Cardiac Cycle Classification

The human heart is a muscular organ roughly the size of a fist that pumps blood throughout the body via a network of arteries and veins. The cardiovascular system comprises the heart, arteries, veins, capillaries, and blood.

The cardiac cycle is when the ventricles and atria contract and relax alternately to circulate blood in the body. It begins at the start of a single heartbeat and ends at the beginning of the next. When the heart first begins to contract in the fourth week of pregnancy, the process begins. The systemic and pulmonary circulations are both involved in this process.

Every cardiac cycle involves diastole in which the heart chambers relax and fill with blood from veins and a systolic phase in which the heart chambers contract and pump blood to the periphery via the arteries. Both the ventricles and atria alternate between systole and diastole stages. 

Structure of The Human Heart

A heart rate, which is normally expressed in beats per minute, depicts the occurrence of a cardiac cycle. Our hearts beat 72 times every minute, resulting in 72 cardiac cycles per minute. Each ventricle pumps out around 70 mL of blood throughout a cardiac cycle, referred to as the stroke volume. The cardiac output is calculated by multiplying the stroke volume by the heart rate. The amount of blood pumped out by the ventricles in a minute is cardiac output. A heart cycle takes 0.8 seconds to complete. Atrial systole lasts 0.1 seconds, whereas ventricular systole lasts 0.3 seconds and cardiac diastole lasts 0.4 seconds. 

The left and right parts of the human heart are divided into four different chambers. Left and right atria are located in two upper chambers, whereas right and left ventricles are located in two bottom chambers. The right ventricle’s main job is to pump deoxygenated blood to the lungs through the pulmonary arteries and trunk. The left ventricle is in charge of pumping freshly oxygenated blood through the aorta to the rest of the body.

What is the Cardiac Cycle?

One heartbeat or one cycle of contraction and relaxation in the cardiac muscles constitutes a cycle. The atria and ventricles contract and relax in response to the heartbeat. Systole refers to the contraction phase, whereas diastole refers to the relaxation phase.

The four Cardiac cycle classifications are atrial systole, atrial diastole, ventricular systole, and ventricular diastole.

Atrial Systole

The atria contract as a result of the SA node-stimulated contraction wave. Because the bicuspid and tricuspid valves are open, blood is driven into the ventricles. It causes blood to flow from the two atria to the respective ventricles, which are in a state of diastole via the two atrioventricular openings. Due to the closure of their apertures into the atria, there is no prospect of blood flowing backwards from the two atria to the respective main veins. The atrial systole becomes the atrial diastole again.

Ventricular Systole

The AVN and AV bundle conduct the action potential to the ventricular side. The HIS bundle distributes it over the whole ventricular musculature, causing the ventricular muscles to contract (ventricular systole) while the atria relax (diastole), coinciding with ventricular systole.

Ventricular systole causes the tricuspid and bicuspid valves to close due to increased ventricular pressure. It arises due to the propagation of a depolarizing electric pulse in the ventricular walls. The rise in ventricular blood pressure at the start of ventricular systole causes the atrioventricular valves to close, preventing blood flow from the ventricles to the atria. Because the semilunar valves of the two arches have already closed, ventricular systole progresses.

As the ventricle contracts, its pressure surpasses the corresponding atrium, causing the atrioventricular valves to close. The pressure is insufficient to open the semilunar valves at the same time. As a result, the ventricles are in isovolumetric contraction, as the ventricle’s total volume (end-diastolic volume) does not vary.

The atria continue to fill as the ventricles return to isovolumetric relaxation. The process begins again and continues indefinitely as long as the person is alive.

Atrial diastole

A few milliseconds (ms) happens before the SA node’s electrical signal reaches the atria. Atria serve as blood conduits, allowing blood to flow freely into the ipsilateral ventricle. Atria also serve as primers, allowing remaining blood to be pumped into the ventricles. Blood enters the right atrium via the superior as well as inferior vena cava and the left atrium via the pulmonary veins during atrial diastole. The atrioventricular valves are closed in the early stages of this phase, and blood pools in the atria.

Ventricular Diastole

The first drop in intraventricular blood pressure causes the semilunar valves to close, producing a second heart sound, or ‘dup’, at the start of ventricular diastole. Because the two atrioventricular valves are already closed, the two ventricles dilate further as closed chambers, resulting in a fast drop in intraventricular blood pressure; to the point where it is less than that of the two atria. At the end of the ventricular diastole, it is followed by the opening of the two atrioventricular valves. The two atria also undergo diastole simultaneously, resulting in atrioventricular joint diastole.

Joint Diastole is the state of the heart when all four chambers are relaxed. 

Double Circulation

The term “double circulation” refers to blood that must flow through the heart twice to complete one cardiac cycle. Blood that has been deoxygenated and blood that has been oxygenated remain separate and do not mix.

Conclusion

The diastolic and systolic phases make up the major Cardiac cycle classifications. Diastole is the filling of heart chambers with blood, and it can be thought of as the heart muscles relaxing. The pumping of pure blood to various regions of the body through the blood arteries is known as systole, and it is the period during which the heart ventricles contract. To proceed with diagnosing and treating individuals with heart failure, it is critical to distinguish between diastolic and systolic dysfunctions. The heart and its surrounding arteries are viewed using ultrasonic waves in echocardiography.