SV CARDIAC OUTPUT

Cardiac Output and Related Concepts

1. Overview of Cardiac Output and Vessels

  • Understanding cardiac output and vessels that transport blood away from the heart is essential for grasping how blood circulation operates in the body.

2. Stroke Volume

  • Definition: Stroke volume is defined as the volume of blood ejected from a ventricle during one contraction.
  • Comparison between Sides: The stroke volume for both the right and left sides of the heart is typically identical in a healthy heart.

3. Stroke Volume Calculation

  • Stroke Volume Formula:
    • extStrokeVolume=extEndDiastolicVolume−extEndSystolicVolumeext{Stroke Volume} = ext{End Diastolic Volume} - ext{End Systolic Volume}
  • End Diastolic Volume (EDV): The volume of blood present in the heart at the end of filling (prior to contraction).
    • This is represented on graphs as the maximum volume after filling.
  • End Systolic Volume (ESV): The volume of blood remaining in the ventricle after contraction.
    • This is measured at the end of ventricular ejection and remains constant during isovolumetric relaxation.

4. Pressure-Volume Loop and Wiggers Diagram

  • These diagrams are useful in visualizing stroke volume by marking EDV and ESV.
  • The pressure-volume loop allows for easy identification of stroke volume contributions during the cardiac cycle.

5. Length-Tension Relationship in Cardiac Muscle

  • Relationship with Skeletal Muscle: Similar to skeletal muscle, where sarcomere length influences generated tension:
    • Optimal length results in maximum tension production.
  • **Graphical Representation:
  • Cardiac muscle operates differently as the normal range of tension can vary significantly with sarcomere length due to physiological conditions in the heart.

6. Cardiac Function Curve

  • Curve Description: The cardiac function curve shows the relationship between the end diastolic volume (on the x-axis) and stroke volume (y-axis).
  • Venous Return Impact: Venous return determines EDV; an increase in venous return increases EDV, thus increasing stroke volume due to the Frank-Starling mechanism.

7. Frank-Starling Mechanism

  • The intrinsic mechanism where increased venous return results in greater stroke volume.
  • Mechanism explanation:
    • Stretching of cardiac muscle increases troponin's affinity for calcium, allowing more cross-bridges to form and resulting in stronger contractions.

8. Venous Return Influences

  • Definition and Importance: Venous return refers to blood returning to the heart through veins.
  • Compliance of Veins:
    • Compliance: Ability of veins to hold more blood without a significant increase in pressure is a critical factor.
    • Capacitance: Veins can be referred to as capacitance vessels due to their compliance.
  • Sympathetic Stimulation Effect:
    • Vascular smooth muscle in veins possesses alpha receptors that, when stimulated, cause constriction and thus decrease compliance, pushing more blood towards the heart.

9. Mechanisms Increasing Venous Return

  • **Skeletal Muscle Pump:
    • When leg muscles contract, they compress veins, moving blood toward the heart aided by valves preventing backflow.
  • Respiratory Pump:
    • Inhalation enlarges the thoracic cavity, reducing pressure and promoting blood return to the heart.

10. Force of Contraction in the Ventricles

  • Sympathetic Stimulation's Role:
    • Increases the force and speed of ventricular muscle contractions.
    • Enhanced contractility results in higher stroke volumes without altering EDV, shifting the cardiac function curve upward.

11. Ionotropic Effects

  • Positive Ionotropic Effect: Increase in contractility due to sympathetic stimulation.
  • Negative Ionotropic Effect: Decrease in contractility resulting from lowered sympathetic stimulation.

12. Combined Effects on Stroke Volume

  • Simultaneous Effects:
    • Both venous return and sympathetic stimulation can occur concurrently, leading to significant increases in stroke volume.

13. Ejection Fraction

  • Definition: Ejection fraction quantifies contractility:
    • Formula: extEjectionFraction=extStrokeVolumeextEndDiastolicVolumeext{Ejection Fraction} = \frac{ ext{Stroke Volume}}{ ext{End Diastolic Volume}}
    • Example Calculation:
    • Start with EDV of 145 mL, stroke volume of 80 mL results in:
      ext{Ejection Fraction} = rac{80}{145} imes 100 = 55 ext{%} (normal for a healthy heart).

14. Effects of Sympathetic Stimulation on Ejection Fraction

  • Increasing sympathetic stimulation raises stroke volume, thus positively affecting ejection fraction (e.g., 80 mL to 90 mL with increased contractility resulting in 62% ejection fraction).

15. Summary of Mechanisms Affecting Cardiac Output

  • Sources of Cardiac Output:
    • Cardiac output (CO) is defined as the volume of blood ejected from each ventricle per minute.
    • Formula: extCardiacOutput=extStrokeVolumeimesextHeartRateext{Cardiac Output} = ext{Stroke Volume} imes ext{Heart Rate}
    • Adjusting stroke volume can change cardiac output through:
    1. Modulation of End Diastolic Volume: via venous return and Frank-Starling mechanism.
    2. Sympathetic Stimulation of Ventricular Muscle: improves contractility.
  • Heart Rate Influence:
    • Sympathetic stimulation speeds heart rate via beta-one receptors; parasympathetic stimulation slows it down via muscarinic receptors.