The heart is a sac enclosed muscular pump located directly behind the sternum. It is somewhat pear shaped, with the apex directed to the left, and it is about the size of the person's fist. The enclosing sac, called the pericardium, is a tough, fibrous structure with its base attached to the diagraphm below.

Essentially, the heart is a mass of cardiac muscle (myocardium) organized into four chambers : the two atrium above and the two ventricles below. The atrium, much smaller than the ventricles and with relatively thin walls, are marked by an ear-shaped, or auricular, appendage. Between the two atria there is a small oval depression (the fossa ovalis) marking the site of an opening (the foramen ovale) in the fetal heart. The right atrium receives blood from the venae cavae (the great veins returning blood to the heart) and empties into the right ventricle through an opening guarded by the tricuspid valve. The left atrium receives the four pulmanary veins carrying blood from the lungs and empties into the left ventricle via the bicuspid (or mitral) valve. The left ventricle, the larger of the two ventricles, forms the apex of the heart.

The secret of the beating heart resides in the sinoatrial (SA) node sequestered in the wall of the right atrium. This tiny bit of nervous tissue, commonly dubbed the "pacemaker" emit electrical impulses at a rate of about 60 -100 per minute, each impulse causing the atria and ventricles to contract. The atria contract first (atrial systole), and then the venticles contract (ventricular systole), the impulse reaching the lower chambers via a special conduction system.

The heart's phenomenal automatism not withstanding, a number of highly influential extracardiac factors influence that organ's activities. Perhaps foremost is blood volume. According to Starling's law, the volume of blood pumped by the heart is normally determined by the volume of blood returned to the heart. This is because cardiac muscle contracts with greater force the more it is stretched- up to a point, of course. Other extracardiac factors are hormones and the autonomic nervous system. The adrenal hormne epinephrine for example, increase both the rate and strengtg of contraction.