How a clot creates dead space
A pulmonary embolus is material, usually a blood clot from a leg vein, that lodges in a pulmonary artery and blocks blood flow to part of the lung. The alveoli downstream still receive air with each breath, but no blood arrives to pick up oxygen. Ventilation continues without matching perfusion, the definition of dead space (a high ventilation-to-perfusion region).
Because gas exchange cannot happen where there is no blood flow, the ventilated but unperfused region contributes nothing useful, and blood is redirected toward the remaining open vessels, which can become relatively overperfused.
Why oxygen levels fall
Hypoxemia in pulmonary embolism arises mainly from the redistribution of blood flow that worsens matching elsewhere, along with reflex changes and, in larger clots, strain on the right heart. Patients often breathe faster, which can lower carbon dioxide even while oxygen remains low.
Pulmonary embolism is a medical emergency with widely varying presentation. This is general educational information about the physiology, not diagnostic guidance; sudden breathlessness or chest pain warrants urgent assessment by a qualified clinician.