Systole is the phase of the cardiac cycle in which the heart muscles are contracted. Systole can be divided into atrial systole in which the atria contract, and ventricular systole when the ventricles contract. The other phase of the cardiac cycle is diastole, when all the muscles relax. Atrial systole moves blood from the atria into the ventricles, while ventricular systole moves blood out of the heart into the pulmonary and systemic circulations. Systole will end as diastole begins, and allows blood to fill the heart again. The following diagram shows systole in the context of the entire cardiac cycle.

Types of Systole

• Atrial Systole
• Ventricular systole

Atrial Systole: Atrial systole particularly refers to one of the phases of the cardiac cycle. During the atrial systole the left and right atria get contracted at the same time and push blood into the right as well as left ventricle respectively.

• The contraction of the atria initiates with the beginning of the late ventricular diastole period. Further, when the ventricles relax, the pressure inside them descends and the atrioventricular valves open.
• This then allows the blood which is filled in the artery to flow into the ventricles thus filling them up to 75%- 80%. 
• At the beginning of the atrial systole, the dominating region of the atria shrinks, building pressure in the atria and thus forcing the left-over blood in the ventricles through the valves or flaps and hence, filling the ventricles with blood.
• Therefore, during this time period of atrial systole the function of the heart is to pump the blood from the atria to the ventricles for further transport of blood. 
• Further, the atrial systole time is calculated from the opening of the atrioventricular valves that is the mitral or bicuspid valve, which is present between the left atrium and left ventricle and tricuspid valve which is present between right atrium and right ventricle.

Ventricular Systole: Ventricular systole is another phase of the cardiac cycle which induces self-shrinkage such that pressure in both the ventricles left and right rises to a level above in the two atrial chambers, thereby enclosing the tricuspid and mitral valves which are then restrained from getting inverted by chordae tendineae and the papillary valves. Ventricular systole is itself the origin of the pulse.

• Ventricular systole can also be defined as the shrinking of the cardiac muscles of the two ventricles. 
• The Ventricular systole is the systole of the heart function that is in charge of taking oxygenated blood to other parts of the body and impure blood consisting of carbon dioxide to the lungs and in return receiving oxygenated one.
• During this process of Ventricular systole, the atrioventricular systole remains close since the beginning in order to prevent any leakage or inverse flow of the blood into the atria. 
• The aortic and the pulmonary valves which are present between the left ventricle and aorta and the right ventricle and pulmonary artery respectively are open during Ventricular systole.
• During this Ventricular systole, the shrinkage on contraction of the ventricles creates a pressure gradient due to which ejection of blood takes place from the ventricles to the pulmonary and the systemic blood circulation. This process is called the ejection phase.

Mechanism of Systole

Mechanisms of systole consist of a combination of electrical impulses generated by the heart’s pacemaker that is sinoatrial node or SA node. Further, the effect of sinoatrial nodes on impulsive muscle cells inside which the interlinkage of the actin and myosin creates a mechanical force which then leads to contraction of cardiac muscles. Let’s understand it step by step:

• The SA node cells get activated by the instant depolarization of the electric potential across their cell membrane.
• This then leads to voltage-gated calcium channels present in the cell membrane to unlock and permit the calcium ions to go through into the cytoplasm of the cardiac muscle cells.
• These ions of calcium then bind to troponin C thus leading to compliant changes in the troponin-tropomyosin protein complex which leads to rapid changes in the myosin-actin interactions which ultimately gives birth to the ventricular systole.
• Then, the electrical activity thus generated by the SA node, which is positioned above the dominated region of the atria, is coordinated by the atrioventricular node to the ventricles.
• Then the cardiac action potential spreads throughout the end part to the small network branches of the Purkinje fibres through the flux of cations passing through the opening junctions interlinking the cytoplasm of the adjacent myocytes.

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By Team Learning Mantras