Write a short note on cardiac cycle.

Cardiac Cycle: The Rhythmic Process of Heart Contraction and Relaxation

The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, encompassing both systole (contraction) and diastole (relaxation) phases of the heart. This rhythmic cycle is essential for pumping blood throughout the body and maintaining systemic circulation.

1. Phases of the Cardiac Cycle:

• Atrial Contraction (Atrial Systole): The cardiac cycle begins with atrial contraction, initiated by the electrical impulses generated by the sinoatrial (SA) node. This contraction forces blood from the atria into the ventricles through the open atrioventricular (AV) valves.

• Ventricular Contraction (Ventricular Systole): Following atrial contraction, the electrical signal reaches the atrioventricular (AV) node, triggering ventricular contraction. The ventricles contract, generating pressure that closes the AV valves and opens the semilunar valves, allowing blood to be ejected into the pulmonary artery and aorta.

• Isovolumetric Relaxation: After ventricular contraction, both the atria and ventricles enter a brief period of relaxation known as isovolumetric relaxation. During this phase, all four heart valves are closed, preventing blood from flowing back into the atria or ventricles.

• Ventricular Filling (Early Diastole): As the ventricles relax, pressure within them decreases, causing blood to flow passively from the atria into the ventricles through the open AV valves. This phase, known as ventricular filling, accounts for most of the diastolic filling of the ventricles.

2. Regulation of the Cardiac Cycle:

• Autonomic Nervous System: The cardiac cycle is regulated by the autonomic nervous system, with sympathetic stimulation increasing heart rate and contractility, while parasympathetic stimulation decreases heart rate and contractility.

• Cardiac Conduction System: The cardiac cycle is coordinated by the cardiac conduction system, which includes specialized structures such as the SA node, AV node, bundle of His, and Purkinje fibers. These structures generate and transmit electrical impulses that coordinate atrial and ventricular contraction.

3. Clinical Significance:

• Electrocardiography (ECG/EKG): Electrocardiography is a diagnostic tool used to assess cardiac function by recording the electrical activity of the heart during the cardiac cycle. Changes in ECG patterns can indicate abnormalities such as arrhythmias, myocardial ischemia, or myocardial infarction.

• Cardiovascular Disorders: Disorders affecting the cardiac cycle can have significant clinical implications, including heart failure, arrhythmias, valvular heart disease, and coronary artery disease. Understanding the mechanisms underlying these disorders is essential for accurate diagnosis and treatment.

In summary, the cardiac cycle is a complex and coordinated process involving the rhythmic contraction and relaxation of the heart chambers to facilitate blood flow throughout the body. Understanding the phases and regulation of the cardiac cycle is crucial for assessing cardiac function, diagnosing cardiovascular disorders, and optimizing patient care.