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Making the world better, one answer at a time. Thin air is caused by low oxygen pressure at high altitudes. The lack of oxygen makes the air less dense. Thin air is caused by low oxygen pressure at high altitudes. The lack of oxygen makes the air less dense.
Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood, where it's taken away to your lungs. Pulmonary Circulation Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again.