Understanding Systemic Vascular Resistance (SVR)

May 21, 2025

Physiology: Systemic Vascular Resistance (SVR)

Introduction

  • SVR (Systemic Vascular Resistance): Also known as Total Peripheral Resistance (TPR)
    • Represents the force exerted on circulating blood by body vasculature.
    • Determined by:
      • Length of blood vessels
      • Diameter of blood vessels
      • Viscosity of blood
    • Important for blood pressure regulation: MAP = CO x TPR (Mean Arterial Pressure = Cardiac Output x Total Peripheral Resistance).

Mechanism

  • Vessel Composition:

    • Tunica Adventitia: Strong fibrous connective tissue, prevents vessel bursting.
    • Tunica Intima: Composed of epithelial cells, regulates vascular tone using prostaglandins and nitric oxide.
    • Tunica Media: Elastic connective tissue and smooth muscle, crucial for blood pressure maintenance.
  • Vascular Tone Regulation:

    • Smooth muscle in tunica media controls vessel diameter.
    • Innervated by the autonomic nervous system, mainly sympathetic system, releasing norepinephrine on adrenergic receptors.
    • Renin-angiotensin-aldosterone system influences SVR by altering kidney blood flow and blood volume through angiotensin II.
  • Effects of Vessel Caliber Changes:

    • Significant changes in blood flow due to vessel diameter adjustments.
    • Arterial system contributes most to SVR due to thick vessel walls.
  • Blood Viscosity:

    • High blood viscosity (e.g., polycythemia) increases SVR.
    • Low blood viscosity (e.g., anemia) decreases SVR.

Related Testing

  • SVR estimation requires accurate blood pressure and cardiac output measurement, often using ultrasound.

Pathophysiology

  • Conditions Affecting SVR:
    • Increased SVR: Diabetes mellitus, hypertension (leading to arteriosclerosis).
    • Decreased SVR: Distributive shock (e.g., anaphylaxis, sepsis) leads to decreased SVR due to cytokine release or loss of sympathetic input.

Clinical Significance

  • SVR impacts clinical situations when blood pressure is too high or low.

    • Vasopressors: Increase SVR by agonizing alpha receptors.
    • Calcium channel blockers: Decrease SVR by preventing smooth muscle contraction.
  • Perfusion Issues:

    • Conditions such as erectile dysfunction involve inadequate vasodilatory signals; treated with PDE4 inhibitors like sildenafil.

Images

  • Atherosclerosis illustrations and smooth muscle contraction visuals included for reference.

References

  • Cited studies and articles supporting the physiological and clinical insights into SVR.