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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

Opening of atrioventricular valves

Rapid rushing of blood from atria into the ventricles

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

Pumps blood into the ventricles

10% of ventricular filling.


Phase 2: Isovolumetric Contraction

Impulse depolarizes the ventricular musculature

Ventricles contract

Rise in intraventricular pressure

Atrioventricular valves close.

(Semilunar valves are already close during this)

Ventricles contract as close chambers

No flow of blood as all valves are close

Volume of ventricles remains the same (thus the name isovolumetric contraction)

Pressure in ventricles increase


Phase 3: Outflow

Increased pressure in ventricles opens the semilunar valves

Initially, there is a rapid ejection

Then there is a slow ejection


Phase 4: Isovolumentric Relaxation

At the end of the outflow, the semilunar valves close

Ventricles relax as close chambers

No flow of blood

Volume remains the same (thus the name isovolumetric relaxation)

Pressure falls

Pressure falls below that in atria

Atrioventricular valves open

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.

 

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