Systems Biology
The cardiac cycle encompasses the mechanical and electrical events of the heart, covering everything from the start of one heartbeat to the beginning of the next. Divided into two main parts, diastole and systole, the cardiac cycle can further be subdivided into seven distinct phases. These phases include: atrial systole, isovolumetric ventricular contraction, rapid ventricular ejection, reduced ventricular ejection, isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling. Throughout these phases, various elements, such as left ventricular pressure and volume, aortic pressure, left atrial pressure, the venous pulse, and the timing of the heart sounds, have characteristic values and change from phase to phase.
Notably, the first phase, atrial systole, is directly preceded by the P wave and represents the final phase of ventricular filling. The subsequent phases encompass ventricular contraction (initiated by the QRS complex), ventricular ejection (initiated when the left ventricular pressure exceeds the aortic pressure), reduced ejection rates (beginning with the T wave on the ECG), and relaxation. This ultimately leads back to the beginning with the next P wave, completing the cardiac cycle. Understanding these individual phases and their impact on the mechanics of the heart contributes to a comprehensive knowledge of the cardiac cycle as a whole.
Lesson Outline
<ul> <li>Cardiac cycle consists of diastole and systole</li> <ul> <li>Diastole: period when the ventricles are relaxing</li> <li>Systole: period when the ventricles are contracting</li> </ul> <li>Seven phases of the cardiac cycle:</li> <ul> <li>1. Atrial systole</li> <ul> <li>Directly preceded by P wave on ECG</li> <li>Atrial contraction and final phase of ventricular filling</li> <li>A wave on the venous pulse curve</li> <li>Possible S4 heart sound (atrial gallop)</li> </ul> <li>2. Isovolumetric ventricular contraction</li> <ul> <li>Begins during QRS complex on ECG</li> <li>AV valves close, producing S1 heart sound</li> <li>Ventricular pressure increases, but volume remains unchanged</li> </ul> <li>3. Rapid ventricular ejection</li> <ul> <li>Left ventricular pressure exceeds aortic pressure, aortic valve opens</li> <li>Blood ejected into the aorta, ventricular volume decreases</li> <li>Atrial filling begins and pressure rises</li> <li>C wave on venous pulse curve</li> </ul> <li>4. Reduced ventricular ejection</li> <ul> <li>Begins with T wave on ECG</li> <li>Ventricular pressure drops, but ejection continues at reduced rate</li> <li>Atrial filling and pressure increase</li> <li>Rising V wave on venous pulse curve</li> </ul> <li>5. Isovolumetric ventricular relaxation</li> <ul> <li>Ventricles relax, semilunar valves close, producing S2 heart sound</li> <li>Ventricular pressure decreases, but volume remains unchanged</li> <li>Dicrotic notch on aortic pressure tracing</li> </ul> <li>6. Rapid ventricular filling</li> <ul> <li>AV valves open, resulting in rapid ventricular filling</li> <li>Ventricular volume increases</li> <li>Possible S3 heart sound</li> <li>Falling phase of V wave on venous pulse curve</li> </ul> <li>7. Reduced ventricular filling (diastasis)</li> <ul> <li>Continued ventricular filling at a reduced rate</li> <li>Length determined by heart rate</li> <li>Increased heart rate shortens this phase</li> <li>Decreased end-diastolic volume and stroke volume due to faster heart rate</li> </ul> </ul> </ul>
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FAQs
The cardiac cycle consists of several stages, which are crucial for the proper functioning of the heart. The main stages are diastole, systole, atrial systole, isovolumetric ventricular contraction, ventricular ejection, and ventricular relaxation. Diastole is characterized by the relaxation of the ventricles and filling with blood, whereas systole refers to the contraction of the heart muscles that results in the ejection of blood from the ventricles. Atrial systole is the contraction of the atria that helps pump blood into the ventricles. Isovolumetric ventricular contraction is the initial phase of ventricular contraction during which the ventricles start building pressure without ejecting blood. Ventricular ejection is the stage in which blood is expelled from the ventricles. Ventricular relaxation marks the end of the cardiac cycle and allows the heart to refill with blood before the next cycle starts.
Atrial systole is an essential part of the cardiac cycle during which the atria contract to pump blood into the ventricles. This phase ensures that the ventricles are filled with the appropriate amount of blood prior to ventricular ejection and represents the final stage of diastole. Efficient atrial systole promotes complete ventricular filling, which is necessary for optimal cardiac output during each heartbeat.
Ventricular ejection is a critical stage of the cardiac cycle during which blood is expelled from the ventricles into the aorta and the pulmonary artery. It is the result of ventricular systole, a strong contraction of the ventricular muscle that increases the pressure within the ventricles. Ventricular ejection allows the heart to provide oxygenated blood to the body's tissues and organs and to return deoxygenated blood to the lungs for reoxygenation. This portion of the cardiac cycle is essential for maintaining systemic circulation and meeting the body's metabolic demands.
Isovolumetric ventricular contraction takes place during the early phase of ventricular systole, just after the closure of the atrioventricular valves (mitral and tricuspid). During this stage, the volume of blood inside the ventricles remains unchanged because both the atrioventricular and semilunar valves are closed. The significance of isovolumetric ventricular contraction lies in its role in generating the necessary pressure within the ventricles to overcome the pressure in the aorta and pulmonary artery, thereby ensuring the opening of the semilunar valves (aortic and pulmonic) and the subsequent ejection of blood from the ventricles.
Ventricular relaxation is the process by which the ventricular muscles return to their resting state following contraction (ventricular systole). It occurs during the initial stages of diastole and marks the beginning of ventricular filling. As the ventricles relax, pressure within them decreases, leading to the opening of the atrioventricular valves (mitral and tricuspid), which in turn allows the blood to flow passively from the atria into the ventricles. Ventricular relaxation is crucial for the proper functioning of the cardiac cycle, as the ventricles must be refilled with blood before the next contraction and ejection of blood can occur. During this time, the coronary arteries also fill with blood, ensuring adequate oxygen and nutrient supply to the myocardium, the heart muscle itself.
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