Uneven alveolar ventilation
Let us regard the lung as a spring that we fix at its top end. The top end carries the full weight and is hence distended. The extent to which it is stretched depends on the spring's weight and its stiffness. Lower parts of the spring carry a progressively smaller portion of the load, and will therefore be stretched less. In terms of the lung this implies that the highest parts of the lung (where they are anatomically located depends on body posture) are subjected to a greater distending force, and therefore have a greater volume per alveolus, than lower parts. By implication pleural pressure decreases from top to basal parts of the lung. This has been confirmed experimentally; also the fact that the pressure gradient can be abolished in weightlessness, and be inverted by having a subject stand on his head, confirms that it is due to gravitational forces acting on the lung.
If all alveolar units have identical pressure-volume characteristics, then compartments in the top will be more extended (greater volume V) than basal lung compartments. As the pressure-volume curve is curvilinear, for the same change in pleural pressure during inspiration the volume change (ΔV) will be greatest in basal parts of the lung, and progressively smaller towards the top. Due to this gravitational effect on the lung the ratio ΔV/V, an index of alveolar ventilation, is smallest in upper lung regions. As regards ventilation the conditions for oxygen supply and carbon dioxide removal are therefore in principle most favorable in dependent lung areas. (Obviously it depends on local circulation whether this advantage is in practice untilized).
- Due to gravitational forces upon the lung alveoli in the top of the lung are more inflated than those in basal parts.
- Due to the curvilinear nature of the pressure-volume curve of lung parenchyma the volume change during inspiration is largest in basal lung compartments.
- The combination of these factors is responsible for the fact that in young healthy adults alveolar ventilation is greatest in basal lung compartments. Consequently there is uneven alveolar ventilation even in a healthy lung, and the conditions for gas exchange are in principle most favorable in the basal parts.