| The Normal 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 (Figure 1). 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.  Figure 1. 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 (Figure 1) 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. |
