CARDIAC CYCLE

The cardiac cycle consists of several phases that describe the events occurring during each heartbeat, while the electrical activity of the heart involves the generation and propagation of electrical impulses that coordinate the con­traction and relaxation of cardiac muscle cells.

4.4.1 Phases of the Cardiac Cycle

A. Diastole:

• Early Diastole:

- Period of ventricular relaxation and filling.

- Blood flows passively from the atria into the ventricles through the open atrioven­tricular valves.

• Late Diastole:

- Atrial contraction (atrial systole) occurs, completing ventricular filling.

- The atria contract, pushing the remaining blood into the ventricles.

B. Systole:

• Isovolumetric Contraction:

- Ventricular contraction begins, causing pressure to rise.

- Atrioventricular valves close (S1 heart sound) as ventricular pressure exceeds atrial pressure, preventing backflow into the atria.

• Ventricular Ejection:

- Ventricular pressure exceeds arterial pres­sure, leading to the opening of the semilu­nar valves.

- Blood is ejected from the ventricles into the pulmonary artery and aorta.

• Isovolumetric Relaxation:

- Ventricular relaxation begins, causing ventricular pressure to fall.

- Semilunar valves close (S2 heart sound) as arterial pressure exceeds ventricular pressure, preventing backflow into the ventricles.

4.4.2 Electrical Activity of the Heart

A. Sinoatrial (SA) Node:

• The SA node is the pacemaker of the heart, located in the right atrium near the opening of the superior vena cava.

• It generates rhythmic electrical impulses that initiate each heartbeat.

• The impulses spread through the atria, caus­ing atrial contraction.

B. Atrioventricular (AV) Node:

• The AV node is located at the junction of the atria and ventricles.

• It delays the transmission of electrical impulses to allow for complete atrial contraction before ventricular contraction.

• The AV node conducts the impulses to the bundle of His.

C. Bundle of His and Purkinje Fibers:

• The bundle of His is a bundle of special­ized cardiac muscle fibers that conduct the impulses from the AV node.

• It branches into the left and right bundle branches, which extend through the interven­tricular septum.

• The Purkinje fibers spread throughout the ven­tricles, transmitting the impulses rapidly and causing coordinated ventricular contraction.

D. Electrocardiogram (ECG):

• An ECG records the electrical activity of the heart, providing valuable information about its rhythm and function.

• P waves represent atrial depolarization, QRS complex represents ventricular depo­larization, and T waves represent ventricular repolarization.

Understanding the phases of the cardiac cycle and the elec­trical activity of the heart is essential for assessing cardiac function and diagnosing various cardiac conditions.

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