Our lives depend upon a membrane called the alveolar-capillary barrier. This membrane varies from 200 to 800 nanometers thick. 1 inch = 25,400,000 nanometers. A red blood cell is 7,000 nanometers in diameter. So this is a very thin line between life and death. This membrane is amazingly strong, and it keeps blood out of the lungs, and air out of the blood. So it is sometimes called the blood-air barrier.
So... if there is a blood-air barrier between our blood vessels and alveoli how does oxygen get into the blood and carbon dioxide get out of the blood? Only on the molecular level. The barrier is designed to only allow the exchange of certain gas molecules! We are constantly only 800 nanometers or less from death. Another amazing precision system designed without any room for error. We are wonderfully made by an all powerful Creator!