Cardiac cycle
The series of events that take place in a single pulse is referred to as the cardiac cycle. It entails the heart chambers contracting (diastole) and relaxing (systole), enabling the pumping of blood throughout the body. Depending on the person’s heart rate, the cardiac cycle contains many stages and repeats between 60 and 100 times per minute on average.
The main phases of the cardiac cycle:
1.Atrial Contraction (Atrial Systole):
During the phase of Atrial Contraction, also known as Atrial Systole, the atria of the heart contract. This contraction occurs in the upper chambers of the heart, known as the atria (right atrium and left atrium). Here are the key points of Atrial Contraction:
Initiation of Contraction: The contraction of the atria is initiated by an electrical impulse generated by the sinoatrial (SA) node, the natural pacemaker of the heart. This impulse spreads through the atria, causing them to contract in a coordinated manner.
Atrial Filling: Prior to contraction, the atria have been filling with blood that has returned to the heart from the body (from the systemic circulation) and the lungs (from the pulmonary circulation). The contraction of the atria helps to push the remaining blood into the ventricles.
Atrial Kick: The contraction of the atria is sometimes referred to as the “atrial kick” because it contributes to the final push of blood into the ventricles before ventricular contraction (ventricular systole) begins.
Atrioventricular (AV) Valve Status: During atrial contraction, the atrioventricular valves (tricuspid valve on the right side and mitral valve on the left side) are open, allowing blood to flow from the atria into the ventricles. The semilunar valves (pulmonary and aortic valves) are closed at this point.
It’s important to note that atrial contraction is just one phase of the cardiac cycle, and it plays a role in ensuring efficient blood flow from the atria to the ventricles. The atrial kick contributes to the filling of the ventricles and helps optimize the amount of blood pumped into the systemic and pulmonary circulations during the subsequent ventricular contraction.
2.Isovolumetric Ventricular Contraction:
Isovolumetric ventricular contraction is a phase in the cardiac cycle during which the ventricles of the heart contract, but there is no change in the volume of blood within the chambers. The term “isovolumetric” means constant volume.
Here’s a more detailed explanation of the isovolumetric ventricular contraction phase:
Beginning of Contraction: The cardiac cycle starts with the atria contracting, which sends blood into the ventricles. As the ventricles receive blood, they begin to contract.
Closure of Atrioventricular Valves: The contraction of the ventricles leads to an increase in pressure within these chambers. When the pressure in the ventricles surpasses that in the atria, the atrioventricular valves (tricuspid and mitral valves) close. This closure prevents the backflow of blood into the atria.
Semilunar Valves Closed: Simultaneously, at the beginning of the isovolumetric ventricular contraction, the semilunar valves (pulmonary and aortic valves) are still closed. These valves separate the ventricles from the pulmonary artery and aorta, respectively.
No Blood Ejection Yet: Despite the contraction of the ventricles, no blood is ejected into the pulmonary artery or aorta during this phase. The closure of the semilunar valves prevents blood from being pumped out into the arteries.
Building Pressure: The ventricles continue to contract, generating pressure within the chambers. This pressure rise is essential for overcoming the resistance in the arteries and eventually opening the semilunar valves for blood ejection.
The isovolumetric ventricular contraction phase is a brief period during which the ventricles are actively contracting, but all the heart valves are closed. This phase ensures that the blood does not flow back into the atria or leak into the major arteries until the pressure generated by ventricular contraction is sufficient to open the semilunar valves and propel blood into the pulmonary artery and aorta during the subsequent phase of ventricular ejection.
3.Ventricular Ejection:
Ventricular ejection is a crucial phase in the cardiac cycle during which the ventricles of the heart contract, leading to the expulsion of blood into the pulmonary artery from the right ventricle and into the aorta from the left ventricle. This phase follows the isovolumetric ventricular contraction and precedes the isovolumetric ventricular relaxation. Let’s delve into the details of ventricular ejection:
Pressure Buildup: As the ventricles contract, intraventricular pressure increases. The pressure in the left ventricle rises to overcome the resistance in the aorta, while the pressure in the right ventricle increases to overcome the resistance in the pulmonary artery.
Semilunar Valve Opening: Once the pressure in the ventricles surpasses the pressure in the pulmonary artery and aorta, the semilunar valves (pulmonary valve in the right ventricle and aortic valve in the left ventricle) open. This allows blood to be ejected from the ventricles into the respective arteries.
Blood Ejection: Blood is forcefully propelled into the pulmonary artery and aorta. The blood that is ejected into the pulmonary artery travels to the lungs for oxygenation, while the blood ejected into the aorta is distributed to the rest of the body.
Volume Reduction in Ventricles: As blood is ejected, the volume of blood in the ventricles decreases. The reduction in volume is known as stroke volume, which is the amount of blood ejected by one ventricle in a single contraction.
Semilunar Valve Closure: Once the ventricles start to relax, the pressure in the pulmonary artery and aorta drops. As a result, the semilunar valves close to prevent the backflow of blood into the ventricles. The closure of these valves produces the characteristic “lub” sound of the heartbeat.
It’s important to note that during ventricular ejection, the atria are in diastole (relaxation), and blood is actively filling the atria in preparation for the next cardiac cycle. The coordinated contraction and relaxation of the heart chambers ensure the efficient circulation of blood throughout the body.
4.Isovolumetric Ventricular Relaxation:
Isovolumetric ventricular relaxation is a phase in the cardiac cycle when the ventricles of the heart begin to relax after the contraction phase (ventricular ejection). During this phase, both the atrioventricular (AV) and semilunar valves are closed, and the volume of blood in the ventricles remains constant. No blood is entering or leaving the ventricles at this moment.
Here’s a more detailed explanation of isovolumetric ventricular relaxation:
Closure of Semilunar Valves: As the ventricles start to relax, the pressure within them decreases. The decrease in pressure causes the semilunar valves (pulmonary and aortic valves) to close. This closure prevents the backflow of blood from the arteries back into the ventricles.
Closure of Atrioventricular Valves: The atrioventricular valves (tricuspid and mitral valves) remain closed during this phase as well. The closure of these valves prevents blood from flowing back into the atria from the ventricles.
No Change in Ventricular Volume: The term “isovolumetric” indicates that the volume of blood in the ventricles remains constant during this phase. Although the ventricles are relaxing, no blood is actively entering or leaving them.
Preparation for Atrial Contraction: Isovolumetric ventricular relaxation sets the stage for the next phase of the cardiac cycle, which is atrial contraction. As the ventricles relax, the atria are filling with blood, preparing for their contraction to push blood into the ventricles.
This phase ensures that the blood pumped into the arteries during ventricular ejection doesn’t immediately flow back into the ventricles. The closure of both the atrioventricular and semilunar valves creates a brief moment of isovolumetric relaxation before the cycle progresses to the next stage.
5.Atrial Relaxation (Atrial Diastole):
Atrial diastole is a phase in the cardiac cycle during which the atria, the upper chambers of the heart, are in a state of relaxation. Diastole is the period of the cardiac cycle when the heart muscle is relaxed and is filling with blood. In the context of the atria specifically, atrial diastole refers to the relaxation of the atrial muscle fibers.
During atrial diastole:
The atria receive blood from the veins that return oxygen-poor (deoxygenated) blood from the body into the right atrium and oxygen-rich (oxygenated) blood from the lungs into the left atrium.
As the atria relax, their volume increases, and blood begins to flow passively from the atria into the ventricles through the open atrioventricular valves (tricuspid and mitral valves).
The filling of the ventricles during atrial diastole is facilitated by the pressure difference between the atria and ventricles. The blood flows into the ventricles because the ventricles are in diastole as well, and their pressure is lower than that of the relaxed atria.
Atrial diastole is an essential part of the cardiac cycle, ensuring the proper filling of the ventricles before the subsequent contraction phase (atrial systole and ventricular systole). The coordinated contraction and relaxation of the atria and ventricles during the cardiac cycle maintain an efficient flow of blood through the heart, allowing for the subsequent ejection of blood into the pulmonary and systemic circulation.
6.Isovolumetric Ventricular Filling:
After ventricular ejection, the ventricles start to relax. The semilunar valves close to prevent blood from flowing back into the ventricles, but the atrioventricular valves remain closed. During this phase, there is no change in the volume of blood in the ventricles because both sets of valves are closed. This period marks the early diastole, and it precedes the opening of the atrioventricular valves for the next round of ventricular filling.
                  The cardiac cycle is a continual series of actions that help the heart pump blood efficiently. It entails the atria and ventricles going through alternate stages of contraction (systole) and relaxation (diastole). The unidirectional blood flow that is ensured by these synchronized activities permits oxygenation of the lungs and the delivery of oxygenated blood to the body’s tissues. To keep the cardiovascular system’s circulation stable and efficient, the cycle repeats repeatedly.