Venous Return

CIRCULATORY SYSTEM OVERVIEW

  • Topic Focus: Venous Return/Cardiac Output

  • Content Sections:

    1. Venous Return

    2. Cardiac Output

    3. Factors Affecting Cardiac Output

    4. Measurement of Cardiac Output

  • Presented by Dr. S B Phiri

VENOUS RETURN

  • Definition: Venous return (VR) is the flow of blood back to the heart.

  • Equilibrium: Under steady-state, venous return equals cardiac output (Q).

    • This is crucial as the cardiovascular system is a closed loop.

    • Prevents blood accumulation in systemic or pulmonary circulations.

  • Interdependence: Cardiac output and venous return may be regulated independently.

Factors Affecting Venous Return

  1. Musculovenous Pump:

    • Rhythmic contraction of limb muscles during activities (e.g., walking) enhances VR through the muscle pump mechanism.

  2. Decreased Venous Capacitance:

    • Sympathetic activation reduces venous compliance and increases venous return via:

      • Increased vasomotor tone

      • Increased central venous pressure

      • Augmented cardiac output (Frank-Starling mechanism).

  3. Respiratory Pump:

    • During inspiration:

      • Negative intrathoracic pressure and positive abdominal pressure create a gradient, pulling blood towards the right atrium.

  4. Vena Cava Compression:

    • Increased resistance in the vena cava (e.g., during Valsalva maneuver) reduces venous return.

  5. Gravity:

    • Standing causes hydrostatic pressure changes:

      • Right atrial pressure decreases.

      • While venous pressure in dependent limbs increases, overall VR can decrease due to lowered cardiac output.

  6. Pumping Action of the Heart:

    • Changes in right atrial pressure affect central venous pressure and, subsequently, venous return.

CARDIAC OUTPUT

  • Definition: Cardiac output (CO) is the volume of blood ejected by the heart per time unit (L/min).

    • Normal resting values: 5 to 6 L/min, about 8% of body weight per minute.

  • Cardiac Index: CO adjusted for body surface area; increases proportionally with body surface area.

    • Average cardiac index is ~3 L/min/m² for a normal 70 kg adult.

  • Calculation: CO = Heart Rate (HR) x Stroke Volume (SV).

Stroke Volume (SV)

  • Definition: Volume of blood ejected from one ventricle per beat (mL/beat).

  • Calculation: SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV).

  • Healthy SV is typically approx. 70 mL for each ventricle.

  • Ejection Fraction (EF): Percentage of blood ejected per contraction; an important metric for heart function and heart failure assessment.

    • Left Ventricular Ejection Fraction (LVEF): Volume pumped by left ventricle per beat / Volume at end of diastolic filling.

    • Affected by overall heart activity; reductions indicate heart failure.

FACTORS AFFECTING CARDIAC OUTPUT

Heart Rate Influences:

  1. Age: Newborns (HR ~ 120 bpm); rates decrease through youth then increase with age.

  2. Exercise: Increases HR and SV during physical activity.

  3. Pathological Rates:

    • Bradycardia: Reduced rate; can be intrinsic or extrinsic causes.

    • Tachycardia: Increased rate; often linked to stress or physiological conditions.

  4. Autonomic Regulation:

    • Controlled by cardiovascular centers in the medulla oblongata (sympathetic and parasympathetic effects).

  5. Broken Heart Syndrome:

    • Emotional events can weaken myocardium without oxygen lack.

  6. Other Influences: Hormonal levels, ion concentrations, temperature, hypoxia, and pH.

Stroke Volume Influences:

  1. Preload: Stretch on ventricles before contraction; increased EDV results in increased SV.

  2. Contractility: Strength of ventricular contraction affected by:

    • Positive inotropic factors (e.g., catecholamines, some drugs).

    • Negative inotropic factors (e.g., certain drugs, hypoxia).

  3. Afterload: Resistance against which ventricles must pump; increased afterload demands stronger contractions.

MEASUREMENTS OF CARDIAC OUTPUT

  1. Direct/Invasive Methods:

    • Fick method

    • Indicator dilution method

  2. Indirect/Non-invasive Methods:

    • Echocardiography

    • Computed Tomography

    • Radionuclide Imaging

  3. Fick’s Principle: Blood flow equals difference in arterial and venous substance concentrations multiplied by blood flow rate.