Question 12(p.2)

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

Discussion

Unpacking this SAQ comment, one is immediately struck by the number of times the examiners "expected" something they did not specifically ask for in their stem, as if the candidates were intended to read their minds. 

Gravity-dependent processes that influence pulmonary blood flow:

• Vertical gradient of pleural pressure:
  • In the upright subject, pleural pressure is more negative in the apices, and less negative in the bases
  • Apical alveoli are more distended than basal alveoli
  • Bases of the lungs are therefore more compliant
• Vertical gradient of pulmonary arterial hydrostatic pressure:
  • Lungs of an adult may be 30cm in height
  • Blood in pulmonary vessels therefore represents a column of blood which exerts a hydrostatic pressure (i.e. at the bottom of it, the pressure would be 30 cm H2O, or 22 mmHg)
  • This increase in hydrostatic pressure tends to recruit capillaries and increase blood flow to the basal regions of the lung.
• Gravity-related changes in lung volume
  • Pulmonary vascular resistance is lowest at FRC
  • At low lung volumes, it increases due to the compression of larger vessels
  • Gravity-induced collapse of lung bases can reduce pulmonary blood flow by:
    • increased intestitial pressure compressing extraalveolar vessels
    • hypoxic pulmonary vasoconstriction
• This separates the pulmonary circulation into four functional regions:
  • Zone 1, where alveolar pressure is higher than arterial or venous pressure;
  • Zone 2, where the alveolar pressure  is lower than the arterial but higher than the venous pressure
  • Zone 3, where both arterial and venous pressure is higher than alveolar
  • Zone 4, where the interstitial pressure is higher than alveolar and pulmonary venous pressure (but not pulmonary arterial pressure)

Changes that result from an acute increase in pressure in the pulmonary vessels, for 30% of the marks (and therefore, only briefly):

• Increased pressure in the pulmonary arteries leads to compensatory changes that buffer the pressure increase by decreasing pulmonary vascular resistance:
  • Dilatation of pulmonary arterioles
  • Distension and recruitment of pulmonary capillaries
  • Thus, decreased pulmonary vascular resistance and increased pulmonary blood flow
  • Also increased pulmonary arterial volume (acting as capacitance vessels)
• These mechanisms can compensate for acute changes in pulmonary arterial pressure until capillary recruitment is exhausted

West, J. B., C. T. Dollery, and A. Naimark. "Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures." Journal of applied physiology 19.4 (1964): 713-724.

Permutt, S., B. Bromberger-Barnea, and H. N. Bane. "Alveolar pressure, pulmonary venous pressure, and the vascular waterfall." Respiration 19.4 (1962): 239-260.

West, J. B., and C. T. Dollery. "Distribution of blood flow and the pressure-flow relations of the whole lung." Journal of Applied Physiology 20.2 (1965): 175-183.

Sobin, Sidney S., et al. "Elasticity of the pulmonary alveolar microvascular sheet in the cat." Circulation Research 30.4 (1972): 440-450.

Johnson Jr, R. L., and C. C. Hsia. "Functional recruitment of pulmonary capillaries." Journal of Applied Physiology 76.4 (1994): 1405-1407.

Hanson, WENDY L., et al. "Site of recruitment in the pulmonary microcirculation." Journal of Applied Physiology 66.5 (1989): 2079-2083.