Vascular resistance concepts

Vascular Resistance - The Big Squeeze

• Total Peripheral Resistance (TPR) or Systemic Vascular Resistance (SVR): The force opposing blood flow across the systemic circulation. It's a primary determinant of afterload.
• Ohm's Law for Circulation: Defines the relationship between pressure, flow, and resistance.
  • Formula: $ΔP = Q × R$
  • Resistance is calculated as: $R = ΔP / Q$
  • ΔP: Pressure gradient (Mean Arterial Pressure - Right Atrial Pressure).
  • Q: Blood flow (Cardiac Output).

⭐ Arterioles are the primary site of vascular resistance and are most responsible for regulating TPR.

Poiseuille's Law - Radius Rules All

Resistance (R) to blood flow is determined by physical factors, mathematically expressed by Poiseuille's Law: $R = \frac{8ηL}{πr^4}$.

• Vessel Radius (r): The most powerful determinant of resistance.

  • Resistance is inversely proportional to the radius to the fourth power ($r^4$).
  • This is the primary mechanism for regulating blood flow in arterioles; small radius changes cause massive shifts in resistance.

• Viscosity (η): A measure of the "thickness" of blood.

  • Mainly determined by hematocrit.
  • ↑ Hematocrit (e.g., polycythemia) → ↑ Viscosity → ↑ Resistance.

• Vessel Length (L): Resistance is directly proportional to vessel length.

⭐ Halving the radius of a vessel increases its resistance by 16-fold, dramatically reducing blood flow.

Circuitry of Circulation - Series & Parallel Paths

• Series Resistance: Blood flows sequentially through vessels arranged one after another (e.g., within a single organ's vascular path).

  • Total resistance is the sum of individual resistances: $R_{total} = R_1 + R_2 + ...$
  • Removing a resistor in series decreases total resistance.

• Parallel Resistance: Blood is distributed simultaneously across multiple parallel vessels (e.g., systemic circulation supplying different organs).

  • The reciprocal of total resistance is the sum of the reciprocals: $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + ...$
  • Adding a resistor in parallel decreases total resistance.

⭐ The parallel arrangement of systemic circulation ensures that total peripheral resistance is less than the resistance of any single organ's vascular bed.

Regulation of Resistance - Control Knobs

Vascular resistance is dynamically regulated by nervous, hormonal, and local factors to control tissue blood flow. Arterioles are the primary site of this regulation.

⭐ In the heart and skeletal muscle during exercise, local metabolic vasodilation (metabolic hyperemia) overrides sympathetic vasoconstriction.

• Poiseuille's law is key: resistance is inversely proportional to the radius to the fourth power (r⁴), making vessel radius the most critical factor in determining flow.
• The arterioles are the principal site of Total Peripheral Resistance (TPR), acting as the main control point for systemic blood pressure.
• Systemic circulation arranges organs in parallel, which significantly ↓ total resistance (1/R_total = Σ1/R_i).
• Resistance in series (e.g., within an organ) is the sum of individual resistances (R_total = ΣR_i).
• ↑ Hematocrit or ↓ temperature leads to ↑ blood viscosity and ↑ vascular resistance.