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The cardiac cycle is described as the process through which the atria and ventricles contract and relax alternately in order to pump blood throughout the body. It begins at the start of one heartbeat and finishes at the start of the next. The process begins as early as the fourth week of gestation, when the heart begins to contract.
Each cardiac cycle consists of a diastolic phase (also known as diastole), during which the heart chambers relax and fill with blood from the veins, and a systolic phase (also known as systole), during which the heart chambers contract and pump the blood to the periphery via the arteries. Both the atria and ventricles alternate between systole and diastole stages. In other words, the ventricles are in systole when the atria are in diastole, and vice versa.
Conducting System of the Heart
Myocardiocytes are a type of cell present in the heart that is capable of generating and spreading electrical activity independently of other cells. They connect via gap junctions (points of permeability) formed by the intercalated discs (where cell walls meet). Because the communication is so efficient, the cells form a syncytium in which ions can flow freely and swiftly between them. Due to this network, the heart muscles contract practically simultaneously.
The sinoatrial node (SA node) is a collection of sub-specialised cells. This area is positioned on the superior lateral wall of the right atrium, near the superior vena cava’s opening. Because the SA node is capable of contracting at a quicker rate than the rest of the heart tissue, it sets the speed of cardiac contraction. As a result, it is referred to as the heart’s pacemaker. Through preferential conductive routes, the SA node is capable of spreading its impulse across the remainder of the right and left atria.
The atrioventricular node (AV node) is a secondary area of concentrated conductive tissue positioned medially and posterior to the tricuspid valve. As with the SA node, the AV node is autonomous and capable of generating an action potential. However, because these cells are slower than those in the SA node, they function in reaction to SA node activity. There are preferential internodal channels that enable the impulse to be transmitted more efficiently to the AV node.
The AV node is coupled to a fibre network that runs down the interventricular septum and then through the ventricle walls. The initial portion of this route is referred to as the His bundle. His bundle then divides into two bundle branches on the left and right. Additionally, the left bundle branch gives rise to left posterior branches, which carry impulses to the left ventricle’s posterior side. Both the left and right bundle branches produce many branches known as Purkinje fibres that supply the ventricular myocardium with oxygen and nutrients.
Phases of Cardiac Cycle
The cardiac cycle begins with a spontaneous action potential in the sinus node, as previously stated. The atria and ventricles respond to this stimulus in a succession of ways. All of these occurrences are “structured” in two distinct phases:
- diastole (when the heart fills with blood)
- as well as systole (when the heart pumps the blood)
Numerous occurrences occur throughout these two phases, which we will discuss in the following paragraphs.
Atrial diastole
Atrial diastole occurs at the start of the cardiac cycle. It occurs a few milliseconds before the SA node’s electrical signal reaches the atria. The atria serve as channels for blood flow into the ipsilateral ventricle. Additionally, they serve as primers for the pumping of remaining blood into the ventricles. Blood enters the right atrium via the superior and inferior vena cava and the left atrium via the pulmonary veins during atrial diastole. The atrioventricular valves are closed during the early stages of this phase, and blood accumulates in the atria.
At some point, the pressure in the atrium exceeds the pressure in the corresponding ventricle on the same side. This pressure differential causes the atrioventricular valves to open, allowing blood to enter the ventricle.
Atrial Systole
An action potential is initiated by the autonomous sinoatrial node and propagated across the atrial myocardium. Electrical depolarization causes synchronous contraction of the atria, which forces any remaining blood from the upper chambers of the heart into the heart’s lower chambers. Atrial contraction results in an additional increase in atrial pressures.
Ventricular Diastole
Both the atrioventricular and semilunar valves are closed during the early phases of ventricular diastole. During this period, the volume of blood in the ventricle remains constant, while the intraventricular pressure falls precipitously. This phenomenon is referred to as isovolumetric relaxation.
The ventricular pressure eventually drops below the atrial pressure, at which point the atrioventricular valves open. This results in the quick filling of the ventricles with blood, which is frequently referred to as rapid ventricle filling. It accounts for the majority of the blood in the ventricle prior to contraction.
From the venae cavae, a tiny amount of blood rushes directly into the ventricles. Any blood remaining in the atria at the end of ventricular diastole is pumped into the ventricle. End-diastolic volume or preload refers to the entire volume of blood present in the ventricle at the end of diastole.
Ventricular Systole
Ventricular systole refers to the period during which the ventricles contract. The atrioventricular node (AV node) receives the electrical impulse shortly after the atria are depolarized. At the AV node, there is a brief delay that permits the atria to contract completely before the ventricles are depolarized. The action potential is conducted from the AV node down the His bundle, and then to the left and right bundle branches (conductive fibres that travel through the interventricular septum and branches to supply the ventricles). Electrical impulses are carried by these fibres via their respective ventricular areas, resulting in ventricular contraction.
When the ventricle begins to contract, the pressure in the ventricle exceeds that in the corresponding atrium, causing the atrioventricular valves to close. Simultaneously, there is insufficient pressure to open the semilunar valves. As a result, the ventricles are in an isovolumetric contraction condition — there is no change in the overall volume of the ventricle (end-diastolic volume).
When the pressure inside the ventricle surpasses the pressure outside the ventricle, the semilunar valves open, allowing blood to exit the ventricle. This is the heart cycle’s ejection phase. The volume of blood remaining in the ventricle at the end of the systole is referred to as the end-systolic volume (afterload, around 40–50 ml of blood).
The stroke volume output is the amount of blood actually discharged from the ventricle. The ejection fraction is the ratio of the stroke volume produced to the end-diastolic volume. It is typically around 60%.
The ventricles re-enter an isovolumetric state, while the atria continue to fill. The cycle begins again and continues indefinitely as long as the individual is alive.
Conclusion
The cardiac cycle is separated into two phases: diastole and systole, which are mutually exclusive. These events take place as the heart beats, moving blood via a network of blood vessels that delivers oxygen and nutrients throughout the body. After contracting to expel blood, the heart enters diastole, which occurs when the heart relaxes after contracting to expel more blood.
