The cardiac cycle involves contraction and relaxation of the atria and ventricles for pumping blood throughout the body. The cardiac cycle begins at one heartbeat and ends at the beginning of another. This process starts at the 4th gestational week when the heart initially begins contracting.
There are mainly two phases of the cardiac cycle the diastolic phase, and the systolic phase. The diastolic phase is where the heart remains in a relaxing state and gradually fills with blood from the veins. And in the systolic phase, the heart contract and gradually pumps blood into the periphery through the arteries.
Arteria and ventricles undergo oscillating states of systole and diastole, which means that when the ventricles are in systole, the atria are in diastole and vice versa. Here in this blog, we will discuss the cardiac cycle phases and psychological principles governing the process.
The Cardiac cycle phases
A spontaneous action potential in the sinus node initiates the process of the cardiac cycle. This stimulus triggers certain functions in the atria and ventricles, and these functions are organized into two phases:
- Diastolic Phase
- Systolic Phase
During these phases, certain events occurred and we will discuss them in detail below:
It is the first occurrence of the cardiac cycle and occurs sometimes before the electrical signal arrives at the atria. Here the atria function as conduits that allow the flow of blood into the ipsilateral ventricle. Atria also pumps residual blood into the ventricles. Blood enters the right atrium via superior and inferior vena cava and left atrium through pulmonary veins during this process of atrial diastole. The atrioventricular valves remain closed initially during this process and blood fills in the atria.
During this phase, there comes a time when the pressure in the atrium increases than the pressure in the ventricles, and this pressure difference allows blood to flow into the ventricles by opening the atrioventricular valves.
Atrial systole involves an action potential that is initiated by the sinoatrial node which transmits through the atrial myocardium. Following this, a simultaneous contraction of the atria occurs due to the electrical depolarization allowing residual blood to flow from the upper chambers of the heart to the lower chambers. There occurs a further increase in the atrial pressure due to the atrial contraction.
Initially, during the ventricular diastole, both the semilunar and atrioventricular valves remain closed. There is no change in the amount of blood during this process, but a precipitous fall in the intraventricular pressure occurs known as isovolumetric relaxation. Gradually the pressure in the ventricles becomes less than the pressure in the artery and this leads to the opening of atrioventricular valves.
Following this blood fills the ventricles that are referred to as rapid filling of the ventricles. This blood remains in the ventricle before it contracts. A small amount of blood flows into the ventricles from the venae cavae. At the end of this phase, residual blood from the artery is pumped into the ventricle. The blood which remains in the ventricle after the completion of this diastole phase is referred to as end-diastolic volume or preload.
The contraction of the ventricles is mainly referred to as ventricular systole. After depolarization of the atria, the electrical arrives at the atrioventricular node. A little delay at the AV node allows atria to contract before depolarization of the ventricles. The action potential moves to the AV node down the bundle of His and specifically to the left and right bundle branches. These fibers transmit electrical impulses via ventricular territories leading to the occurrence of ventricular contraction.
The atrioventricular valves close when the ventricles begin to contract due to the increase of pressure in the atria. But at the same time, the pressure is not enough for opening the semilunar valves. So the ventricles remain in a state of isovolumetric contraction, as there is absolutely no change in the overall volume of the ventricle.
The semilunar valves open, and blood leave the ventricle when the ventricular pressure increases in comparison to the pressure in the pressure outside. This phase is referred to as the ejection phase of the cardiac cycle. The blood left in the ventricle after the completion of the ventricular systole is known as end-systolic volume. On the other hand, the blood ejected from the ventricle is referred to as stroke volume output. The ratio of end-systolic volume and the stroke volume output is known as ejection fraction and is amounts to approximately 60%.
The cardiac cycle is the process of contraction and relaxation of the arteries and ventricles for transmitting blood throughout the body and is highly dependent on certain factors and disruption of any of these can be extremely fatal. For example, an electrolyte imbalance in the body can highly affect the phases of the cardiac cycle.