Factors of Stroke Volume

Factors Affecting Stroke Volume and Heart Rate

Overview

  • Stroke volume (SV) is influenced by factors that change heart rate and the volume of blood pumped by the ventricles per minute.

  • Key factors that influence stroke volume:

    • Preload

    • Contractility

    • Afterload

    • Other contributing factors

Anatomy and Function of the Heart

  • Ventricular Volume and Wall Stretching:

    • Case study of identical twins Jack and Tom.

    • Jack's ventricles stretch more than Tom's, indicating a greater volume of blood received.

  • Analogy: Ventricles compared to balloons; more stretch = more blood received.

  • Force of Contraction:

    • The time taken for the ventricular wall to contract is crucial.

    • Contractility increases with stretch, akin to rubber bands that snap back more forcefully when stretched.

Differences between Jack and Tom

  • Thicker Ventricular Myocardium:

    • Jack's myocardial walls are thicker than Tom's due to aerobic exercises.

    • Physiological Cardiomegaly is the thickening of the ventricular myocardium through regular exercise.

  • Hypertrophy of Cardiac Muscle Cells:

    • Cardiac muscle cells generally do not divide. Instead:

    • Increased stimulation leads to greater protein synthesis, causing each cell to become thicker over time (hypertrophy).

    • Result of hypertrophy: stronger ventricles with more force generation capacity.

Key Concepts Defined

  • Preload: The extent of ventricular wall stretch before contraction.

    • Jack's preload is increased due to aerobic exercises.

  • Contractility: The force of contraction generated by the ventricles.

    • Higher preload and physical conditioning lead to higher contractility in Jack compared to Tom.

  • End Diastolic Volume (EDV): The volume of blood in the ventricle at the end of diastole (relaxation).

Factors Increasing Stroke Volume

  1. Cardiac Cycle Dynamics:

    • Ventricles must relax to receive blood during:

      • Atrial systole

      • Ventricular filling

    • If heart rate exceeds 160 beats per minute, stroke volume tends to decrease.

    • At very high heart rates (>200 bpm), the cardiac cycle becomes too short for effective filling of the ventricles (i.e., diastolic phase is minimized).

  2. Consequences of High Heart Rate:

    • Rapid ventricular contractions do not allow enough time for the ventricles to relax effectively.

    • Failure to fill the ventricles leads to a potential stroke volume of zero.

    • Cardiac output = heart rate × stroke volume; therefore, if stroke volume = 0, then cardiac output = 0, which poses serious health risks.

  3. Venous Return:

    • The volume of blood returning to the heart from the veins is critical in increasing EDV and stroke volume.

    • Diagram: Illustration of how increasing venous return raises the heart's blood volume, contributing to stroke volume.

    • Importantly, veins, especially those in the lower limbs, must bring blood back effectively to augment the ventricular blood volume.

Final Equation for Increasing Stroke Volume

  • To increase stroke volume, ensure the following:

    • High EDV

    • High preload

    • High contractility

    • High venous return

    • Sufficient time for ventricular diastole to allow filling of blood.

Summary of Factors Necessary for Optimal Stroke Volume

  • A strong and efficient heart capable of pumping significant amounts of blood must maintain:

    • Relaxed ventricles during adequate diastole

    • Sufficient venous return to keep EDV elevated

    • Increased load (preload) and contraction capacities (contractility).