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The cardiac cycle is the sequence of contraction and relaxation of heart muscle, that ensures the most efficient pumping action of the heart is maintained. In essence the contraction of the heart is top down, bottom up with the atria acting as pump primers, whose function is to ensure that the ventricles are stretched before they contract. As the process is a continuous cycle there is no real beginning and end, rather a continual moving from phase to phase. However for the purposes of describing the phase, it is usual to begin from the moment after blood has been ejected from the heart.

To be able to understand the cycle it is essential to learn that the atrio-ventricular (AV) valves separate the atria from the ventricles and the semilunar (SL) valves separate the ventricles from the arteries (aorta and pulmonary).

The following explanation details the movement of blood, the valve position, and the state of the heart muscle throughout the cycle.

  1. The first phase is called isometric relaxation. All the valves are closed and the atria and ventricles are relaxed (in diastole). Blood is flowing into the atria from the pulmonary veins and the venae cavae. When this blood fills the atria then the pressure in the atria begins to rise
  2. This causes the AV valves to open (because atrial pressure exceeds ventricular pressure) and blood flows through the atria into the ventricles. This phase is known as the rapid filling phase. The atria and ventricles remain relaxed (in diastole) and the SL valves remain closed.
  3. The blood flow into the heart slows then ceases as the pressure in the heart equals that in the veins supplying it. There is then a brief period of diastasis where there is no movement of blood at all.
  4. Atrial systole is the next phase. The atria contract (systole) causing blood to be forced into the ventricles, the main purpose of this is to stretch the wall of the ventricles because this allows them to contract more forcibly (this is a feature of all muscles, we use it when we run stretch and contract). The ventricles are still in diastole and the SL valves are closed.
  5. As soon as Atria relax (diastole) the pressure in the atria drops below that of the ventricles so the AV valves snap shut (making the "lub" sound of the heart beat). The ventricles now begin to contract (systole) there is a brief period of isometric contraction where the ventricles contract causing the pressure in the ventricles to increase but until it exceeds the pressure in the aorta and pulmonary artery no blood moves as the SL valves are closed. Quickly the pressure in the ventricles exceeds that in the arteries and this forces the SL valves open and ejection occurs. The ventricles are contracting (systole), the atria are relaxed (diastole), the AV valves are shut and the SL valves are open. Once the contraction of the ventricles is complete they relax causing the pressure in the ventricles to drop so the SL valves close returning the heart to the phase of isometric relaxation.

This information is summarised in Table 2, below

Table 2 : Cardiac Cycle
Cardiac Cycle

The sequence of the cardiac cycle is determined entirely by events within the heart (intrinsic). The sequence of contraction of the atria and ventricles determines the state of the valves, and the flow of blood that occurs during the cardiac cycle. The heart pacemaker, and conduction system, controls the contraction of the atria and ventricles.

Figure 5 : The regulation of the cardiac cycle
The regulation of the cardiac cycle

The control of the cardiac cycle is said to be myogenic, the heartbeat is initiated from within the cardiac muscle itself. The heart pacemaker is the sino-atrial node (SAN), this is a group of specialised cardiac muscle cells, situated in the wall of the right atrium near where the vena cava enters. It determines the rate of the heartbeat. An impulse spreads from the SAN to both atria causing them to contract almost simultaneously. This impulse also reaches a similar atrio ventricular node (AVN) which lies between the two atria at the top of the septum (the wall between the two ventricles). At the AVN there is a slight time delay before the impulse is conducted. The impulse is conducted from the AVN along the Purkinje fibres (collectively making the bundle of His) through the septum to the apex of the heart. The impulse now travels from the apex of the ventricles upwards, causing the ventricle to contract, thus forcing the blood into the arteries.

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