Systole Heart Function: Meaning, Types, Mechanism

The systole is a segment of the cardiac cycle during which some chambers of the heart muscle contract or shrink after blood is refilled. Also, the cardiac cycle thus involves various events, patterns of shrinkage and relaxations of the heart thus to complete one full heartbeat. The resultant output of the cardiac cycle is the measure of the rate of flow of blood through blood vessels. Systole can also be defined as the period of contraction of the ventricles of the heart which results in the ejection of blood into the aorta and pulmonary trunk of heart. In this article, we will read more about the cardiac cycle, it’s phases, types of systoles, mechanism and clinical importance of systole.

Key Terms: Cardiac Cycle, Heart, Blood, Systole, Diastole, Aorta, Ventricles, Pulmonary Arteries

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Cardiac Cycle

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The cardiac cycle is the duration consisting of all the events from one heart contraction to the start of the next contraction of the heart. Further, it is defined as a series of alternating contraction and relaxation of the ventricles and atria in order to pump blood throughout the body.

These alternate contractions or cycles start with the depolarization of the SA node which is then succeeded by the atrial systole for 0.1 second, then ventricular systole for 0.3 seconds, followed by the diastole of the complete heart by 0.4 seconds. If the heart rate of a person is 75 bpm, then duration for one complete cardiac cycle is given as 60 / 75= 0.8 s. The duration is inversely proportional to heart rate.

• The main function of the heart is to pump the blood through pulmonary and systemic circulation thus circulating throughout the body.
• The contraction of the heart leads to fluctuations in the pressure leading to movement of blood.
• The blood so pumped flows from the high pressure region to the low-pressure region.
• Events in both the ventricles are the same, but with low pressure gradients in the right portion.
• Though the relaxations occur at the same time at the time of diastole of complete heart for 0.4 seconds, this is not same in the event of atrial.

Schematic Representation of Blood Flow in Human Heart

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Phases of Cardiac Cycle

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There are basically two phases of cardiac cycle:

• Atrial Event: The duration of systole and diastole is as follows:
• Systole: 0.1 s.
• Diastole: 0.7 s.
• Ventricular Event: The duration of systole and diastole is as follows:
• Systole: 0.3 s.
• Diastole: 0.5 s.

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Systole

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Systole can be stated as a period of contraction of the ventricles of the heart that happens to be in between the first and second heart sound of the cardiac cycle. In order to understand systole, just imagine a simple picture of the blood flowing through the heart. The heart consists of four main chambers, namely, Left Atrium, Right Atrium, Left Ventricle and Right Ventricle. The aorta then collects the blood from the lungs and pulmonary trunk or vena cava. They then transport the blood to particular ventricles by contraction through specialised valves to resist the inverse flow of blood. The ventricles then contract and send for the oxygenation of blood to lungs and into the aorta as well for the transportation of oxygenated blood to all the body systems. This particular contraction of the heart chambers after refilling the blood under electrochemical stimulus is called systole.

Systole

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Types of Systole

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There are basically two types of systole:

1. Atrial Systole
2. 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.

Atrial Systole

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.

Ventricular Systole

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Mechanism of Systole

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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.

Cardiac Cycle

Also Read: Difference between Heart Rate and Pulse Rate

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Clinical Importance of Systole

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The very important thing that determines the health of a living body is the reading of its blood pressure. The left ventricular systole helps in attaining the measurement of the blood pressure in the larger arteries of the left ventricle of the heart. Due to this, a pulse is measured with the beginning of the ventricular systole.

• Finally, in medical usage, the blood pressure is signified in terms of the systolic and diastolic pressure separated by a slash. For example, Blood pressure of 110/70 mm Hg is normal in an adult. In which the systolic blood pressure is 110 mmHg and during the diastolic blood pressure it is 70 mm Hg.
• So, the blood pressure given above is an example of the appropriate blood pressure of a healthy adult living human being. Thus, it signifies the importance of systole heart function in the life of a living human being.

Systolic vs Diastolic Blood Pressure

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Things to Remember

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• In a healthy heart, the ventricles pump out oxygenated rich blood to all the parts of the body and in return atria receives deoxygenated blood back in.
• Normal ejection fraction is between 50-70 % of blood which is pumped out during each contraction. You feel normal during any activity.
• Borderline ejection fraction is between 41-49 % of blood which is pumped out during each contraction. You may notice some symptoms during activity.
• Reduced ejection fraction is between at or less than 40% of blood which is pumped out during each contraction. You may notice symptoms even while at rest.
• People with prehypertension are at higher risk of developing heart disease.
• The duration is inversely proportional to heart rate.

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