Systole

The systole /ˈsɪstəliː/ is the part of the cardiac cycle during which some chambers of the heart muscle contract after refilling with blood. The term 'systole' originates from New Latin via Ancient Greek συστολή (sustolē): from συστέλλειν (sustellein, 'to contract') via [σύν (syn, 'together') + στέλλειν (stellein, 'send'). The use of systole, 'to contract', is very similar to the use of the English term 'to squeeze'. The systole /ˈsɪstəliː/ is the part of the cardiac cycle during which some chambers of the heart muscle contract after refilling with blood. The term 'systole' originates from New Latin via Ancient Greek συστολή (sustolē): from συστέλλειν (sustellein, 'to contract') via [σύν (syn, 'together') + στέλλειν (stellein, 'send'). The use of systole, 'to contract', is very similar to the use of the English term 'to squeeze'. The mammalian heart has four chambers: the left atrium above the left ventricle (lighter pink, see graphic), which two are connected through the mitral (or bicuspid) valve; and the right atrium above the right ventricle (lighter blue), connected through the tricuspid valve. The atria are the receiving blood chambers for the circulation of blood and the ventricles are the discharging chambers. When, in late ventricular diastole, the atrial chambers contract, they send blood down to the larger, lower ventricle chambers. When normal flow is completed, the ventricles are filled and the valves to the atria are closed. The ventricles now perform systole isovolumetrically, which is contraction while all valves are closed—ending the first stage of systole. The second stage proceeds immediately, pumping oxygenated blood from the left ventricle through the aortic valve and aorta to all body systems, and simultaneously pumping oxygen-poor blood from the right ventricle through the pulmonic valve and pulmonary artery to the lungs. Thus, the pairs of chambers (upper atria and lower ventricles) contract in alternating sequence to each other. First, atrial contraction feeds blood into the ventricles, then ventricular contraction pumps blood out of the heart to the body systems, including the lungs for resupply of oxygen. Cardiac systole is the contraction of the cardiac muscle in response to an electrochemical stimulus to the heart's cells (cardiomyocytes). Cardiac output (CO) is the volume of blood pumped by the left ventricle in one minute. The ejection fraction (EF) is the volume of blood pumped divided by the total volume of blood in the left ventricle. Atrial systole occurs late in ventricular diastole and represents the contraction of myocardium of the left and right atria. The sharp decrease in ventricular pressure that occurs during ventricular diastole allows the atrioventricular valves (or mitral and tricuspid valves) to open and causes the contents of the atria to empty into the ventricles. The atrioventricular valves remain open while the aortic and pulmonary valves remain closed because the pressure gradient between the atrium and ventricle is preserved during late ventricular diastole. Atrial contraction confers a minor-fraction addition to ventricular filling, but becomes significant in left ventricular hypertrophy, or thickening of the heart wall, as the ventricle does not fully relax during its diastole. Loss of normal electrical conduction in the heart—as seen during atrial fibrillation, atrial flutter, and complete heart block—may eliminate atrial systole completely. Contraction of the atria follows depolarization, represented by the P wave of the ECG. As both atrial chambers contract—from the superior region of the atria toward the atrioventricular septum—pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular valves. At the start of atrial systole, during ventricular diastole, the ventricles are normally filled to about 70 – 80 percent of capacity by inflow from the atria. Atrial contraction, also referred to as the 'atrial kick,' contributes the remaining 20–30 percent of ventricular filling. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole. The two ventricles are isolated electrically and histologically (tissue-wise) from the two atrial chambers by electrically impermeable collagen layers of connective tissue known as the cardiac skeleton. The cardiac skeleton is made of dense connective tissue which gives structure to the heart by forming the atrioventricular septum—which separates the atria from the ventricles—and the fibrous rings which serve as bases for the four heart valves. Collagen extensions from the valve rings seal and limit electrical activity of the atria from influencing electrical pathways that cross the ventricles. These electrical pathways contain the sinoatrial node, the atrioventricular node, and the Purkinje fibers. (Exceptions such as accessory pathways may occur in this firewall between atrial and ventricular electrical influence but are rare.) Cardiac rate control via pharmacology is common today; for example, the therapeutic use of digoxin, beta adrenoceptor antagonists, or calcium channel blockers are important historical interventions in this condition. Notably, individuals prone to hypercoagulability (abnormality of blood coagulation) are at decided risk of blood clotting, a very serious pathology requiring therapy for life with an anticoagulant if it cannot be corrected.