Oxygen moves down its concentration gradient from the alveolar air space, across the respiratory membrane, and into pulmonary capillaries. The carbon dioxide content of atmospheric air is relatively low. Because our cells make carbon dioxide, the partial pressure of carbon dioxide in the blood arriving from the systemic circuit is higher than it is in the inspired alveolar air. Therefore, carbon dioxide moves from blood, across the respiratory membrane, and into the alveolar air space. The exchange of gases between the body’s interstitial fluids and the external environment is called external respiration.
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