Transport In Mammals - Session 5

The cardiac cycle is the complete sequence of events that occur during one heartbeat. It is a vital process that ensures the heart effectively pumps blood to all parts of the body. The cardiac cycle consists of two main phases. These phases are called diastole and systole.

During diastole, the heart is in a state of relaxation. The heart prepares to receive and fill with blood. This phase can be further broken-down into several events. During atrial diastole, both atria are in diastole. In this state, they are relaxed and allow blood to flow into them.

The right-atrium receives deoxygenated blood from the body. The left-atrium receives oxygenated blood from the lungs. Towards the end of the atrial diastole, the atria contract. This contraction pushes the remaining blood into the adjacent ventricles. This phase is called atrial systole.

While the atria are contracting and relaxing, the ventricles remain in diastole. They are in a relaxed state.This allows them to expand and fill with blood. Ventricular diastole is essential for receiving an adequate preload of blood before the upcoming contraction.

Systole is the phase when the heart contracts to pump blood into the arteries. It can be further divided into two sub-phases. At the beginning of ventricular systole, the ventricles contract. The contraction causes a rise in ventricular pressure. The aortic and pulmonary valves remain closed. This closure prevents blood from being ejected into the arteries. This initial phase is termed as isovolumetric contraction because there is no change in the ventricular volume.

As the ventricles continue to undergo contraction and their pressure surpasses the pressure in the aorta and the pulmonary artery, the semilunar valves open. This allows blood to be forcefully ejected from the ventricles into the systemic circulation and the pulmonary circulation. This phase is called ventricular ejection. This phase is responsible for pushing blood out of the heart and into the major arteries.

Following ventricular ejection, the ventricles move into the diastolic phase, which includes two additional sub-phases. These sub-phases are isovolumetric relaxation and ventricular filling.During isovolumetric relaxation, the ventricles begin to relax, and their pressure decreases. However, the semilunar valves remain closed as the ventricular pressure falls below that of the aorta and pulmonary artery. Like the isovolumetric contraction phase, there is no change in ventricular volume during isovolumetric relaxation.

After isovolumetric relaxation, the ventricular filling occurs. As the ventricles continue to relax, their pressure drops further. The atrioventricular valves open. This allows blood to flow from the atria into the ventricles. Ventricular filling is crucial for refilling the ventricles with blood and preparing them for the next cardiac cycle.

The cardiac cycle is a continuous and rhythmic process. It repeats with each heartbeat to ensure that blood is constantly circulated throughout the body. The entire sequence of events is tightly regulated by electrical signals generated by the sinoatrial node and the atrioventricular node.

The sinoatrial node and the atrioventricular node are two important components of the cardiac conduction system. Cardiac conduction system controls the rhythm and coordination of the heart's contractions. The sinoatrial node is often referred as the natural pacemaker of the heart. It is a small cluster of specialized cardiac muscle cells. It is located in the right-atrium of the heart, near the opening of the superior vena cava.

The sinoatrial node generates electrical impulses that initiate each heartbeat. These impulses are responsible for setting the heart's rhythm by determining the rate at which the heart contracts. The electrical impulses produced by the sinoatrial node spread throughout the atria. This causes the atria to undergo contractraction and push blood into the ventricles.

The atrioventricular node is another cluster of specialized cardiac muscle cells. It is located in the right-atrium, near the septum that separates the atria from the ventricles.The atrioventricular node acts as a relay station in the cardiac conduction system. It receives the electrical impulses generated by the sinoatrial node. It briefly delays these impulses before transmitting them to the ventricles.

This delay is important because it allows the atria to undergo contraction and push blood into the ventricles before the ventricles themselves contract. This coordinated contraction sequence ensures efficient blood pumping. It prevents atrial and ventricular contractions from occurring simultaneously, which could result in inefficient blood flow.

The Purkyne tissues or Purkinje fibers undertake a vital role in the cardiac conduction system. They work alongside the sinoatrial node and atrioventricular node to coordinate and regulate the electrical activity of the heart. The Purkinje fibers are specialized cardiac muscle fibers located in the ventricles. They have a unique property of fast conduction. This allows them to transmit electrical signals quickly.

The electrical signals generated by the sinoatrial node pass through the atrioventricular node and the bundle of His. The Bundle of His is a bundle of specialized fibers. From there, the electrical signals are transmitted into the Purkinje network. The Purkinje fibers then rapidly distribute these impulses throughout the ventricles. The rapid transmission of electrical signals via the Purkinje fibers ensures that both ventricles undergo contraction in a coordinated and synchronized manner.