The purpose of the electrical system of the heart is to coordinate the pumping of the four chambers of the…. The heart is divided into four chambers. The two upper chambers are called the atria. The right atrium and left…. A normal heartbeat produces a regular, identifiable pattern: the P wave, the QRS complex, and the T wave. Similar to the aortic valve, the pulmonary valve opens in ventricular systole, when the pressure in the right ventricle exceeds the pressure in the pulmonary artery.
When ventricular systole ends, pressure in the right ventricle drops rapidly, and the pressure in the pulmonary artery forces the pulmonary valve to close. The closure of the pulmonary valve also produces a sound, however it is softer than the aortic sound because the blood pressure in the right side of the heart is lower compared to the left side, due to the differences between pulmonary and systemic circulation. Valves are vulnerable to several conditions that impair their normal functions.
Two of the most common problems with the semilunar valves are stenosis and regurgitation. Valve stenosis refers to narrowing of the valves, which prevents the valve from opening fully, causing an obstruction of blood flow. Valve stenosis is often caused by calcium buildup and scarring from rheumatic fever, and may cause cardiac hypertrophy and heart failure.
Valve regurgitation is backflow through the valves that occurs when when they cannot close completely. It is the cause of most heart murmurs, and is generally a minor problem, but if severe enough, it can cause heart failure. This quantity is referred to as stroke volume. Stroke volume will normally be in the range of 70—80 mL. Since ventricular systole began with an EDV of approximately mL of blood, this means that there is still 50—60 mL of blood remaining in the ventricle following contraction.
This volume of blood is known as the end systolic volume ESV. Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG.
It too is divided into two distinct phases and lasts approximately ms. During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch small dip seen in blood pressure tracings.
The semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called the isovolumic ventricular relaxation phase , also called isovolumetric ventricular relaxation phase see image below.
In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles.
Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed see image below.
The cardiac cycle is complete. Figure 2 illustrates the relationship between the cardiac cycle and the ECG. Figure 2. Initially, both the atria and ventricles are relaxed diastole. The P wave represents depolarization of the atria and is followed by atrial contraction systole.
Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation. In a normal, healthy heart, there are only two audible heart sounds : S 1 and S 2. In both cases, as the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed.
There is a third heart sound, S 3 , but it is rarely heard in healthy individuals. The venous return to the right ventricle RV increases during inspiration due to negative intrathoracic pressure and P2 is even more delayed, so it is normal for the split of the second heart sound to widen during inspiration and to narrow during expiration.
Clinically, this is more remarkable with slow heart rates. The third heart sound S3 represents a transition from rapid to slow ventricular filling in early diastole. S3 may be heard in normal children. The fourth heart sound S4 is an abnormal late diastolic sound caused by forcible atrial contraction in the presence of decreased ventricular compliance.
In these cases, the split is usually wide and "fixed" with no difference between inspiration and expiration due to fixed RV volume see ASD section. In both conditions, the aortic valve A2 closes after the pulmonary valve P2.
Since the respiration only affects P2, its effect in paradoxical splitting is the opposite of normal, i. Murmurs are additional sounds generated by turbulent blood flow in the heart and blood vessels. Murmurs may be systolic, diastolic or continuous. Systolic murmurs are the most common types of murmurs in children and based on their timing within systole, they are classified into:. The murmur is heard shortly after S1 pulse.
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