EMS Update: Anatomy & Physiology of the Heart
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The cardiac cycle is defined as the series of events that occur during a single heartbeat. It consists of two phases: systole and diastole. Systole refers to a period of contraction by the heart muscle, whereas diastole refers to a period of relaxation by the heart muscle. The heart does not undergo the cardiac cycle as one unit. Rather, the atria and ventricles act as separate units, entering systole and diastole at different times. However, both atria and both ventricles act simultaneously.
The duration of these events during a typical cardiac cycle is approximately 0.8 seconds. Because the series of contractions and relaxations of the different chambers of the heart occur cyclically, there is no real "beginning" or "ending" of the cardiac cycle. However, for the purpose of discussion, we will artificially label atrial systole, or atrial contraction, as the "beginning" of the cycle.
Atrial Contraction
Just prior to atrial contraction, both the atria and ventricles are relaxed. The pulmonic and aortic valves connecting the ventricles to the major arteries are closed. However, the atrioventricular valves that connect the atria to the ventricles are open. During this period of relaxation, blood flows continually from the veins into the atria, filling these chambers. Some of this blood passes through the open atrioventricular valves to the ventricles. When the atria contract, they force the remaining blood contained in them to flow into the ventricles. By the end of atrial contraction, the ventricles contain a full supply of blood, while the atria contain virtually none.
Ventricular Systole
Ventricular systole occurs only a fraction of a second after atrial contraction. As the ventricles begin to contract, the pressure within them quickly exceeds that within the atria, forcing the atrioventricular valves to close. This action prevents a backward flow of blood (regurgitation) from being forced into the atria from the ventricles. As ventricular contraction continues, the pressure within the ventricles reaches a point where it exceeds that in the aorta and the pulmonary arteries. At this point, the aortic and pulmonic valves open, and the blood from the ventricles is ejected through these valves into the aorta and pulmonary artery, respectively.
At about the same time that the ventricles enter systole, the atria begin to relax. During this period, blood flows into the left atrium from the pulmonary veins and into the right atrium from the superior and inferior vena cavae. However, this blood remains in the atria during ventricular systole, since the high pressure in the ventricles during its contraction forces the atrioventricular valves to remain closed.
Ventricular Diastole
When ventricular diastole begins, the ventricles start to relax and the pressure within the ventricles decreases. Once the ventricular pressure becomes lower than the pressure in the aorta and the pulmonary artery, the pulmonic and aortic valves close, preventing regurgitation of blood into the ventricles. As the ventricles fully relax, the ventricular pressure becomes lower than the pressure in the atria. This allows the atrioventricular (mitral and tricuspid) valves to open.
Because the ventricles are now in diastole and the atrioventricular valves are open, some of the blood that has been flowing into the atria flows through the open valves into the ventricles. The ventricles reach about 80% of their capacity before the atria begin to contract and the cardiac cycle is repeated.
Regulation of the Cardiac Cycle
In order for the heart to effectively complete each cardiac cycle, cardiac muscle cells must contract more or less simultaneously. The coordination of cardiac muscle contractions is made possible by two factors. First, the cells in cardiac muscle are tightly interwoven, so that muscle contraction spreads rapidly throughout the heart. Second, the heart contains specialized cardiac muscle cells that are organized into a conducting system, spreading muscle contraction impulses throughout the heart at regular intervals.
The heart has the unique ability to beat (contract) on its own. Although in real life it is assisted in this function by nerves and hormones in the blood, it still functions even when removed from these influences. This is best illustrated by the donor organ in heart transplantations.
The mechanism by which the heart generates and transmits the signal to contract is quite complex. Actually, a minute electrical current of about 2 millivolts is generated and passes down from its origin through the conducting system of the heart, causing muscular contraction of each chamber as it passes through it. The impulse or action potential normally arises in a specialized group of cells located in the wall of the right atrium called the sinoatrial (SA) node. Normally, an electrical difference (potential difference) exists between the inside and outside of all cells, which is due to the differences in electrical charges inside the cell from those outside the cell. The impulse is initiated by passage of electrical charges across the membrane (covering) of the cell, causing a change in the potential difference and creating an action potential.
Leaving the SA node, the impulse passes through both atria causing them to contract thus helping blood pass into their respective ventricles. On its way to the ventricles, the action potential next encounters the atrioventricular (AV) node, another group of specialized cells. From there the impulse passes down an anatomical pathway called the Bundle of His which branches out and spreads throughout both ventricles (Purkinje fibers), resulting in contraction of the ventricles.
Figure 5 illustrates the components of the conducting system of the heart.
Figure 5: The Conducting System of the Heart
Anatomy & Physiology of the Heart
Overview of the Cardiovacular System
The Cardiac Cycle
Hemodynamics of the Cardiac System