Cardiac cycle Phases
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The heart is an organ that has a pump function. With each heartbeat there is an increase in pressure in the circulatory system, thus causing a pressure difference and guaranteeing blood flow to all organs.
The cardiac cycle is the sequence of electrical and mechanical events, which repeat with each heartbeat. This has two main phases, systole, where the heart muscle contracts, and diastole, where it relaxes.
The phases of the cardiac cycle happen in the same way in the right and left chambers; but there is a difference in pressures and volumes. For practical purposes, we are going to explain this cycle in the left cavities.
In the left ventricle, there is a residual volume of approximately 50 milliliters of blood from the previous cardiac cycle; and has a pressure of 2 to 3 millimeters of mercury. Initially, blood enters from the pulmonary veins to the left atrium; filling this cavity until an increase in atrial pressure is generated; which exceeds that of the ventricle; Therefore, the atrioventricular valves open, thus initiating the so-called ventricular filling satge or inflow stage.
When 2/3 of the ventricular filling time has occurred; The sinus node generates an electrical impulse, which initiates atrial systole, which is represented by the P wave on the electrocardiogram. This atrial contraction causes an increase in ventricular pressure; while the atrial pressure drops little by little; because the atrium is evacuating; until finally it empties completely and relaxes. For the above; The pressure in the ventricle becomes greater than that in the atrium, causing the atrioventricular valve to close; which causes the first noise or S1. This is how the ventricular filling stage ends with an end-diastole volume of approximately 130 milliliters.
To remember: the ventricular filling stage begins with the opening of the atrioventricular valves, and ends with their closure.
However. Blood must leave the left ventricle into the systemic circulation, and for this to occur; the ventricle must overcome the pressure of the aorta; so the electrical impulse travels from the sinus node to the atrioventricular node; causing ventricular depolarization, represented by the QRS complex of the electrocardiogram. This is how ventricular systole begins, which increases the pressure of the ventricle without changes in volume, because the valves are closed for the moment. This period is known as the isovolumetric contraction stage.
When the left ventricle exceeds the pressure of the aorta of approximately 70 millimeters of mercury, it causes the aortic valve to open; starting the ejection stage; in which blood begins to flow from the left ventricle to the aorta, progressively decreasing ventricular volume; but at the same time increasing its pressure to approximately 120 millimeters of mercury; because it continues to contract. Later comes the moment when the ventricle already has little blood, and the strength of the contraction begins to gradually decrease, and is represented by the T wave of the electrocardiogram.
Meanwhile, the aorta distends, and its pressure increases to approximately 120 millimeters of mercury; thus equalizing the pressure of the ventricle(which is already decreasing progressively), causing a retrograde flow, and thus the closure of the aortic valve that causes the second sound or S 2, thus ending the ventricular ejection phase.
To remember: the Isovolumetric Contraction stage begins with the closure of the atrioventricular valves and ends with the opening of the semilunar valves. And the ejection stage begins with the opening of the semilunar valves and ends with their closing.
Finally, the ventricle stops contracting, and there is an abrupt drop in pressure, returning to the initial 2 to 3 millimeters of mercury, and with approximately 50 millimeters of blood remaining from each cycle. This period is called the isovolumetric relaxation stage; which lasts until the new opening of the atrioventricular valve; where a new cycle begins.
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Bibliography
- Guyton. Medical Physiology.
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