The alveoli themselves, grape-like clusters of membrane on the lung wall, collapse because blood cannot get through the pulmonary artery. The artery has constricted in an effort to shunt blood to areas of the lungs where there is more oxygen. Unfortunately, those don't exist. This is called ventricular fibrillation. The more irregularly the heart beats, the less blood it moves and the faster life functions decline.
Adults die faster.
The heart beats less and less effectively until, after several minutes, there's no movement at all. Only the brain is alive. If the person were defibrillated at that moment, he might possibly survive. He could be given cardiopulmonary resuscitation, put on a respirator, and coaxed back to life. Still, the body is doing everything it can to delay the inevitable.