Let’s dedicate this week to RESPIRATORY PHYSIOLOGY principles
• Pulmonary vascular resistance (PVR):
- Pulmonary circulation is a low-resistance circulation; PVR is about 1/10th of that of systemic vascular resistance.
- PVR is lowest when the lungs are at FRC (at the end of quiet expiration).
- Pulmonary capillary bed contributes about 40% to the PVR. This is in contrast to the systemic circulation where capillaries do not contribute much to the vascular resistance.
- Ohm’s law: Q = (delta P)/R
- Hence, R = (P1 – P2)/Q
(Q is blood flow though the lungs; taking cardiac output as 6 L/min, blood flow through the lung would be 6 L/min.)
PVR = [(P1 – P2)]/6 L/min.
P1 = Pulmonary artery pressure (mean) = 14 mm Hg; P2 = left atrial pressure = 8 mm Hg.
Hence,
PVR = (14 mm Hg – 8 mm Hg)/6 L/min
= 1.0 mm Hg/L/min.
[Ref: Berne & Levy. Physiology. 6th edi.]