The human heart is one of the most vital organs of the circulatory system that pumps blood through blood vessels. The human heart and the cardiac cycle is important as it helps in understanding how oxygen and nutrients are carried to body parts. The heart is also responsible for removing metabolic waste and CO2 from cells, further excreted out of the body. The human heart cardiac cycle is a sequence of mechanical and chemical events from one heartbeat to the start of the next heartbeat. The cardiac cycle explains how blood pumps cyclically through contraction (systole) and relaxation (diastole). 

Importance of the Human Heart 

• The human heart is triangular-shaped and is the size of the left-hand closed fist. It is generally 9 cm in width and 12 cm in length.
• It is enclosed in a double-walled sac known as the pericardium, and the pericardial fluid separates the pericardium and heart wall. The heart is made of cardiac muscle fibres.
• The human heart consists of three layers: epicardium, myocardium, and endocardium. 

1. Epicardium is an external layer that protects the outer section. 
2. Myocardium is the thickest and middle layer made of cardiac muscle cells. 
3. Endocardium is the innermost layer covering blood valves and preventing blood clots.

Human Heart and the Cardiac Cycle Components 

The human heart consists of two upper chambers, forming the right and left atria. Two lower chambers form the right and left ventricles.

• The left auriculo-ventricular is where the left atrium opens into the left ventricle, and the right auriculo-ventricular is where the right atrium opens into the right ventricle.
• Arteries are blood vessels that carry oxygenated blood from the heart to different body parts. However, only the pulmonary artery carries deoxygenated blood from the right side of the heart to the lungs.
• Veins carry deoxygenated blood from various organs to the heart. However, only the pulmonary vein carries oxygenated blood from the lungs to the heart.
• The heart consists of four valves which are:

1. Tricuspid valve: protects the opening between the right atrium and right ventricle.
2. Bicuspid valve: protects the opening between the left atrium and left ventricle.
3. Aortic semilunar valves: are located at the point of origin of an aorta from the left ventricle.
4. Pulmonary semilunar valves: are located at the right ventricle’s opening and into the pulmonary artery.

What is the Cardiac Cycle of the Heart? 

The human heart and the cardiac cycle help in understanding the process of blood circulation and how the heart pumps blood. It is the heart’s function from the start of one heartbeat to the beginning of the next, consisting of two periods – diastole (relaxation that fills the heart with blood) and systole (contraction that pumps the blood).

Following are the human heart cardiac cycle phases.

Atrial Systole

• Blood enters the right atrium through the inferior and superior vena cava during this process. Blood enters from the lungs through pulmonary veins into the left atrium. 70% of the blood flows into the ventricles.
• The atrium walls contract and push the remaining 30% of the blood into the ventricles.
• The blood flows from the atria into the ventricles during contraction through the atrioventricular valves. The semilunar valve closure prevents blood flow from the ventricles back into the atrium.

Atrial Diastole

It is the second sequence of the human heart and the cardiac cycle after the atrial systole. As the blood from the lungs enters the left and right atria, it is followed by simultaneous atrial relaxation and ventricular contraction.

Ventricle Systole

• This is the period of contraction of the ventricles leading to an increase in ventricular pressure. As this happens, the atrioventricular valves close, preventing the blood’s backflow to the atria.
• In the ventricular contraction, the blood depleted of oxygen passes from the right ventricle to the pulmonary artery. The pulmonary valve prevents the backflow of blood into the ventricle.
• A strong ventricle contraction leads to the opening of the semilunar valves, resulting in the pumping of the blood from the ventricles to the lungs via the pulmonary artery.
• The pulmonary artery further carries deoxygenated blood to the lungs, where it collects oxygen to return to the heart’s left atrium.

Ventricular Diastole

• This phase follows the ventricular systole, where the ventricular pressure decreases. It leads to the closure of the semilunar valves to prevent backflow from the pulmonary artery and aorta.
• Simultaneously, it opens the atrioventricular (tricuspid and mitral) valve to allow blood flow from the atria to the respective ventricles.
• The atria and ventricles relax during the ventricular diastole phase, which opens the atrioventricular valves. The opening leads to the return of the oxygen-depleted blood to the heart from various body parts.

Conclusion 

The human heart is the circulatory system’s crucial organ, initiating blood circulation and pumping blood through the body. The human heart and its cardiac cycle help understand how the circulatory system works in humans. As the human heart pumps blood, the cardiac cycle is a sequence of events starting from one heartbeat to the beginning of the next heartbeat. The human heart cardiac cycle defines oxygenated blood flow from the heart to all body parts and deoxygenated blood flow from the lungs to the heart.