Ninja nerd cardiac cycle

Overview of Cardiac Cycle

  • The cardiac cycle encompasses all mechanical events associated with blood flow through the heart.
  • On average, it takes approximately 0.8 seconds to complete one cycle.

Major Components of the Cardiac Cycle

  • Atrial vs. Ventricular Pressure
    • Importance of pressure differences in determining valve actions.
  • Arterial vs. Ventricular Pressure
    • Pressure relationships affecting blood flow dynamics.
  • Atrioventricular (AV) Valves
    • Function and role during different phases.
  • Semilunar Valves (SLV)
    • Includes pulmonary and aortic semilunar valves; their role in blood ejection.
  • Electrocardiogram (EKG) Correlation
    • Identification of specific heart actions with components on the EKG waveform.

Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole

  • Definition:
    • Diastole means relaxation; thus, this phase relates to ventricular relaxation.
  • Blood Flow Into the Heart:
    • Blood returns to the heart from:
      • Inferior vena cava
      • Superior vena cava
      • Coronary sinus
      • Pulmonary veins (e.g., left atrium).
  • Valves Open:
    • As blood accumulates in the atria, atrial pressure exceeds ventricular pressure, leading to the opening of the AV valves (tricuspid in the right heart and bicuspid or mitral in the left heart).
  • Passive Blood Flow:
    • Approximately 70-80% of blood flows into the ventricles passively, aided by gravity (no contraction).
  • Pressure Relationships:
    • Atrial pressure is greater than ventricular pressure.
  • EKG Representation:
    • The SA node fires, initiating atrial depolarization, shown on the EKG as the P wave.

2. Isovolumetric Contraction (Systole)

  • Definition:
    • Ventricles begin contracting, resulting in a lifelong increase in pressure without a change in volume (no blood leaves yet).
  • Pressure Dynamics:
    • Aortic pressure: Approximately 80 mmHg.
    • Pulmonary trunk pressure: Around 7-10 mmHg.
    • Average ventricular pressure during contraction is about 60 mmHg.
  • Valves Status:
    • AV valves close due to increased ventricular pressure, resulting in S1 or 'Lub' sound upon closure.
    • Semilunar valves remain closed as the pressure is still less than in the aorta/pulmonary trunk.
  • Pressure Changes:
    • Ventricular pressure rises to approximately 120 mmHg for the left ventricle and 25-27 mmHg for the right ventricle.
  • EKG Representation:
    • Ventricular depolarization during this period shown as the QRS complex on the EKG.

3. Ventricular Ejection

  • Definition:
    • Blood is ejected from ventricles into the arteries.
  • Pressure Relationships:
    • Ventricular pressure is now greater than arterial pressure, resulting in the opening of semilunar valves.
  • Ejection Phase:
    • Semilunar valves open, allowing blood to flow into the aorta and pulmonary trunk.
  • Valves Status:
    • AV valves remain closed due to higher pressure in the ventricles compared to the atria.
  • EKG Representation:
    • The ongoing ventricular contraction is shown by the QRS complex.

4. Isovolumetric Relaxation

  • Definition:
    • Ventricles begin to relax after expelling blood, leading to a decrease in ventricular pressure.
  • End Systolic Volume (ESV):
    • A small volume of blood remains in the ventricles after contraction, defined as ESV.
  • Pressure Dynamics:
    • Aortic pressure rises due to blood flow, exceeding ventricular pressure once more.
    • Dicrotic notch occurs as semilunar valves snap shut, producing the S2 or 'Dub' sound.
  • Valves Status:
    • Both semilunar and AV valves close briefly, leading to another moment when all valves are shut.
  • EKG Representation:
    • Ventricular repolarization occurs, represented on the EKG as the T wave.

Conclusion

  • The cardiac cycle concludes as the ventricles relax and begin filling again, perpetuating the cycle for another 0.8 seconds.
  • Continually supported by pressure gradients between atria, ventricles, and arteries to guide blood flow and valve function.