The Cardiac Cycle

The Cardiac Cycle

Definition of Cardiac Cycle

The cardiac cycle refers to the period between the start of one heartbeat and the beginning of the next. This cycle is characterized by a sequence of mechanical events that are triggered by electrical signals.

Duration of Cardiac Cycle

The typical duration of a cardiac cycle lasts about 800 milliseconds (msec).

Phases of the Cardiac Cycle

The cardiac cycle consists mainly of two phases:

• Systole (Contraction): This phase involves the ejection of blood from the heart.

• Diastole (Relaxation): This phase entails the filling of the heart with blood.

Atrial Systole

• During atrial systole, the atria contract, which helps to push blood into the ventricles.

Ventricular Systole

• In ventricular systole, the ventricles contract, leading to the ejection of blood into the arteries.

Diastole

• Diastole is the period of relaxation that follows systole, allowing the heart chambers to refill with blood.

Timing in the Cardiac Cycle

• The cycle can be broken down into specific timing intervals, specifically:
    - Atrial Diastole: Continuing for roughly 800 ms.
    - Atrial Systole: Lasts about 100 ms.
    - Ventricular Systole: Completing within approximately 270 ms.
    - Ventricular Diastole: Lasting around 530 ms.

Detailed Timing Breakdown

• The entire cardiac cycle of 800 msec involves:
    - Atrial systole: 100 ms
    - Atrial diastole: 700 ms
    - Ventricular systole: approximately 270 ms
    - Ventricular diastole: approximately 530 ms

Blood Volume During the Cardiac Cycle

• During the diastole phase, each ventricle retains some blood, as not all of it has been pumped out. This volume is approximately 60 ml of blood in each ventricle.

• It is during this time that an additional 45 ml of blood enters each ventricle through passive filling, culminating in a total volume of approximately 105 ml in each ventricle (60 ml retained + 45 ml from atria).

Atrial Systole Contribution

• During the atrial systole, the contraction of the atria forces the remaining approximately 25 ml of blood into each of the ventricles. At this point, the end-diastolic volume (EDV) in each ventricle reaches about 130 ml:
    - The volume breakdown is:
      - 60 ml retained from previous contraction
      - 45 ml passively filled from atria
      - 25 ml actively filled from atria during contraction

Ventricular Systole

The ventricular systole phase further comprises two critical phases:

• Isovolumetric Contraction: This occurs first during ventricular systole where pressure rises enough to close the atrioventricular (AV) valves but is insufficient to open the semilunar valves.

• Ventricular Ejection: Following the isovolumetric contraction, pressure builds sufficiently to open the semilunar valves, and blood is ejected from the ventricles. Each ventricle ejects approximately 70 ml of blood during this phase, known as the stroke volume:
    - Volume before contraction: 130 ml (end-diastolic volume)
    - Volume after contraction: 60 ml (end-systolic volume)

Ventricular Diastole

• Ventricular diastole consists of two phases:
    - Early Ventricular Diastole: The heart muscle relaxes, and the atria fill with blood; however, no blood enters the ventricles because all valves are closed at this moment.
    - Late Ventricular Diastole: Pressure rises in the atria sufficiently to open the AV valves, allowing blood to move passively from the atria into the ventricles.

Heart Sounds

Heart sounds correspond with the closing of heart valves:

• S1 (“lub”): This sound results from the closing of the AV valves during ventricular systole.

• S2 (“dub”): This sound is produced by the closing of the semilunar valves during ventricular diastole.

These fundamental processes constitute the intricate and coordinated events of the cardiac cycle, essential for maintaining effective blood circulation throughout the body.