Lecture Notes on Pressure, Resistance, and Flow

Introduction

• Discussion on three main topics: pressure, resistance, and flow.
• Aim: Understand these concepts thoroughly.

Blood Flow Pathway

• Heart: The starting point for blood circulation.
• Aorta: Largest artery, leading to others like the brachial artery.
• Journey to Fingertip:
  • Blood flows from aorta → brachial artery → arterial → capillary bed.
  • Blood becomes deoxygenated in the second half of the capillary bed.
  • Then flows into venule → vein → vena cava (superior or inferior).

Blood Pressure

• Important to check pressures at different points:
  • Start (aorta), branch points, end of the system.
• Pressure Values:
  • Aorta: ~95 mmHg
  • Brachial artery: ~90 mmHg
  • Arteriol: ~80 mmHg
  • Capillaries: Drop significantly (~30 mmHg end, ~20 mmHg middle, ~10 mmHg entry)
  • Vena cava: ~5 mmHg
• Units: Millimeters of mercury (mmHg).

Understanding Resistance

• Total Body Resistance: Key question - what is it?
• Equation: Pressure (P) = Flow (Q) x Resistance (R)
  • Delta P is change in pressure (start pressure - end pressure).
  • Q is blood flow, measured in volume per minute (e.g., liters/min).

Calculating Flow

• Flow (Q) = Stroke volume x Heart rate
  • Stroke volume: Volume per heartbeat.
  • Heart rate: Beats per minute.
• Example: For 70 kg person:
  • Stroke volume: 70 ml/beat
  • Heart rate: 70 beats/min
  • Flow: ~5 liters/min

Calculating Total Body Resistance

• Equation Setup:
  • Delta P: 95 (aorta) - 5 (vena cava) = 90 mmHg
  • Flow (Q): 5 liters/min
  • Resistance (R): Unknown
• Solving for R:
  • 90 mmHg = 5 liters/min x R
  • R = 18 mmHg.min/liter

Conclusion

• This equation (P = Q x R) helps solve the problem of determining total body resistance.
• Understanding the relationships between pressure, flow, and resistance can clarify complex circulatory dynamics.