[1][clarification needed] A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused.
[2] Intrapulmonary shunting is the main cause of hypoxemia (inadequate blood oxygen) in pulmonary edema and conditions such as pneumonia in which the lungs become consolidated.
[5] In addition, some of the smallest cardiac veins drain directly into the left ventricle of the human heart.
This drainage of deoxygenated blood straight into the systemic circulation is why the arterial PO2 is normally slightly lower than the alveolar PO2, known as the alveolar–arterial gradient, a useful clinical sign in determining the cause of hypoxemia.
If the A-a gradient is significantly higher than the result of this equation, the person likely has a disorder of the alveoli, causing oxygen to be unable to diffuse into the blood.
If the person has a normal A-a gradient but still has hypoxemia, then there is probably a cause unrelated to the alveoli, such as hypoventilation, obstructive lung disease, or shunting.An irregular distribution of ventilation can occur in asthma, bronchiolitis, atelectasis, and other conditions,[9] which have the effect of reducing the amount of oxygen present in some alveoli relative to others.
[citation needed] When alveoli fill with fluid, they are unable to participate in gas exchange with blood, causing local or regional hypoxia, thus triggering vasoconstriction.
A decrease in perfusion relative to ventilation (as occurs in pulmonary embolism, for example) is an example of increased dead space.
Although ventilation at that area is unaffected, blood will not be able to flow through that capillary; therefore, at that zone there will be no gas exchange.