Lecture Notes: Blood Flow and Vascular Resistance

Understanding Blood Flow

• Definition: Blood flow refers to the volume of blood traveling through a blood vessel, an organ, or the entire body over a period of time.
• Measurement: Typically measured in liters per minute.

Factors Affecting Vascular Resistance

• Vascular Resistance: The opposition encountered by blood flow due to various factors.

Blood Viscosity

• Definition: Viscosity is akin to the fluid's thickness or stickiness.
• Proportional Relationship: Resistance is directly proportional to viscosity (represented by the Greek letter η).
• Example: Maple syrup (high viscosity) vs. orange juice (low viscosity).
• Effects on Blood:
  • High viscosity due to proteins and cells.
  • Conditions affecting viscosity:
    • Polycythemia: Increases viscosity.
    • Anemia: Decreases viscosity.

Blood Vessel Length

• Proportional Relationship: Resistance is directly proportional to the blood vessel length.
• Growth Impact: As a child grows, vessel length increases, raising peripheral resistance.

Blood Vessel Radius

• Inversely Proportional: Resistance is inversely proportional to the radius to the fourth power.
• Radius Changes: Can change minute-to-minute, especially in arterioles.
  • Vasoconstriction: Decreases radius, increases resistance.
  • Vasodilation: Increases radius, decreases resistance.

Key Equations

• Resistance Equation: [ R = \frac{8ηL}{πr^4} ]
• Blood Flow Equation: [ Q = \frac{ΔP}{R} ]

Application Example

• Blood Flow Change: If a carotid artery is blocked by half:
  • Radius decreases to one-half.
  • Resistance increases 16 times.
  • Blood flow decreases from 300 mL/min to 19 mL/min.

Total Resistance in Blood Vessels

Series Resistance

• Configuration: Vessels in a row, one after another.
• Calculation: Add individual resistances to get total resistance.

Parallel Resistance

• Concept: Vessels split and rejoin.
• Calculation: [ \frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \ldots ]_

Example Calculations

• Series and Parallel: Adding parallel resistances first, then series:
  • Parallel: Convert reciprocals, sum, and invert for total.
  • Series: Add remaining resistances.

Systemic vs. Pulmonary Circulation

• Systemic Circulation: Higher resistance (approx. 10 mmHg/min/L).
• Pulmonary Circulation: Lower resistance (approx. 1 mmHg/min/L).

Recap

• Vascular Resistance Equation: Affects blood flow, directly proportional to viscosity and length, inversely to radius to the fourth.
• Resistance Calculation:
  • Series: Sum of resistances.
  • Parallel: Sum of inverses.

Final Note

• Understanding these concepts is crucial for clinicians in diagnosing and treating circulatory issues.