The Principle of the Heartbeat

In a healthy person the heart beats without ceasing. Its contraction and relaxation allow all tissues of the body to take up oxygen and nutrients from the blood and to release carbon dioxide and metabolic wastes into it. If the heart fails, not only our health but even our life may be threatened.

So how does the heart beat?

To understand the principle, we must first know an important structure of the heart—the sinoatrial node (SA node). The SA node is a structure composed of a group of special cardiac muscle cells; it is the normal pacemaker of our heart. It lies in the deep surface of the epicardium in the upper third of the terminal groove at the junction of the right atrium and the superior vena cava; it is not easily visible to the naked eye on the epicardial surface. Its long axis is roughly parallel to the terminal groove, and the human SA nodal artery consistently passes through its center.

The SA node is elliptical or semilunar, about 10-15 mm long and 2-3 mm wide, about the size of a soybean. Small as it is, its role is vital.

The cells of the SA node are mainly pacemaker cells (P cells) with a few transitional cells (T cells). Around these cells are rich autonomic nerve endings (sympathetic and parasympathetic), which regulate heart rate. The SA node is also surrounded by abundant collagen fibers forming a network scaffold that maintains its stability.

The SA node itself needs a blood supply to function; its blood comes from the SA nodal artery. In about 60% of people the SA nodal artery arises from the right coronary artery, and in about 40% from the left coronary artery.

Its special cellular composition and structure allow the SA node to release regular electrical impulses (excitation) that control the frequency and rhythm of the heartbeat.

The muscle tissue of the SA node is cardiac muscle, not voluntary muscle, and is not controlled by our conscious brain; we cannot will our heart to beat faster or slower. But the SA node is controlled by our autonomic nervous system, so when we are nervous the heartbeat speeds up. We often say that meeting someone or experiencing something gave us an "electric" feeling; there is some truth to this. When we are excited, the SA node releases an unusual electrical impulse.

The SA node is an important part of the cardiac conduction system, but not the whole of it. The cardiac conduction system consists of the SA node, internodal tracts, atrioventricular (AV) node, atrioventricular bundle, left and right bundle branches, and the Purkinje fiber network. The excitation (electrical impulse) generated by the SA node is conducted onward through these structures.

However, the exact pathway by which SA nodal excitation reaches the left and right atria and the AV node has long been debated. The internodal tracts currently accepted between the SA and AV nodes were only a hypothesis proposed by James and others in the early 1960s; they still lack sufficient morphological evidence. Much in this field awaits exploration.

According to James's hypothesis, the internodal tracts conduct between the SA and AV nodes; there is also an interatrial band connecting the left and right atria. There are three internodal tracts: anterior, middle and posterior, all arising from the SA node and descending to the upper and posterior margins of the AV node. Like power cords, they conduct the SA node's electrical signal to the AV node.

The AV node is a flat structure in the central part of the AV junction; strictly speaking it is a region rather than a single node. Its importance in the conduction system is second only to the SA node; it is the obligatory pathway by which electrical signals pass from atria to ventricles. It produces the AV delay, so that atrial and ventricular muscles contract in sequence, preventing mixing of atrial and ventricular blood. Many arrhythmias arise in the AV junction.

After reaching the AV node, the electrical signal continues along the atrioventricular bundle. The atrioventricular bundle, also called the bundle of His, arises from the anterior end of the AV node, passes through the right fibrous trigone, runs between the muscular part of the interventricular septum and the right fibrous trigone, descends along the postero-inferior border of the membranous part of the septum, and finally divides there, together with fibers of the left bundle branch, into the left and right bundle branches.

The branches of the left and right bundle branches weave under the endocardium into the endocardial Purkinje fiber network, distributed mainly in the middle and lower interventricular septum, the apex, the lower part of the papillary muscles and the lower part of the free ventricular wall. Under the endocardium, Purkinje branches enter the inner ventricular wall at right or obtuse angles, forming the intramural Purkinje network, and finally connect with contractile myocardium.

Together these structures form a complete conduction system that carries the electrical impulses from the SA node to every part of the heart, enabling periodic diastolic and systolic contraction.

Scientists divide the cardiac cycle into four phases: cardiac cycle 1, cardiac cycle 2, cardiac cycle 3 and cardiac cycle 4.

Cardiac cycle 1 is atrial diastole and also the start of ventricular diastole: oxygen-poor blood from the systemic circulation and oxygen-rich blood from the lungs enter the right and left atria respectively. Cardiac cycle 2 is late ventricular diastole: the atria contract and blood is pumped into the left and right ventricles. Cardiac cycle 3 is ventricular systole: the left ventricle contracts and ejects blood into the aorta; the right ventricle contracts and ejects blood into the pulmonary artery. In cardiac cycle 4, the ventricles begin to relax again and blood once more enters the left and right atria.

This process repeats, cycle after cycle, throughout our lives, supplying us with a constant stream of oxygen and nutrients, carrying metabolic wastes out of the body, and sustaining our life. Without the tireless work of the heart, we would be without vitality.

Different people have different cardiac function. Those with good cardiac function pump blood efficiently, supplying the body with essential nutrients and oxygen; they are more energetic, fatigue less, and study and work more efficiently. People who fatigue easily often have insufficient cardiac blood supply, are more prone to cardiovascular and cerebrovascular disease, and are more troubled by mental problems.