The Cardiac Cycle

The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, including both the contraction (systole) and relaxation (diastole) of the heart muscle. The cardiac cycle is coordinated by a series of electrical signals that regulate the rhythmic contraction and relaxation of the heart chambers.

Coordinated Contraction and Relaxation

The cardiac cycle is coordinated by the electrical conduction system of the heart, which includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers. This conduction system generates and conducts electrical impulses that regulate the contraction and relaxation of the heart chambers.

The cardiac cycle can be divided into two main phases: systole and diastole.

Systole
During systole, the ventricles of the heart contract, pumping blood out of the heart into the pulmonary and systemic circulations. Systole can be further divided into two phases: atrial systole and ventricular systole.

1. Atrial Systole:
- The electrical impulse generated by the SA node spreads through the atria, causing them to contract and pump blood into the ventricles.
- The atrioventricular (AV) valves, including the tricuspid and mitral valves, open to allow blood flow from the atria to the ventricles.
- This phase contributes to the final filling of the ventricles before they contract.

2. Ventricular Systole:
- The electrical impulse reaches the ventricles, causing them to contract.
- The AV valves close to prevent backflow of blood into the atria.
- The semilunar valves, including the pulmonic and aortic valves, open to allow blood to be ejected from the ventricles into the pulmonary and systemic circulations.
- This phase is responsible for the main pumping action of the heart.

Diastole
During diastole, the heart chambers relax and fill with blood. Diastole can also be divided into two phases: isovolumetric relaxation and ventricular filling.

1. Isovolumetric Relaxation:
- After ventricular systole, the ventricles relax, and the semilunar valves close to prevent backflow of blood into the ventricles.
- The AV valves remain closed during this phase.
- No blood flows into or out of the ventricles, hence the term "isovolumetric."

2. Ventricular Filling:
- As the ventricles relax, the pressure within them drops below the pressure in the atria.
- The AV valves open, allowing blood to flow from the atria into the ventricles.
- This phase accounts for the majority of ventricular filling.

Summary
The cardiac cycle is a coordinated sequence of events that ensures the efficient pumping of blood throughout the body. During systole, the heart contracts to pump blood, while during diastole, it relaxes and fills with blood. This process is regulated by electrical signals that originate in the SA node and spread through the heart's conduction system. Understanding the cardiac cycle is crucial for comprehending the functioning of the heart and the circulation of blood in the body.