Heart Function

Basic Anatomy and Function of the Heart

The sole purpose of the heart is to pump blood. The heart is constructed of a unique type of muscle, called cardiac muscle, which is specially designed to contract continuously for an entire lifetime. The contractions of cardiac muscle cause blood to be pumped to all the body's organs, including the lungs, brain, kidneys, muscles, and heart itself. Unlike skeletal muscle (found in the arms, legs, and elsewhere throughout the body) that can stop contracting and take a rest, the heart muscle must contract tirelessly during every moment of life.

The cardiac muscle is shaped to form four cardiac chambers (see figure). Each of the two upper chambers is called an atrium, and each lower chamber is called a ventricle. Each chamber of the heart has its own function. The right atrium receives the "used up" blood from all parts of the body. This blood is depleted of oxygen but contains high levels of carbon dioxide from metabolism in the body's tissues. The right atrium contracts and pumps this blood through the tricuspid valve into the right ventricle. The blood is then pumped from the right ventricle through the pulmonic valve into the pulmonary artery, which delivers the blood to the lungs. In the lungs the blood takes on oxygen and gives-up its carbon dioxide. During normal breathing carbon dioxide is expelled from the lungs while oxygen is taken in. The newly oxygenated blood travels from the lungs through the pulmonary veins to the left atrium. The oxygenated blood is pumped from the left atrium, through the mitral valve, into the left ventricle. The left ventricle is the more important of the two lower chambers, and it is supplied with blood through three coronary arteries. Finally, during contraction of the left ventricle, the oxygen-rich blood is pumped through the aortic valve into the aorta and branching arteries to be delivered to all the organs of the body, including the brain, heart, kidneys, and muscles. The body tissues utilize the oxygen for fuel and deliver the waste carbon dioxide back to the blood, which returns to the right atrium through veins, and the cycle continues.

Schematic illustration of the anatomy of the normal heart. The heart consists of four chambers. The two upper chambers are called the atria, while the two lower chambers are referred to as ventricles. The tricuspid and mitral valves separate the right atrium and left atrium from the right ventricle and left ventricle, respectively. The normal cardiac electrical circuit includes the sinoatrial (SA) node, atrioventricular (AV) node, His bundle, bundle branches, and Purkinje fiber network. Activation of the heart using the normal electrical circuit is called sinus rhythm. Conduction of electrical impulses through the normal electrical circuit triggers the ventricles to contract and pump blood to the lungs (right ventricle) or the body as a whole (left ventricle). See text for complete discussion.

The heart has an electrical system (see figure) that both triggers the heart chambers to pump in a coordinated fashion, as well as regulates the rate at which pumping occurs. To do this, the heart has its own built-in "spark plug" or pacemaker called the sinus or sinoatrial (SA) node located in the upper portion of the right atrium. It sends out electrical impulses at regular intervals to both the right and left atria. In general, SA node impulses are generally emitted at a rate of 40 to 150 impulses/min. The SA node "fires" at a rate that is exquisitely sensitive to the metabolic needs of the body. During vigorous exercise, the sinus node firing rate will be very high because large volumes of oxygenated blood are needed by the body tissues. On the other hand, while sleeping the sinus firing rate will be much slower because less oxygenated blood is needed by the tissues. The SA node electrical impulses cause the right and left atrium to contract and pump blood into the ventricles in a coordinated fashion.

From the sinus node the electrical impulse travels to the atrioventricular (AV) node located in the middle of heart near the junction of all four chambers. The AV node is a critical structure because it acts as the "toll booth", regulating the flow of electrical "traffic" from the atria to the ventricles. The AV node "protects" the ventricles from very rapid electrical activity in the atria - such as occurs during atrial fibrillation - by limiting the number of impulses that can conduct to the ventricles. The impulse conducts through the AV node to reach the His bundle. The His bundle, in turn, bifurcates into a right bundle branch and a left bundle branch. Once in the ventricles, the impulse is conducted to individual cardiac muscle cells via the extensively branching Purkinje fiber network. By following these pathways, the original cardiac impulse originating in the SA node is transmitted to individual cardiac muscle cells in the ventricles, thereby triggering them to contract virtually simultaneously and forcing the blood out of the ventricular chambers. This synchronized sequence of atrioventricular contraction occurring under the influence of the sinus node and using the heart's normal electrical circuit is termed sinus rhythm. Sinus rhythm also provides for atrioventricular synchrony that maximizes the amount of blood pumped per each cardiac cycle. AV synchrony means that the ventricles only pump blood after they have been maximally filled from above by the coordinated atrial contractions. This AV synchrony optimizes cardiac output and produces the greatest efficiency of the cardiovascular system.

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