Systemic Vascular Resistance

What is it?

Systemic vascular resistance (SVR) is the resistance to blood flow through the systemic circulation.
It is influenced primarily by the diameter and tone of small arteries and arterioles.

How does it work?

Vasoconstriction:

• Blood vessels narrow.
• Resistance to blood flow increases.
• Blood pressure generally increases if cardiac output remains constant.
  Vasodilation:
• Blood vessels widen.
• Resistance to blood flow decreases.
• Blood pressure generally decreases if cardiac output remains constant.

Factors Affecting SVR

• Vessel Diameter: Narrower vessels create greater resistance.
• Blood Viscosity: Thicker blood increases resistance.
• Vessel Length: Greater vessel length increases resistance.
• Autonomic Nervous System: Sympathetic stimulation commonly increases arteriolar constriction.
• Hormones: Angiotensin II and other vasoactive substances regulate vascular tone.

Relationship to Blood Pressure

Simplified relationship:
Mean Arterial Pressure ≈ Cardiac Output × Systemic Vascular Resistance
This approximation assumes central venous pressure is small relative to arterial pressure.

• Higher SVR generally increases blood pressure when CO remains constant.
• Lower SVR generally decreases blood pressure when CO remains constant.
• Blood pressure depends on the interaction between SVR, CO, and other physiological factors.

Why does it matter clinically?

Changes in SVR contribute to blood pressure regulation.
Increased SVR may occur with:

• Sympathetic nervous system activation.
• Vasoconstriction associated with cold or stress.
• Some forms of hypertension.
  Decreased SVR may occur with:
• Certain medications.
• Widespread vasodilation during distributive shock.
• Some causes of hypotension.
  These conditions may affect SVR through several mechanisms.

Related

• Cardiovascular System
• Blood Pressure
• Cardiac Output
• Hypertension
• Hypotension
• Orthostatic Hypotension

Sources

• Anatomy and physiology — cardiovascular circulation

Flashcards

What happens to SVR when arterioles constrict?02-mechanisms / anatomy-physiology / cardiovascular

Answer

Resistance increases.

What happens to pressure when SVR rises and cardiac output stays constant?02-mechanisms / anatomy-physiology / cardiovascular

Answer

Arterial pressure generally increases.

What vessel property has a major influence on SVR?01-foundations / anatomy-physiology / cardiovascular

Answer

Arteriolar diameter and tone.