Physiology of the Cardiac Cycle

The cardiac cycle is the sequential, repetitive events occurring in the heart. It is composed of systole and diastole of both the atria and ventricles. The heart rate denotes the occurrence of the cardiac cycle. 

What is the cardiac cycle in simple explanation?

The series of steps that allow the heart to pump in and pump out blood is known as a cardiac cycle. There are 7 phases of the cardiac cycle covered by its two major phases. These two phases are the muscle relaxation phase or diastole and the muscle contraction phase or systole.  

The heart beats seventy-two times in a  minute. It means that it states that seventy-two cardiac cycles take place in a  minute. If you calculate this rate, the entire cycle becomes roughly 0.8 seconds. Let us learn about the cardiac cycle in detail.

Physiology of the Cardiac Cycle

There are four chambers in the human heart, comprising left and right halves. The left and right atria comprise two upper chambers, and the left and right ventricles from the lower chamber. Newly oxygenated blood is pumped to the body by the left ventricle through the aorta. 

Sequential Phases in the Cardiac Cycle Process:

There are two fundamental cardiac cycle phases. These two primary phases cover the more minor 7 phases of the cardiac cycle.

Diastolic Phase

The description of the cardiac cycle takes place from late diastole. This takes place when the ventricles and atria are in a state of relaxation. Also, the opening of the Atrioventricular valves takes place. Below are the various more minor phases of the diastolic phase of the cardiac cycle:

1. Slow ventricular filling – The intraventricular pressure is slightly lower than the pressure within the atria. Therefore, the opening of AV valves takes place, thereby facilitating a slow flowing of blood from the atrium to the ventricle.
2. Atrial Contraction –  A spontaneous depolarization of the Sino Atrial node pacemaker cells takes place. This generates an action potential. The electrical impulse that results from this is conducted very rapidly throughout the atria. This triggers atrial contraction. 

• As a result of atrial contraction, ejection of most of the remaining arterial blood occurs via the valves into the ventricles. This has only 10% of ventricular filling in a state of rest.
• During the exercise phase, the shortening of the diastole takes place. This contraction makes up about 40% of ventricular filling. The generation of the pressure takes place during atrial contraction. The transmission of this pressure takes place along the venae cavae and pulmonary veins. This is because they lack valves.

1. Isovolumetric relaxation –  After the closure of the semilunar valves, it takes a short period for the ventricles to come to a state of relaxation. This way, its pressure reduces to go lower than that of the atria. Throughout late systole and isovolumetric relaxation, a gradual rise of the atrial pressure occurs because of the venous return. This return takes place from the venae cavae and the lungs.
2. Rapid ventricular filling – Once the exceeding of atrial pressure occurs to the ventricular pressure, the opening of the AV valves happens. 

Systolic Phase

Below are the various more minor phases of the systolic phase of the cardiac cycle:

Isovolumetric Contraction – The action potential continues through the AV node and its conduction takes place via the ventricles. This takes place by a system known as the His–Purkinje system, represented on the ECG by the QRS complex.  

Rapid Ejection – Once the exceeding of the ventricular pressure occurs in comparison to the pressure in the pulmonary artery and aorta, the opening of the semilunar valves happens. As such, the ejection of the blood takes place from the ventricles. 

Reduced Ejection: This phase happens exactly after 200 milliseconds after the QRS and the initiation of ventricular contraction. Here, the falling of the ventricular pressure takes place slightly so that it goes below the outflow tract pressure. Nevertheless, the outward flow still occurs because of the blood’s kinetic energy.   

Stroke Volume

Stroke Volume refers to blood volume whose ejection takes place from the LV per heartbeat. Before the contraction, the blood volume in the LV is the left ventricular end-diastolic volume (LVEDV). Moreover, the volume of blood that remains in the LV after contracting is the left ventricular end-systolic volume (LVESV). It is essential to determine stroke volume when analysing the cardiac heart cycle. 

Stroke volume = EDV – ESV.

What is the cardiac cycle of the heart?                

       S.A. Node (Pacemaker)

                                      ↓

                        Inter nodal pathway

            ↓

                             A.V. Node

                                     ↓

                           Bundle of His

                                    ↓

                  Purkinje fibres (Rt & Lt)

Conclusion

The cardiac cycle refers to the sequence of various stages or events occurring within the heart. The beating of the heart takes place seventy-two times within sixty seconds. A cardiac cycle diagram helps learn about it in a detailed manner. The diastolic and systolic phases are two major phases in this cycle. The diastolic phase consists of rapid ventricular filling, atrial contraction, slow ventricular filling, and isovolumetric relaxation. In contrast, the systolic phase consists of a rapid ejection, reduced ejection, and isovolumetric contraction.