Heart Cycle|Cardiac Cycle.
The heart beats 72 times per minute and the cardiac cycle refers to the cardiac events related to blood flow occurring in the heart during one complete heart beat. This blood flow is directed by the systole and diastole of the atria and the ventricles and the pressure changes which determine the action of the heart valves and the direction of blood flow through the heart chambers.
Cardiac Cycle Phases.
The events of the heart cycle described below constitute one heart beat and the time duration is less than one second. You can take any stage as the start of the cardiac cycle, but for convenience let us sequence it as explained below. The duration of each phase of the cardiac cycle is taken at rest.
Diagram of cardiac cycle and its phases.
Click on image to enlarge.
Diastasis.
Diastasis is a relatively passive period of slow ventricular filling just prior to the atrial systole. Major ventricular filling takes place during this phase of diastasis. The pressure in the veins of the heart (vena cavae on the right and the pulmonary veins on the left) is more than that in the atria and therefore atrial filling begins.
The AV valves are also open and blood flows passively into the relaxed ventricles. The semilunar valves of the aorta and the pulmonary artery are closed because the pressure in the arteries is more than in the ventricles. Blood therefore collects in the ventricles and does not flow into the arteries.
Duration. This phase lasts for 0.19 seconds and ends abruptly with the onset of the atrial systole.
On ECG. It is indicated as P wave on ECG.
Atrial Contraction.
After the diastasis and during late ventricular diastole, the atria contract. This causes increase in intra atrial pressure by about 5 mm of Hg. Due to atrial contraction, blood is ejected into the ventricles. Only 20% of ventricular filling takes place during this phase, the major filling taking place during diastasis.
The pressure in the ventricles has now increased and is more than that in the atria. This forces the AV valves to snap shut, thereby preventing back flow of blood into the atria.
Duration. This phase lasts for 0.11 seconds.
On ECG. This is indicated by the beginning of the QRS complex.
Ventricular Isometric Contraction.
This phase indicates the onset of ventricular contraction. The aortic and the pulmonary valves are closed and the AV valves are on the verge of closing or are already closed. The volume of blood is constant but due to onset of ventricular contraction, the pressure in the ventricles begins to rise rapidly.
Since the heart muscle cannot shorten during this period it is called the isometric contraction or the isometric phase of systole. The pressure in the aorta at this stage is 80 mm of Hg. The closure of the AV valves causes turbulence of blood which is heard as the first sound – the lub of lub-dub.
Duration of this phase is 0.5 seconds.
On ECG. This phase is indicated on ECG just after the onset of the QRS complex
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