Lecture Notes: Blood Flow and Resistance

Introduction

• Equation for blood flow related to Ohm's Law.
• Voltage = Current x Resistance.
• Pressure gradient akin to voltage.
• Blood flow akin to current.
• Resistance remains constant.

Key Concepts

Pressure Gradient

• High pressure on one side and low pressure on the other leads to blood flow towards lower pressure.
• Greater pressure gradient results in higher flow.

Vessel Resistance

• Tiny vessels have higher resistance than larger vessels.
• Resistance inversely proportional to the radius of the vessel.
  • Radius is the primary determinant of flow and resistance:
    • Larger radius equates to more flow.
    • Smaller radius equates to less flow.
• Flow and resistance change significantly with radius change.
  • Doubling the radius increases flow by 16 times.
  • Resistance decreases by a factor of 16 when the radius is doubled.

Cross-Sectional Area

• Capillaries have the greatest total cross-sectional area due to their abundance.
• Total cross-sectional area > Aorta, despite the small size of individual capillaries.
  • Capillaries = Greater Total Cross-Sectional Area.
  • Aorta = Lower Total Cross-Sectional Area.

Blood Velocity

• Capillaries have the lowest blood velocity due to large total cross-sectional area.
• Aorta has high blood velocity due to low total cross-sectional area.

Resistance in Blood Vessels

• Arterioles are primary regulators of total peripheral resistance.
• Series Circuits:
  • Total resistance is the sum of individual resistances.
  • Constant blood flow through all resistors.
• Parallel Circuits:
  • Total resistance calculated using reciprocal of individual resistances.
  • Constant pressure across all parallel branches.

Flow Equation

• Q (flow) = Change in Pressure / Resistance.
• Resistance involves:
  • Radius to the fourth power
  • Viscosity (n) and Length on the denominator.

Viscosity

• High viscosity (thicker blood) reduces flow.
• Affected by conditions such as polycythemia (higher viscosity) and anemia (lower viscosity).

Length of Blood Vessel

• Longer vessels increase resistance due to more interactions with vessel walls.

Conclusion

• Understanding flow dynamics is crucial in medical applications.
• Radius and total cross-sectional area are key factors influencing flow and resistance.