In many cases, the arterial oxygen saturation will decrease with disease of the lung. The decrease may be caused by various factors:
1. Diminished ventilation may result from (a) stenosis of the air passages, e. g., stenosis of the larynx and bronchi due to inflammatory processes; (b) a decreased movement of the thoracic wall caused by weakness of the respiratory muscles, as with rickets and (c) a decreased function of the central nervous regulation, often found in very young children. The consequence of this disturbance of ventilation is diminution of the alveolar oxygen tension. Under these conditions the blood cannot take up sufficient oxygen, and the arterial oxygen saturation decreases (hypoxic hypoxemia).
2. There may be decreased diffusion into the pulmonary capillaries. This occurs if the alveolar epithelium is swollen or covered by exudate.
Seriously lessened saturation may result if considerable circulation is maintained through parts in which the oxygen