Which of the following statements best explains how opening up collapsed alveoli improves oxygenation?
A. Alveoli need oxygen to live.
B. Alveoli have no effect on oxygenation.
C. Collapsed alveoli increase oxygen demand.
D. Gaseous exchange occurs in the alveolar membrane.
Correct Answer: D. Gaseous exchange occurs in the alveolar membrane.
Gaseous exchange occurs in the alveolar membrane, so if the alveoli collapse, no exchange occurs, Collapsed alveoli receive oxygen, as well as other nutrients, from the bloodstream. Collapsed alveoli have no effect on oxygen demand, though by decreasing the surface area available for gas exchange, they decreased oxygenation of the blood.
Option A: Ventilation is 50% greater at the base of the lung than at the apex. The weight of fluid in the pleural cavity increases the intrapleural pressure at the base to a less negative value. As a result, alveoli are less expanded and have higher compliance at the base, resulting in a more substantial increase in volume on inspiration for increased ventilation.
Option B: The exchange of both oxygen and carbon dioxide is perfusion limited. Diffusion of gases reaches equilibrium one-third of the way through the capillary/alveolar interface. Deoxygenated blood from the pulmonary arteries has a PVO2 of 40 mmHg, and alveolar air has a PAO2 of 100 mmHg, resulting in a movement of oxygen into capillaries until arterial blood equilibrates at 100 mmHg (PaO2).
Option C: Gas exchange in the alveoli occurs primarily by diffusion. Traveling from the alveoli to capillary blood, gases must pass through alveolar surfactant, alveolar epithelium, basement membrane, and capillary endothelium.
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