Compliance of blood vessels

Compliance Basics - Vessel Stretchiness 101

• Definition: A vessel's ability to stretch in response to pressure; the change in volume (ΔV) for a given change in pressure (ΔP).
• Formula: $C = \Delta V / \Delta P$
• Vessel Types:
  • Veins: High compliance. Act as capacitance vessels, holding large blood volumes at low pressure.
  • Arteries: Low compliance. Stiff walls withstand high pressures.
• Physiological Changes:
  • Aging: ↓ Arterial compliance (arteriosclerosis) → ↑ systolic BP & pulse pressure.

⭐ Due to high compliance, veins contain ~65% of the body's total blood volume, acting as the primary circulatory reservoir.

Arteries vs. Veins - The Great Divide

• Compliance ($C$): A vessel's ability to distend for a given change in pressure. Governed by the equation $C = \Delta V / \Delta P$.
• Arterial system is a low-volume, high-pressure, low-compliance system.
• Venous system is a high-volume, low-pressure, high-compliance system.

⭐ In veins, a large volume change results in only a small pressure change. Sympathetic stimulation constricts veins (venoconstriction), decreasing compliance and mobilizing this blood reservoir to increase venous return and cardiac output.

Clinical Correlation - Aging Pipes & Pressure

• Arteriosclerosis (Arterial Stiffening): With age, elastin fibers ↓ and are replaced by stiffer collagen in large arteries like the aorta.
  • This leads to a significant ↓ in arterial compliance.
• Hemodynamic Impact:
  • Systolic: Stiff arteries accommodate stroke volume poorly, causing a sharp ↑ in Systolic Blood Pressure (SBP).
  • Diastolic: Reduced elastic recoil leads to a ↓ in Diastolic Blood Pressure (DBP).
  • Result: A widened Pulse Pressure ($PP = SBP - DBP$).
• Clinical Presentation: Isolated Systolic Hypertension (ISH).
  • Defined as SBP ≥ 140 mmHg with DBP < 90 mmHg.

⭐ Increased aortic stiffness results in a faster pulse wave velocity (PWV), an independent predictor of cardiovascular morbidity and mortality in hypertensive patients.

High‑Yield Points - ⚡ Biggest Takeaways

• Compliance (C = ΔV/ΔP) is the vessel's ability to stretch in response to pressure; it is the inverse of elastance (stiffness).
• Veins are the most compliant vessels, acting as the primary blood reservoir (capacitance vessels).
• Arteries are less compliant, serving as pressure reservoirs to maintain diastolic blood flow.
• Aging and atherosclerosis ↓ arterial compliance, making arteries stiffer.
• Decreased compliance causes an increase in systolic and pulse pressure.