What is cardiac cycle? Explain the stages of it.
in long way
Answers
Question
- What is cardiac cycle? Explain the stages of it.
- in long way
Answer
- The cardiac cycle includes two phases: diastole and systole. In the diastole phase, blood returns to the heart from the superior and interior vena cava and flows into the right atrium. The pressure in the right atrium increases as blood flows into it.
Answer:
Cardiac Cycle
Cardiac sounds and murmurs that arise from turbulence or vibrations within the heart and vascular system may be innocent or pathologic. It is important to understand the timing of events in the cardiac cycle as a prerequisite to understanding heart murmurs. The relationship between the normal heart cycle and that of the heart sounds is noted in Fig. 8.2.
The cardiac cycle begins with atrial systole, the sequential activation and contraction of the 2 thin-walled upper chambers. Atrial systole is followed by the delayed contraction of the more powerful lower chambers, termed ventricular systole. Ventricular systole has 3 phases:
1.
Isovolumic contraction: the short period of early contraction when the pressure builds within the ventricle but has yet to rise sufficiently to permit ejection
2.
Ventricular ejection: when the ventricles eject blood to the body (via the aorta) and to the lungs (via the pulmonary artery)
3.
Isovolumic relaxation: the period of ventricular relaxation when ejection ceases and pressure falls within the ventricles
During ventricular contraction, the atria relax (atrial diastole) and receive venous return from both the body and the lungs. Then, in ventricular diastole, the lower chambers relax, allowing initial passive filling of the thick-walled ventricles and emptying of the atria. Later, during the terminal period of ventricular relaxation, the atria contract. This atrial systole augments ventricular filling just before the onset of the next ventricular contraction.
The sequence of contractions generates pressure and blood flow through the heart. The relationship of blood volume, pressure, and flow determines opening and closing of heart valves and generates characteristic heart sounds and murmurs.
The cardiac cycle begins with the atrium and ventricle in a relaxed state. During diastole, blood flowing from central veins fills the atrium and partially fills the ventricle, passing through the sinus venosus, the sino-atrial (SA) canal and the atrio-ventricular (AV) canal. Atrial contraction (atrial systole) then ejects blood from the atrium and into the ventricle, which is still in diastole, and this completes ventricular filling. The contributions of these two phases of ventricular filling vary among species, with sometimes only ∼50% occurring via atrial contraction. In addition, the contribution of passive ventricular filling may decrease at high heart rates when the diastole is shorter.
Atrial relaxation (atrial diastole) occurs while the ventricle is contracting (ventricular systole). However, before ventricular contraction starts, the propagation of the action potential from the atrium is briefly delayed in the AV canal. This AV delay ensures that blood has sufficient time to move from the atrium and complete ventricular filling. Ventricular contraction ejects blood into the outflow tract past an opened bulbo-ventricular (BV) valve. The cardiac cycle is completed when the ventricle relaxes (ventricular diastole).