In size and in shape the heart resembles a closed fist. Each day the heart, a marvel of efficiency and dependability, pumps nearly 2000 gallons of blood usually with no maintenance beyond a sensible diet and lifestyle.
The heart is a hollow muscular organ, which receives blood from the veins and propels it into the arteries. It has four cavities - two receiving chambers (atria) and two ejecting chambers (ventricles). The left atrium receives oxygenated blood from the lungs and delivers the blood to the left ventricle. The left ventricle (the heart's main pumping chamber) forces the blood via the aorta (the body's main artery) through the arterial system to the capillaries. As blood flows through the capillaries, oxygen and nutrients are exchanged for waste and carbon dioxide in body tissue. The capillaries drain into the veins and the oxygen-depleted blood flows from the network of veins to the right atrium of the heart. The right atrium passes blood from the veins to the right ventricle, which then pumps blood into the lungs to be oxygenated.
Four major valves, functioning like one way doors, keep blood flowing in one direction. Blood enters the right ventricle from the right atrium through the right atrioventricular (tricuspid) valve and leaves through the pulmonary valve. Blood enters the left ventricle from the left atrium through the left atrioventricular (mitral) valve and leaves to the aorta through the aortic valve.
The heart as a working muscle requires a constant supply of nutrient blood. The left and right coronary arteries, which emerge from the aorta, carry oxygen rich blood to the heart itself. The left main coronary artery divides into the left anterior descending artery, which travels down the front of the heart, and the left circumflex artery that circles around the left side and back of the heart. The right coronary artery travels to the right side and back of the heart.
The heart is a pump that works in cycles, similar to the cylinder of an automobile engine. During one phase of each cycle, the heart chambers fill; during the next phase of the cycle the muscular walls of the heart contract and the chambers empty. The atria contract somewhat earlier than the ventricles to allow the ventricles, which are downstream from the atria, to fill more completely.
To accomplish this pumping, the heart spontaneously and repetitively generates electrical impulses, which organize and trigger the sequence of heart muscle contractions during each heartbeat. The pattern and timing of the sequence determines the rhythm. Arrhythmia, a disruption in this normal rhythm, may therefore prevent the heart from effectively pumping blood to the lungs and body.
Normally, the electrical impulse to generate a heartbeat originates high in the right atrium at the sinoatrial (SA) node, a group of special non-muscle cells. The impulse leaves the SA node to spread radially across both the left and right atria causing them to contract simultaneously. The signals converge at the atrioventricular (AV) node, the relay station or electrical gate between the otherwise electrically isolated atria and ventricles. The delay provided by the AV node enables both atria to empty completely before the electrical impulse reaches the ventricles.
After the AV node organizes and slows down the impulse, the impulse enters a network of nerve like fibers (His-Purkinje system) designed to carry the signal to the tip of the heart. The His bundle, the topmost part of the rapidly conducting His-Purkinje system, penetrates the electrically insulating fibrous layer between the atria and ventricles and delivers the impulse to the ventricular side. The impulse then follows the His bundle as it divides into a left and a right bundle branch. Further branching of the Purkinje fibers allows the electrical impulse to distribute rapidly throughout the ventricular muscle and provide orderly ventricular contraction (heartbeat) and pumping of blood. After contracting, the heart muscle cells dissipate the electrical impulse and prepare to receive the next impulse. When the SA node generates a new electrical signal, the cycle repeats.
If for some reason the heart's SA node, fails to produce an impulse, other heart tissues are able to produce impulses as well. However, the spontaneous firing rate of these subsidiary pacemakers is slower than the SA node's rate. Failure of the SA node to generate an impulse may result in decreased cardiac output and blood flow.
As suggested above, the heart's pumping efficiency depends upon synchronous, organized contraction of the chambers. The (AV) node, responsible for the carriage of the impulse from the atria to the ventricles, limits the number of impulses that can pass through it in a given time period, preventing the ventricles from being driven at excessive rates when atrial disease produces abnormal impulses.
The AV node's delaying and screening effects are under the control of the autonomic nervous system and hormones. At rest, under the influence of "decelerator" nerves (parasympathetic, or "vagal" nerves), inhibitory effects dominate, resulting in delaying and screening of impulse transmission. During exercise, "accelerator nerves" (sympathetic or adrenergic nerves) are activated, and "decelerator" nerve activity is suppressed, which facilitates AV node signal transmission necessary for achieving rapid heart rates. If the atrial rate is excessive, however, impulses entering the AV node from the atria are extinguished and do not continue on down to the ventricles. This feature is important in atrial disease states.
Electrical signals rate of travel depends upon the heart tissue through which they are passing. These signals, although electrical, travel at a rate much slower than electricity through copper wires. The heart's specialized conduction tissues transmit signals relatively rapidly; heart muscle conducts signals more slowly.
Electrical impulses cannot cross or be conducted by the "skeleton" of the heart, an inert fibrous structure that separates the atria from the ventricles and to which the valves are attached. The His bundle, which traverses the cardiac skeleton, represents the only normal electric connection between the atria and ventricles. In certain disease states, extra connections or "accessory pathways" are present and cross the cardiac skeleton. These pathways can allow the inappropriate transmission of signals from the atria to the ventricles or the ventricles to the atria, resulting in "short circuits".
Just as fibrous tissue blocks signal transmission between the atria and the ventricles, fibrous tissue that arises as a result of scarring within the atria or ventricles blocks signal transmission within the chambers. For example, when healing from a myocardial infarction occurs, scar tissue replaces or mixes in with normal heart muscle. This juxtaposition of tissues can result in the formation of a short circuit within the damaged heart wall.