Systole.
Systole, per say, refers to the contraction of the heart. Usually it is used to refer to the contraction of the ventricles and more particularly the left ventricle.
The heart is divided into two halves – right heart and left heart – by the septum of the heart. Each half of the heart is divided into upper and lower chambers. The heart, therefore has four chambers – two upper chambers and two lower chambers.
The upper chambers are called the atria (plural for atrium) and the lower chambers are called the ventricles.
The cardiac systole comprises of the contraction of the atria followed by the contraction of the ventricles. When the atria contract, they push blood into the ventricles through the atrioventricular valves. When the ventricles are filled with blood, they contract and pump blood into the aorta (on left side) and the pulmonary trunk (right side). The aorta carries blood to all parts of the body while the pulmonary artery carries blood to the lungs. See sequence of blood flow through heart.
Cardiac systole is the result of an electrochemical stimulus to the cardiomyocytes (cells of heart muscle) which leads to the contraction of the specialized heart muscle.
Atrial Systole.
Atrial systole refers to the contraction of the atria which occurs late in ventricular diastole (dilatation or relaxation of the ventricles). One reason that drives blood from the atria to the ventricles is the fall of pressure in the ventricles during ventricular diastole (when the ventricles are emptied of blood). Due to this fall in pressure in the ventricles, the AV valves open and allow the blood to flow into the ventricles.
80% of the blood flows passively from the atria to the ventricles during diastasis (read cardiac cycle) and so the atria do not have to contract much as there isn’t much blood to push during the atrial systole which occurs during late ventricular diastole. This additional flow of blood during the atrial contraction is called the atrial kick.
Systolic pressure in the atria increase by 5 mm of Hg during atrial contraction. Duration of the atrial systole is 0.1 second.
Both the atria contact at the same time and constitute the atrial systole. The time period of the atrial systole is the time between opening of the AV valves (mitral valve on left and tricuspid valve on right) and their closure.
The atrial contraction or depolarization is initiated by the electrical impulse initiated by the SA node and which spreads to both the atria. This is indicated by the ‘P’ wave on ECG.
Ventricular Systole.
Ventricular systole refers to the contraction of the ventricle and indicates the origin of the pulse. During ventricular systole, the pressure in the ventricles rises above that in the atria. This causes the AV valves to snap shut – mitral valve on the left side and the tricuspid valve on the right side. Increased ventricular pressure does not cause the AV valves to prolapse in to the atria because these valves are anchored to the papillary muscles in the ventricles by the chordae tendineae.
The ventricular systole lasts for 0.3 to 0.4 seconds.
Pressure in the ventricles continues to rise due to ventricular contraction and increases beyond that in the aorta on the left side and the pulmonary artery on the right side. This causes the semi lunar valves to open – aortic valve on the left side and the pulmonary valve on the right side. This is the ejection phase during which about 80 to 100 cubic centimeters of blood leaves each ventricle.
During ventricular systole, the systolic arterial blood pressure is maximum. It is 120 mm of Hg in the aorta and 26 mm of Hg in the pulmonary artery.
Isovolumic Phase.
The isovolumic phase of the ventricular systole is defined as the time interval between closure of the AV valves and the opening of the semilunar valves.
Ejection Phase.
The ejection phase of the ventricular systole corresponds to the time interval between opening of the semilunar valves and their closure.
Systole and Heart Sounds.
The first heart sound S1 (the lub of lub-dub) is caused by the turbulence of blood due to the closure of the AV valves. This occurs during systole (contraction of ventricles of the heart).
The second sound S2 (the dub of the lub-dub) is caused by the closure of the aortic and the pulmonary valve at the end of the ventricular systole.
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