the heart by large veins, and the ventricles are where blood collects ready to be sent back around the body. The left ventricle pumps blood around the body, whereas the right ventricle pumps blood around the pulmonary system, so oxygen can diffuse from the air into the blood, and carbon dioxide can diffuse into air to be breathed out. Contraction of the heart is called systole, and relaxation is called diastole. During systole, the atria contracts first, which is followed, almost immediately, by the ventricles.