Discuss the factors affecting pulmonary vascular resistance.
A good answer to this question required a discussion that focused on the following key
Recruitment and distension of pulmonary capillaries. Helps limit pulmonary vascular
resistance as pressure and flow increase.
Lung volume. Lung inflation is thought to have a dual effect, expanding large vessels
by traction while compressing smaller vessels and resistance is dependent upon lung volume,
being lowest at approx FRC.
Gravity. Perfusion is distributed down a vertical gradient in the lung, reflecting the
balance between intra-alveolar pressure and the distending vascular pressure. (West’s Zones)
Oxygen. Hypoxia causes pulmonary vasoconstriction. Diverts blood away from
hypoxic regions of the lung.
Hypercapnia and acidosis vasoconstriction
Autonomic innervation. Alpha -adrenergic stimulation - vasoconstrictor,
Beta-adrenergic stimulation - vasodilatation, Parasympathetic stimulation - Vasodilatation
Local mediators. Vasoconstrictors (Serotonin, histamine, endothelins) and
Vasodilators (NO, prostacyclin, isoprenaline)
The majority of these points could be efficiently explained through the use of graphs/figures,
all of which are commonly found in the majority of physiology books that include respiratory
physiology. Those candidates who failed to do so were also those who scored fewer points
with this question.
Syllabus - B1i2b
Reference – Respiratory Physiology, the Essentials, West 5th Ed Chp4 p33-44