During alveolar gas exchange, respiratory gases are exchanged between the air in the alveoli and the blood in the capillaries that surround them. Oxygen and carbon dioxide must diffuse through the respiratory membrane, which is composed of the squamous cells forming an alveolar wall and the squamous cells forming a capillary wall.
Alveolar air has a higher concentration of oxygen and a lower concentration of carbon dioxide than does the capillary blood. Because molecules tend to move from an area of higher concentration to an area of lower concentration, oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses from the blood into the alveolar air.
Blood entering a capillary network of an alveolus is oxygen poor and carbon dioxide rich. Following the gas exchange, blood leaving the capillary is oxygen rich and carbon dioxide poor.
After blood has been oxygenated, it returns to the heart and is pumped throughout the body to supply the tissue cells through systemic gas exchange. Blood in the systemic capillaries supplying body tissues contains a higher concentration of oxygen and a lower concentration of carbon dioxide than the tissue cells. Therefore, oxygen diffuses from the blood into the interstitial fluid before entering the tissue cells and carbon dioxide diffuses from the tissue cells into the interstitial fluid before entering the blood. In this way, cells are supplied with oxygen for their metabolic activities, and carbon dioxide, which is produced by cellular metabolism, is removed.
Blood entering a systemic capillary network at the tissue level is oxygen rich and carbon dioxide poor. Following gas exchange, blood leaving the systemic capillary is oxygen poor and carbon dioxide rich.