Atelectasis is composed of 78% nitrogen and 21% oxygen. Since oxygen is exchanged at the alveoli-capillary membrane, nitrogen is a major component for the alveoli's state of inflation. If a large volume of nitrogen in the lungs is replaced with oxygen, the oxygen may subsequently be absorbed into the blood, reducing the volume of the alveoli, resulting in a form of alveolar collapse known as absorption atelectasis. It is usually associated with accumulation of blood, fluid, or air within the pleural cavity, which mechanically collapses the lung. This is a frequent occurrence with pleural effusion, caused by congestive heart failure (CHF).
High-altitude adaptation in humans Tibetans, Andeans and Ethiopians is indeed an observable instance of the process of natural selection in acting on favourable characters such as enhanced respiratory mechanisms in humans. Humans are naturally adapted to lowland environment where oxygen is abundant. When people from the general lowlands go to altitudes above , with atmospheric pressure 74% of normal they experience mountain sickness, which is a type of hypoxia, a clinical syndrome of severe lack of oxygen. Complications include fatigue, dizziness, breathlessness, headaches, insomnia, malaise, nausea, vomiting, body pain, loss of appetite, ear-ringing, blistering and purpling of the hands and feet, and dilated veins.