External respiration is the process of exchanging oxygen and carbon dioxide between the air inside the alveoli and the blood in the capillaries of the lungs. Air inside the alveoli contains a higher partial pressure of oxygen compared to the blood in the capillaries. Conversely, blood in the lungs' capillaries contains a higher partial pressure of carbon dioxide compared to the air in the alveoli. These partial pressures cause oxygen to diffuse out of the air and into the blood through the respiratory membrane. At the same time, carbon dioxide diffuses out of the blood and into the air through the respiratory membrane. The exchange of oxygen into the blood and carbon dioxide into the air allows the blood leaving the lungs to provide oxygen to the body’s cells, while depositing carbon dioxide waste into the air.
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