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In this article, we will discuss the cardiac cycle, with reference to the relationship between blood pressure changes during systole and diastole and the opening and closing of valves. Moreover, we will also elucidate the roles of the sinoatrial node, the atrioventricular node, and the Purkinje tissue in the cardiac cycle. So, let us get started.
The Cardiac Cycle
The cardiac cycle refers to the sequence of a single heartbeat coming to an end to the start of the next
The Cardiac Cycle Stages
All the events that happen within the heart come together to become the cardiac cycle. We can say that the cardiac cycle is a sequence of contractions that guarantees that the blood is flowing in the right direction.
The cardiac cycle is divided into the following three stages:
- Cardiac diastole: The whole heart is relaxed
- Atrial diastole: It is also referred to as ventricular diastole
- Ventricular systole: It is also called atrial diastole
Cardiac Diastole
The whole heart is relaxed in cardiac diastole. It means that both the atria and ventricles are relaxed and the blood enters at low pressure through the veins, the pulmonary vein, and the vena cava into the atria. The blood pressure starts increasing because of the blood flow in the atria. It results in the opening of the AV valves which allow the blood to enter the ventricles.
Atrial Systole
At this stage of the cardiac cycle, the atria contracts when they are approximately half (50%) empty. It guarantees that the entire blood is emptied from the atria and enters the ventricles. It results in a slight increase of pressure within the ventricles, which shuts off the AV valves to prevent the backflow of blood back to the atria.
Ventricular Systole
The next stage is ventricular systole where the contraction of ventricles occurs from the bottom of the heart also referred to as the heart’s apex and upwards. Now, there is a further increase in pressure in the ventricles above the pressure inside the arteries (aorta and pulmonary artery). Because of the change in pressure, blood can flow out through the semilunar valves and allows for the blood to leave.
Valves Action and Pressure Changes
We already know that mammals have a double circulatory system. In mammals, the blood remains within the blood vessels to ensure that the pressure is maintained and regulated. The graph below indicates the changes in pressure that occur inside the heart during a single cardiac cycle.
Graph illustrating pressure variations in a single cardiac cycle - Image Source: A Level Biology- The changes in pressure occur in the left side of the heart
- The atrial pressure shown in the above figure has the lowest variations in pressure. Because of the thin walls of the atrium, the pressure is always relatively lower and hence cannot create a force to enhance the pressure. The atria fill up the blood which results in a small increase in blood, however, the amount declines again when the left AV valve opens to allow some of the blood to enter the ventricles.
- The ventricular pressure indicated by the green line is low at the beginning, however, then it undergoes a huge change in pressure during ventricular systole because as the atria contracts, the ventricles fill with the blood. It causes the shutting off of the left AV valves and a dramatic rise in pressure because of the thick muscular walls of the ventricles. With the rise in pressure, the aorta blood is passed through the semilunar valves and enters the aorta. After that, the pressure declines as the ventricles are empty and the walls are relaxed.
- The pressure in the aorta remains high at the beginning which is indicated by the red line in the graph.
- It is critical to remember the important time when the valve opens and closes. The AV valve closes when the ventricular pressure surpasses the atrial pressure as indicated in the figure above. The opening of the AV valve will only occur again when the ventricular pressure is below the atrial pressure.
- When we look above at the top of the graph, we come to know when the semilunar valves open and close. The semilunar valve opens when the ventricular pressure transcends above the aortic pressure. On the other hand, the semilunar valve will close when the ventricular pressure is below the pressure within the aortic artery.
The Cardiac Cycle – Coordination and Regulation
For the proper functioning of the heart, the events that occur during the cardiac cycle must have fine control and balance. The heart tissue is myogenic which implies that it would start its own contraction, enabling the heart to contract without requiring a body connection. If the cardiac muscle was left to contract by itself then the heart could face some potential issues. Because the atria contract quicker than the ventricle, this can result in fibrillation. Therefore, a way to control the cardiac cycle is required.
There are two nodes in the heart that keep the cardiac cycle running correctly.
- Sinoatrial Node (SAN)
- Atrioventricular node (AVN)
Sinoatrial Node (SAN)
A heartbeat begins at the region of tissue known as the sinoatrial node (SAN). The SAN is present above the right atrium. The SAN plays the role of the heart pacemaker and guarantees that the heart beats at a consistent regular rate. It is accomplished when SAN gives out regular electrical signals that spread throughout the heart and the atrial muscles, causing the contraction of the atria (atrial systole). This is what begins the contraction of the atria.
Atrioventricular Node (AVN)
The second node is referred to as the atrioventricular node (AVN) which is present near the AV valve. The function of AVN is to pass the electrical signal to the middle of the heart which is also referred to as the septum. At the AVN, there is a delay in the electrical pulse which ensures that the atria are fully empty before the control of the ventricles. After that, the electrical pulse passes the signal into highly insulated fibres known as the “Bundle of His”. The “Bundle of His” transfers the electrical signal to the bottom of the heart, also known as the apex. At the heart’s apex, the “Bundle of His” divides into two.
Purkinje Fibers
The non-insulated fibres known as Purkinje fibres spread up the walls of the ventricle and result in the contraction of ventricles from the heart’s bottom, so that the blood can be forced out from the ventricles, out of the semilunar valves, and towards the pulmonary artery and the aorta.
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