Describe the gravity dependent processes which affect pulmonary blood flow (70% of marks).

Describe the changes that result from an acute increase in pressure in the pulmonary vessels (30% of marks).

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College Answer

Most candidates quite correctly approached this from the perspective of “West’s zones of the
lung. A clear description of the relationship between pulmonary arterial, venous and alveolar
pressures producing the classical 3 zones was expected, along with situations which may alter
the normal balance between the 3 zones, e.g. changing posture or airway pressure.
Additional points were awarded for candidates describing ‘zone 4’ or alternate theories of
V/Q distribution.
Whilst most candidates described recruitment and distension with respect to changing
pulmonary arterial pressure, candidates were also expected to correctly state that an increase
in pulmonary artery pressure is only observed when these processes are exhausted, or in the
setting of pulmonary vascular disease, then describing the subsequent effects of pulmonary
hypertension on the heart and circulation.
The vascular tree is distensible, that is, a change in pressure will produce a corresponding
increase in dimension of the blood vessels:

Vascular distensibility = increase in volume / Increase in pressure x original volume

Increased pressure delivered to the arterioles causes dilation and decreases resistance to flow,
increasing flow by as much as twice what would be expected due to pressure alone
The veins are 6 to 10 x as distensible as arteries owing to the structural differences in their
respective walls. A notable exception is the pulmonary circulation where arteries are
approximately ½ as distensible as veins; this buffers pressure changes transmitted to the
alveolar capillaries and also permits the arteries to adopt a reservoir function.
Pulmonary vascular resistance is also a function of lung volume. At extremes pulmonary
capillaries are linearly stretched and collapse as may occur with hyperinflation. At very low
lung volumes, extra-alveolar blood vessels become compressed and flow reduces (Zone 4).
Use of appropriate graphs to illustrate some of the above points would have been desirable..
Syllabus Ref: B1i 2, B1k 2. a,c,i
Suggested Reading: Nunn’s Applied Respiratory Physiology / A B Lumb & J F Lunn - 6th ed
- Chapters 7,8