Cardiac Cycle: Systole, Diastole & Blood Flow
Cardiac cycle is a sequence of events that takes place in the heart during each heartbeat.
Movements of Cardiac Chambers:
Systole: contraction.
Diastole: relaxation.
When not mentioned explicitly, the terms refer to the movement of ventricles.
Phase 1: Inflow
Atrioventricular valves are open.
Semilunar valves are close.
Blood flows from atria into the ventricles.
Filling occurs in three parts:
1. Rapid Ventricular Filling
At the beginning of the ventricular diastole, pressure in ventricles is low due to previous isovolumetric relaxation, and pressure in atria is high due to accumulation of blood
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Opening of atrioventricular valves
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Rapid rushing of blood from atria into the ventricles
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70% of ventricular filling occurs during this phase.
2. Diastasis
Slow filling of ventricles as blood flows from veins to atria to ventricles.
20% of ventricular filling occurs during this phase.
3. Atrial Systole
Atria contract
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Pumps blood into the ventricles
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10% of ventricular filling.
Phase 2: Isovolumetric Contraction
Impulse depolarizes the ventricular musculature
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Ventricles contract
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Rise in intraventricular pressure
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Atrioventricular valves close.
(Semilunar valves are already close during this)
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Ventricles contract as close chambers
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No flow of blood as all valves are close
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Volume of ventricles remains the same (thus the name isovolumetric contraction)
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Pressure in ventricles increase
Phase 3: Outflow
Increased pressure in ventricles opens the semilunar valves
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Initially, there is a rapid ejection
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Then there is a slow ejection
Phase 4: Isovolumentric Relaxation
At the end of the outflow, the semilunar valves close
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Ventricles relax as close chambers
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No flow of blood
↓ Volume remains the same (thus the name isovolumetric relaxation)
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Pressure falls
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Pressure falls below that in atria
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Atrioventricular valves open
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Next cycle begins with inflow
Durations
Ventricular systole: 0.3 sec
Ventricular diastole: 0.5 sec
Total duration: 0.8 sec (at heart rate of about 75 beats/min)
Atrial systole: 0.1 sec. Occurs during last 0.1 second of ventricular diastole.
Atrial diastole: 0.7 sec.