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.