Understanding the Heart Contraction Cycle Quiz

The Heart Contraction Cycle: From Diastole to Systole

Overview

Understanding the heart contraction cycle is essential for comprehending the cardiovascular system's functioning. It involves the coordinated activity of various structures within the heart, including the atria and ventricles, as well as the valves that control blood flow between these chambers. In this article, we delve into the details of the heart contraction cycle, focusing on the stages of diastole and systole and their roles in maintaining efficient cardiac output.

Diastole

Diastole is the phase of the heart contraction cycle where the heart expands and fills with blood. During this phase, the atria (upper chambers) and ventricles (lower chambers) both distend due to the pressure difference between the chambers. There are two main types of diastole: early diastole and late diastole.

Early Diastole

Atrioventricular (AV) node plays a crucial role in coordinating the timing of electrical signals between the atria and ventricles. As the AV node receives electrical impulses from the sinoatrial (SA) node, it sends them out at a slower pace to the ventricles via the bundle of His and Purkinje fibers. This delay ensures proper synchronization between the atrial and ventricular contractions.

Late Diastole

As the ventricles start filling with blood, they remain relatively relaxed. However, the presence of papillary muscles within the ventricles helps prevent the backward flow of blood from the ventricles to the atria. These muscles contract slightly, further reducing the risk of regurgitation.

Systole

Systole is the phase of the heart contraction cycle where the heart contracts and forces blood through the circulatory system. It is divided into three stages: ventricular contraction, ventricular ejection, and diastolic runoff.

Ventricular Contraction

After reaching maximal size during diastole, the ventricles begin to contract due to the force of the actin-myosin interaction. This process is facilitated by calcium ions entering the myocytes, causing the sarcomeres in the heart muscle to shorten. As the ventricles contract, the pressure within them rises rapidly, pushing blood against closed semilunar valves towards the pulmonary artery and aorta.

Ventricular Ejection

When the atrial pressures increase due to the closing of the tricuspid and mitral valves, the pulmonary and aortic valves open to allow blood to flow out of the ventricles. The opening of these valves creates a very brief period of time when the pressure inside the ventricles is momentarily lower than the pressure outside of them. This transient drop in pressure allows blood to leave the ventricles efficiently without any significant backflow into the heart.

Diastolic Runoff

Once the ventricular ejection is complete, the pressure inside the ventricles decreases, allowing the atrial pressures to take over again. The result is another rapid rise in atrial pressures, which in turn opens the tricuspid and mitral valves. The semilunar valves close after ejection, preventing the blood from flowing back into the ventricles.

Throughout the entire heart contraction cycle, the valves play a vital role in ensuring that blood flows in only one direction. They maintain this unidirectional flow by adjusting their positions based on the pressure differences across them.