In order to understand what is systole, it is important to get a simple picture of the blood flow through the heart. The heart consists of four chambers: the left and right atrium and the left and right ventricle. The atria collect the blood from the lungs and the venae cavae (circulation in veins). They then pass the blood to the respective ventricles by contraction through specialised valves to prevent the backflow of the blood. Once the ventricles are filled the valves are closed. The ventricles then contract and then pump the blood into the pulmonary system i.e. to the lungs for oxygenation of the blood and into the aorta through which the fresh oxygenated blood is distributed to all the body systems. This specialised contraction of the heart chambers after refilling the blood under a certain electrochemical stimulus is known as systole.
Systole Definition and its Types
From the given explanation in the introduction to what is systole, the systole definition can be given as - The contraction of the cardiac muscles (heart muscles) which make up the four chambers of the human heart, in response to an electrochemical stimulus to the heart’s cells (or cardiomyocytes) is known as systole. Thus, during systole heart function guides it to contract to pass forward the blood after refilling its chambers. Hence, in order to very simply define systole, it can be stated as a simple contraction of the heart muscles. Since there are two kinds of chambers present in a human heart, the cardiac systole (or the heart systole) is of two types, which are the atrial systole and the ventricular systole. The answers to the questions now arising in the mind - what is atrial systole and what is ventricular systole are briefly explained as follows:
The atrial systole i.e. the contraction of the atria begins with the starting of the late ventricular diastole period (relaxing of cardiac muscles in ventricles after pumping out the blood). As the ventricles relax, the pressure inside them decreases and the atrioventricular valves (valves in between the atria and the ventricles) open. This allows the blood-filled in the atria to flow into the ventricles filling the ventricles of up to 70% - 80%. At the start of the atrial systole, the superior region of the atria contracts building pressure in the atria and forcing the remaining blood into the ventricles through the valves, now filling the ventricles with blood. Therefore, in this duration of atrial systole heart function is to pump blood from the atria to the ventricles for further transport. The atrial systole time is calculated from the opening of the atrioventricular valves - the mitral or bicuspid valve (in between the left atrium and left ventricle) and the tricuspid valve (in between the right atrium and the right ventricle). It is best displayed by the following diagram:
From the given systole definition, to define systole of the ventricles it can be easily stated as the contraction of the cardiac muscles of the two ventricles. The ventricular systole is the systole heart function responsible for the transport of oxygenated blood to all the body parts and sending carbon dioxide containing blood to the lungs to get oxygenated. During ventricular systole, the atrioventricular valves remain closed from the start in order to avoid any leakage or flow of blood back into the atria. The pulmonary and the aortic valves in between the right ventricle and the pulmonary artery and the left ventricle and the aorta respectively are open during ventricular systole. So, what happens during ventricular systole is that the contraction of the ventricles creates a pressure gradient due to which the blood is ejected out from the ventricles to the pulmonary and the systemic blood circulation. This is known as the ejection phase.
After the ejection phase, the ventricles return to diastole or relaxed mode. The ventricular systole time starts from the pulmonary valve-open to valve-closed in the right ventricle, and from the aortic valve-open and valve-closed in the left ventricle. Also, the ventricular systole is considered to be the starting of the pulse which is usually measured by universally accepted methods of touch and eye. This is best represented by the given below figure:
A Wiggers diagram is shown below which graphically represents the duration of the ventricular systole and various activities and changes that are taking place in the heart during the cardiac cycle.
Mechanism of Systole
The heart systole is a combination of electrical impulses generated by the heart’s pacemaker (sinoatrial node or SA node) and its effect on the excitable muscle cells inside which the interaction of the actin and myosin creates a mechanical force which in turn leads to the contraction of cardiac muscles.
The SA node cells are activated by the spontaneous depolarization of the electric potential across their cell membrane. This causes the voltage-gated calcium channels present in the cell membrane to open and permit the calcium ions to pass through into the cytoplasm of the cardiac muscle cells. These calcium ions in turn bind to troponin C leading to conformational changes in the troponin-tropomyosin protein complex which creates a cascade of changes in the myosin-actin interactions which finally generates the contraction during the ventricular systole.
The electrical activity generated by the SA node, which is located above the superior region of the atria, is coordinated by the atrioventricular node to the ventricles. The cardiac action potential spreads throughout distally to the small network branches of the Purkinje tree through the flux of cations passing through the gap junctions connecting the cytoplasms of the adjacent myocytes.
Clinical Importance of Systole
Blood pressure is a very important reading that determines the health of a living body. The left ventricular systole allows the measurement of the blood pressure in the larger arteries of the left ventricle of the heart. Because of this, a pulse is measured with the starting of the ventricular systole. Conclusively, for medical purposes, the blood pressure is mentioned in terms of the systolic and diastolic pressure separated by a slash. For example, if the blood pressure is stated as 120/80 mm Hg, it means that the pressure in the ventricles during systole is 120 mm Hg and during the diasol is 80 mm Hg. Also, the blood pressure in the given example is the appropriate blood pressure of a healthy adult human being. Thus it underlines the importance of systole heart function in the life of a living human being