Fig. 1.

Normal ventilation and perfusion are responsible for gas exchange in alveoli (a). Alveolar sac filled with edema and exudates produces hypoxemia by decreasing the alveolar and arterial oxygen level result in shunt (b). Pulmonary embolism develops high ventilation in proportion to perfusion produce a dead space (c). Hypoperfusion to the apparently normal parenchyma due to vasoconstriction of pulmonary blood vessels results in poor gas exchange (d) and hyperperfusion through dilated capillaries to alveolar sac filled with edema and exudates result in shunt (e)