The heart is the centre of the circulatory system. It is a muscular hollow organ which works as a high precision suction and pressure pump. The heart consists of a left and right half; each half containing two chambers: an atrium and a ventricle. The atria receive the blood that flows from the general circulation back to the heart and convey it into the ventricles. The right ventricle pumps blood into the pulmonary circulation which supplies the lungs, and the left ventricle into the systemic circulation which supplies the body.

In the horse the heart rate at rest is between 30 and 40 beats per minute and can, during great exertion, climb to 250 beats a minute. With every heartbeat 1-1.5 litres of blood are expelled; this results in a maximal cardiac output of over 300 litres/minute.

The rhythmic action of the heart is controlled by an electrical pacemaker, the sinus node. This is located in the wall of the right atrium. From there the electrical stimulus spreads via a conduction system over the entire heart. The electrical activity can be depicted by means of an electrocardiogram (ECG). The action of the heart is divided into a contraction phase or systole in which first the muscles of the atria contract to push the blood into the ventricles, then the muscles of both ventricles contract and expel the blood out into the pulmonary and systemic circulation. This is followed by a relaxation phase or diastole, during which the returning blood flows passively into the atria.