Cardiac Cycle Overview

The cardiac cycle consists of all mechanical events involving blood flow through the heart's chambers, averaging about 0.8 seconds.

Key Components of the Cardiac Cycle

Definition: Ventricular relaxation and filling with blood.
Blood Flow:
- Blood returns via inferior/superior vena cava and pulmonary veins into the atria.
- Atrial pressure > Ventricular pressure → AV valves (tricuspid and mitral) open.
- Passive filling: 70-80% of blood flows into ventricles without contraction.
EKG Connection: This phase includes the P wave, indicating atrial depolarization leading to contraction pushing the remaining 20% of blood into the ventricles.

Definition: Ventricles begin to contract, increasing pressure.
Pressure Dynamics:
- Ventricular pressure < Arterial pressure → Semilunar valves (SLVs) remain closed.
- AV valves close → Produces S1 (Lub) sound.
EKG Connection: Ventricular depolarization continues, reflected in the QRS complex.

Definition: Blood is ejected from the ventricles into the pulmonary trunk and aorta.
Pressure Dynamics:
- Ventricular pressure > Arterial pressure → SLVs open, allowing blood flow.
- AV valves remain closed.
EKG Connection: Similar to the previous phase, represented by the QRS complex.

Definition: Ventricles relax after contraction; pressure decreases.
Pressure Dynamics:
- Atrial pressure < Ventricular pressure → AV valves remain closed.
- SLVs close → Produces S2 (Dub) sound due to backflow and closure.
EKG Connection: Ventricular repolarization is shown by the T wave.

After the iso-volumetric relaxation phase, the cycle begins anew with the ventricles filling as they return to mid to late ventricular diastole.