Cardiology Basics


This is the human heart:

The heart is a muscle about the size of your fist. It sits in the middle mediastinum, aligned between thoracic vertebrae T5-T8, with the apex is slightly to the left of the sternum. A protective sac called the pericardium surrounds the heart; two layers comprise the pericardium, the parietal pericardium (the outer layer) and the serous pericardium (the inner one). The serous pericardium is also known as the epicardium, and it is the heart wall’s outermost layer. Surrounded by the epicardium is the myocardium (heart muscle) itself. The heart is composed of two types of cells, cardiac myocytes (specialized muscle cells that contract) and pacemaker cells, the latter of which form the electrical nodes. The heart is a four chambered structure; the upper two chambers are called the atria and the lower two are called the ventricles; the right atria and right ventricle form the right part of the heart while their left counterparts form the ‘left heart’; a septum (wall) separates the ‘left heart’ from the ‘right heart.’ Valves connect the atria and ventricles; the tricuspid valve connects the right atria to the right ventricle, and the mitral valve connects the left atria to the left ventricle; cartilaginous chordae tenae attach the atrioventricular valves to papillary muscles on the ventricular walls, which ideally prevent prolapse on systole (contraction). Also located between the atria and ventricles in the interatrial septum is the the atrioventricular node, a small collection of pacemaker cells responsible for conducting electricity from the atria to the ventricles. The atrioventricular node is connected to the Atrioventricular (AV) Bundle or (Bundle of His), a specialized group of myocytes that separates into fibrous fascicular branches, leading then to the Purkinje fibers, which are the mitochondria-dense conducting tissue situated in the ventricular walls. Blood is carried to and from the heart via blood vessels.

When it contracts, the heart pumps blood. It does its very important job in two sub-systems, the pulmonary circuit and the systemic one. In the pulmonary circuit, the heart sends deoxygenated blood through the pulmonary artery in the right ventricle and receives oxygenated blood from the lungs in its left atrium via the pulmonary vein. The systemic circuit supplies the rest of the body (including the heart itself) with blood. Blood exits the heart through the aorta where it travels through arteries and capillaries to the rest of the tissue; the body’s tissues are thereby supplied with oxygen. Veins return deoxygenated blood to the heart; the blood reenters the heart through venae cave located in the heart’s right atrium. A muscle in need of its own oxygen, the heart takes some of the blood released from the aorta and dispenses it to itself via the coronary arteries; if/when these arteries become clogged or otherwise obstructed, tissue death occurs in the part of the heart supposedly supplied by the coronary artery in question, causing what is known as a myocardial infarction or heart attack.

Cyclic, electrically-powered contractions of the myocardium are what we know as ‘heartbeats’. It all begins in the sinoatrial node where the electrical impulse originates. Action potential is generated and the atria contract, pushing blood through their valves into the ventricles. The atria relax, and the ventricles fill with blood. The electrical signal travels to the atrioventricular node where it is immediately sent through the Bundle of His to Purkinje fibers in the ventricular walls, which upon receiving this stimulating signal, systole, pumping blood out of the heart and into the arteries. The ventricles relax, and almost instantaneously the process repeats.