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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.
Rating Newest Oldest. Best Answer: This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillaries are so small the red blood cells need to partially fold into bullet-like shapes in order to pass through them in single file. Source(s): http://en.wikipedia.org/wiki/Capillary.