The fourth heart sound, S 4 , results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle. A few individuals may have both S 3 and S 4 , and this combined sound is referred to as S 7. Figure 3. In this illustration, the x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure. The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent flow of blood.
Murmurs are graded on a scale of 1 to 6, with 1 being the most common, the most difficult sound to detect, and the least serious. The most severe is a 6.
Phonocardiograms or auscultograms can be used to record both normal and abnormal sounds using specialized electronic stethoscopes. During auscultation, it is common practice for the clinician to ask the patient to breathe deeply. This procedure not only allows for listening to airflow, but it may also amplify heart murmurs. Inhalation increases blood flow into the right side of the heart and may increase the amplitude of right-sided heart murmurs.
Expiration partially restricts blood flow into the left side of the heart and may amplify left-sided heart murmurs. Figure 4 indicates proper placement of the bell of the stethoscope to facilitate auscultation. Figure 4. Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard. The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.
Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract atrial systole , following depolarization of the atria, and pump blood into the ventricles. The ventricles begin to contract ventricular systole , raising pressure within the ventricles.
When ventricular pressure rises above the pressure in the atria, blood flows toward the atria, producing the first heart sound, S 1 or lub. As pressure in the ventricles rises above two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax ventricular diastole , and pressure within the ventricles drops.
As ventricular pressure drops, there is a tendency for blood to flow back into the atria from the major arteries, producing the dicrotic notch in the ECG and closing the two semilunar valves. The second heart sound, S 2 or dub, occurs when the semilunar valves close.
When the pressure falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart cycle. The valves prevent backflow of blood. Failure of the valves to operate properly produces turbulent blood flow within the heart; the resulting heart murmur can often be heard with a stethoscope.
Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Search for:. Critical Thinking Question Describe one cardiac cycle, beginning with both atria and ventricles relaxed. Show an Example Answer The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.
The atria begin to contract following depolarization of the atria and pump blood into the ventricles. SoftChalk About this book Chapter Index.
Phases of the Cardiac Cycle: Isovolumetric ventricular contraction a-b : This phase marks the beginning of systole and starts with the appearance of the QRS complex on the EKG and the closure of the AV valves at point a. With all valves closed, the ventricle generates positive pressure without any change in its volume isovolumetric to overcome the semilunar valves resistance that open at point b.
Rapid ejection b-c : As the semilunar valves open at point b , there is a rapid ejection of blood due to increased ventricular contractility. The arterial pressure increases until reaching it maximum at point c. Reduced ejection c-d : This phase marks the beginning of ventricular repolarization as depicted by the onset of the T wave on the EKG.
Repolarization leads to a rapid decline in ventricular pressures and hence the reduced rate of ejection. Contraction, therefore, is said to be "isovolumic" or "isovolumetric. Some individual fibers contract isotonically i. Therefore, ventricular chamber geometry changes considerably as the heart becomes more spheroid in shape; circumference increases and atrial base-to-apex length decreases.
The rate of pressure increase in the ventricles is determined by the rate of contraction of the muscle fibers, which is determine by mechanisms governing excitation-contraction coupling. Just after the peak of the c wave is the x'-descent. Cardiovascular Physiology Concepts Richard E.
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