Blood Flow and the Cardiac Cycle

It is fundamental that blood moves from areas of higher pressure to lower pressure. Coordinated cyclic contraction of the heart chambers and competent valvular function are necessary to generate the dynamic force that propels oxygen and nutrient rich blood to all the tissues of the body.

The Wiggers diagram (right) is an animated representation of the cardiac cycle. It provides a comprehensive overview of the complex interplay between electrical activity, pressure changes, volume changes, and valve function during a single cardiac cycle.

Key Features:

• Pressure Changes:
  • Shows the variations in pressure within the left atrium, left ventricle, and aorta throughout the cardiac cycle.
  • Depicts how these pressure changes drive the movement of blood.
• Volume Changes:
  • Illustrates changes in left ventricular volume during diastole (filling) and systole (ejection).
• Valve Function:
  • Demonstrates the timing of valve opening and closing (mitral valve, aortic valve) in relation to pressure and volume changes.
• Electrocardiogram (ECG):
  • Correlates the electrical events of the heart (represented on the ECG) with the mechanical events (pressure and volume changes).
• Heart Sounds:
  • Relates the timing of heart sounds (S1 and S2) to the closing of the valves.

The Cardiac cycle

Diastole:

• The relaxation phase of the cardiac cycle.
• The atria and ventricles fill passively with blood.
• Atrioventricular (AV) valves are open, allowing blood to flow from the atria to the ventricles.
• Semilunar valves (aortic and pulmonary) are closed, preventing backflow from the arteries into the ventricles.
• Ventricular filling continues, stretching the heart muscle.

Systole:

• The contraction phase of the cardiac cycle.
• Atrial systole: the brief contraction of the atria, can contribute approximately 10% to ventricular filling volume at rest. During rapid heart rates, the ventricles have less time for passive filling, increasing the importance of the contribution from atrial systole (up to 40%)
• Ventricular systole: The powerful contraction of the ventricles.
  • AV valves close to prevent backflow into the atria.
  • Semilunar valves open, allowing blood ejection from the ventricles into the aorta and pulmonary artery.

Cardiac Chambers

Venous Return:

• Tissue capillary pressure exceeds that in the venules, the pressure drives fluid into the low-pressure venous system.
• Venous return to the right atrium is facilitated by:
  • Skeletal muscle contractions
  • Respiratory pump
  • One-way valves within veins
  • Gravity (particularly in the lower extremities)

Right Atrial Function

• Diastole:
  • Blood flows unobstructed from the vena cava into the right atrium.
  • Right atrial diastolic filling pressure is similar to the central venous pressure (CVP).
  • The typical range for both right atrial diastolic pressure and CVP is between 2-6 millimeters of mercury (mmHg).
• Systole:
  • RA systolic pressure remains elatively low (around 0-5 mmHg) during ventricular systole.
  • Tricuspid valve closes, preventing backflow into the right atrium.
  • Blood is ejected through the pulmonic valve into the pulmonary artery.

Right Ventricle

• Diastole:
  • Right ventricular (RV) diastolic pressure is low (0-5 mmHg).
  • Blood flows from the right atrium into the RV through the open tricuspid valve.
• Systole:
  • RV systolic pressure increases (20-30 mmHg).
  • Tricuspid valve closes, preventing backflow into the right atrium.
  • Blood is ejected through the pulmonic valve into the pulmonary artery.

Left Atrial Filling and the Frank-Starling Mechanism

• Diastolic pulmonary vein pressure is very low, reflecting the low-pressure of the pulmonary circulation.
• Blood flows passively from the left atrium into the left ventricle.
• Frank-Starling Mechanism:
  • Within physiological limits, the force of ventricular contraction is directly proportional to the initial length of the cardiac muscle fibers.
    • The more the heart muscle sarcomeres are stretched during diastole (preload), the more forcefully they can contract.
    • Remember: atrial systole can contribute about 10% to the left ventricular end diastolic volume (LVEDV) and at elevated heart rates, up to 40% of LVEDV.
  • Increased preload can increase both contractility and stroke volume of the left ventricle.

Left Ventricular Ejection

• The left ventricle (LV) generates the highest pressure in the systemic circulation during systole (100-140 mmHg).
• Aortic Pressure:
  • Aortic diastolic pressure typically ranges from 60-90 mmHg.
  • Afterload: The pressure the left ventricle must overcome to eject blood into the aorta.
• Pressure Gradient: The pressure gradient between the LV and the aorta drives blood flow through the aortic valveAortic Pressure Rise: Ejection of blood from the LV raises aortic pressure.
• Aortic Pressure Rise: Ejection of blood from the LV raises aortic pressure.
• Systemic Circulation: Blood flows through the systemic circulation, with pressure gradually dissipating across the capillary beds.

References

Guyton A.C. (1986). Textbook of Medical Physiology. London: W. B. Saunders

Tkacs, Nancy C., et al. (2020).  Advanced Physiology and Pathophysiology: Essentials for Clinical Practice. Springer Publishing Company.

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