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Cardiovascular System: Cardiac Cycle

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Cardiovascular System: Cardiac Cycle CARDIOVASCULAR SYSTEM: CARDIAC CYCLE For: Semester II CC2TH/ GEN 2TH Prepared and compiled by: OLIVIA CHOWDHURY DEPARTMENT OF PHYSIOLOGY May 2, 2020 OLIVIA CHOWDHURY SURENDRANATH COLLEGE CARDIAC CYCLE The sequence of events that occur from the beginning of one heart beat to the beginning of the next beat is known as the cardiac cycle. The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and their performance is carefully synchronised to ensure blood is pumped efficiently to the body. May 2, 2020 OLIVIA CHOWDHURY DURATION OF CARDIAC CYCLE If normal Heart Rate is 75 beats/min Duration of one (1) beat = 60/75 = 0.8 sec Effect of heart rate: . As HR increases– cardiac cycle (CC) duration decreases. If HR is 200, CC = 60/200 = 0.3 sec. Duration of each phase decreases but diastole duration decreases more than systole duration. May 2, 2020 OLIVIA CHOWDHURY PHASES OF THE CARDIAC CYCLE Atrial cycle (0.8 sec): . Atrial systole (0.1) . Atrial diastole (0.7) Ventricular cycle (0.8 sec): . Ventricular systole (0.3) . Ventricular diastole. (0.5) .During atrial systole, the ventricles are relaxed .During ventricular systole, the atria are relaxed May 2, 2020 OLIVIA CHOWDHURY ATRIAL CYCLE Atrial Systole Blood normally flows continually from great veins into atria Narrowing of origin of great veins-decreasing venous return 75% flows directly from atria into ventricles through the open AV valve before the atria contracts. 25% of filling of ventricles – during atrial contraction Intraatrial pressure rises Intraventricular pressure rises May 2, 2020 OLIVIA CHOWDHURY Atrial Diastole Coincides with ventricular systole & most of the ventricular diastole. Atria relax – gradual filling of atria – atrial pressure slowly increases. Blood flows into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. May 2, 2020 OLIVIA CHOWDHURY VENTRICULAR CYCLE Ventricular systole (0.3 sec) • Phase of Isovolumetric (Isometric) Contraction • Phase of ventricular ejection. 1. Rapid phase 2. Slow phase. May 2, 2020 OLIVIA CHOWDHURY Ventricular Systole Isovolumic Contraction Phase: The AV valves close and the atrial pressure shows slight, but sharp increase Due to sudden closure of AV valve the intraventricular pressure increases. With all the valves being closed, the ventricles become closed chambers. So, the ventricles contract isometrically, i.e., without change in the length of its muscle fibers. May 2, 2020 OLIVIA CHOWDHURY Ejection Phase: Semilunar valves (aortic, pulmonary) open at the beginning of this phase- blood is pumped out of ventricles & ventricular volume decreases rapidly. First 2/3 rapid ejection phase -70% of blood is pumped out Intraventricular pressure increases markedly as a result of continuous ventricular contraction; this pressure is slightly higher than that of in aorta & pulmonary arteries Last 1/3 slow ejection phase- remaining 30% of blood is pumped out Pressure in the ventricles fall below the pressure in the arteries. Blood in the arteries begins to flow back toward the ventricles. At the end of ejection, the semilunar valves close. May 2, 2020 OLIVIA CHOWDHURY VENTRICULAR CYCLE Ventricular Diastole (0.5 sec) . Protodiastole . Isovolumic or Isometric Relaxation phase. Rapid passive filling phase. Reduced filling & Diastasis May 2, 2020 OLIVIA CHOWDHURY Protodiastole Lasts for 0.04 sec Ventricular systole ends- ventricles relax Intraventricular pressure drops rapidly – blood tends to flow from vessels to ventricles semilunar valves May 2, 2020 OLIVIA CHOWDHURY Isovolumic Relaxation Phase Lasts for 0.06 sec Begins with closure of semilunar valves. A-V valves not yet open–ventricles relax as closed chamber – as volume remains same–Iso-volumic relaxation. Ends with opening of A-V valves May 2, 2020 OLIVIA CHOWDHURY Rapid Filling Phase: Lasts 0.1 sec Once the AV valves open, blood that has accumulated in the atria flows rapidly into the ventricles Ventricular volume increases Reduced Filling Phase: Lasts 0.2 sec Rest of blood that has accumulated in the atria flows slowly into the ventricles. Ventricular volume increases more slowly now. The ventricles continue to fill with blood until they are nearly full. May 2, 2020 OLIVIA CHOWDHURY This slow filling of ventricles is called diastasis. SUMMARY May 2, 2020 OLIVIA CHOWDHURY THANK YOU!! May 2, 2020 OLIVIA CHOWDHURY.
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