Physics of Flow and Pressure: Blood as a Fluid Notes

Circulatory System Structure

  • Pulmonary circuit: blood flow between heart and lungs.
  • Systemic circuit: blood flow between heart and body.
  • Blood flows from high to low pressure.

Key Concepts

  • Flow (Q): Volume flow rate (e.g., cm³/sec).
  • Pressure (ΔP): Force per unit area.
  • Resistance (R): Opposition to flow.
  • Compliance: Ability of a vessel to expand. 𝑄 = ΔP/R

Fluid Properties

  • Fluids: Liquids and gases.
  • Density (ρ\rho): Mass (m) per unit volume (V). ρ=m/V\rho = m/V

Pressure

  • Pressure (P): Force (F) per unit area (A). P=F/AP = F/A
  • Unit: Pascal (Pa), 1 Pa = 1 N/m².

Fluid Pressure Characteristics

  • Pressure acts perpendicular to the surface.
  • Pressure is the same in every direction at a given depth.

Pascal’s Principle

  • Applied external pressure increases pressure at every point within the fluid by that amount.

Hydrostatic Pressure

  • Pressure due to the weight of fluid above a certain point.
  • P=ρghP = \rho g h (where ρ\rho is density, g is acceleration due to gravity, and h is depth).

Gauge Pressure

  • Difference between absolute pressure (P) and atmospheric pressure (PA).
  • PG=P–PAPG = P – PA

Measuring Pressure

  • Manometer: Measures pressure using fluid height difference.
  • P=PA+ρgΔhP = PA + \rho g \Delta h

Measuring Blood Pressure (BP)

  • Systolic: Maximum pressure when heart contracts.
  • Diastolic: Minimum pressure when heart relaxes.