How does gravity affect blood flow to the lungs?

Lung perfusion¹ increases linearly from the top to the bottom of the lungs (Figure 15.1, lung perfusion line). The difference in perfusion at the top and bottom of the lung can be explained by the effect of gravity on the alveolar volume, which in turn determines the pulmonary capillary pressure. The difference in pulmonary capillary pressure between the lung apex and base is equivalent to the hydrostatic pressure exerted by a column of blood. The distance from apex to base is 30 cm, so the pressure difference is 30 cmH₂O (equivalent to 22 mmHg). The pulmonary circulation is a low-pressure system: mean pulmonary artery pressure (MPAP) is typically just 15 mmHg. A pressure difference of 22 mmHg between the top and the bottom of the lungs is therefore potentially significant (this is discussed further in Chapter 16).

Figure 15.1 Perfusion, ventilation and the V̇/Q̇ relationship.

The regional differences in lung perfusion are altered by:

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  Exercise. When cardiac output (CO) increases, MPAP also increases. The difference in capillary hydrostatic pressure between the lung apex and base therefore becomes less significant; blood is distributed more evenly throughout the lung.

  
  
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  Body position. When a patient is supine, the vertical difference between the apex and base is abolished. Instead, the anterior lung becomes vertically higher than the posterior lung. For the same reasons as above, perfusion of the posterior lung becomes greater than that of the anterior lung. Similarly, in the lateral position, the dependent lung (the lowermost) has a greater perfusion than the non-dependent lung (the uppermost).

What is the effect of gravity on alveolar ventilation?

The effect of gravity on alveolar ventilation V̇_A is discussed in detail in Chapter 20. In summary:

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  The weight of the lung parenchyma results in intrapleural pressure being more negative at the apex than the base. At functional residual capacity (FRC), the apical alveoli are nearly fully inflated, whereas the basal alveoli are hardly inflated at all.

  
  
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  At FRC, the basal alveoli have a greater compliance (i.e. a greater increase in volume per unit pressure applied) than the apical alveoli, as they are less distended by the weight of the lung parenchyma.

  
  
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  During inspiration, intrapleural pressure becomes more negative, which causes the volume of the basal alveoli to increase more than the apical alveoli. V̇_A therefore increases from apex to base (Figure 15.1, alveolar ventilation line).

What is meant by the term ‘ventilation–perfusion ratio’?

For ideal gas exchange, the ventilation and perfusion to each alveolus should be matched, giving exactly the right amount of V̇_A to fully oxygenate all the passing blood; that is, a ventilation–perfusion ratio V̇/Q̇ = 1. Too little ventilation would lead to partial oxygenation of blood, whereas too much ventilation is unnecessarily wasteful of respiratory effort. However, normal lung perfusion is 5 L/min (i.e. normal CO) and V̇_A is 4 L/min.² The average V̇/Q̇ ratio is therefore 0.8.

Whilst the global V̇/Q̇ ratio in healthy lungs is 0.8, there is considerable regional variation. Owing to the direct and indirect effects of gravity respectively, both ventilation and perfusion are increased in the lung bases compared with the apices. However, there is a greater effect on perfusion than on ventilation, as represented by the steeper gradient of the lung perfusion line in Figure 15.1. The V̇/Q̇ ratio therefore increases from the bottom to the top of the lungs:

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  In the bases, the V̇/Q̇ ratio is approximately 0.6. As perfusion is greater than ventilation, blood may leave the pulmonary capillaries without being fully oxygenated, resulting in a right-to-left shunt. A greater amount of O₂ is extracted from the alveoli, resulting in a low alveolar O₂ tension (P_AO₂).

  
  
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  In the apices, V̇/Q̇ > 3. As ventilation is proportionally greater than perfusion, blood leaving the apical pulmonary capillaries is fully oxygenated. However, as only a small volume of blood passes by the apical alveoli, little gas exchange takes place: P_AO₂ is high and P_ACO₂ is low.