why does expired air usually contain more water then inspired air?

by Guest1417  |  8 years, 5 month(s) ago

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why does expired air usually contain more water then inspired air?

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  1. amomipais82
    The main difference between inspired air and expired air lies in gas composition: inspired air contains more oxygen than expired air, and expired air contains more carbon dioxide than inspired air. These differences reflect the use of oxygen by cells during metabolism and the release of the waste product carbon dioxide.

    The partial pressure of oxygen in the air (PO2) at standard atmospheric pressure is 159 mm mercury (Hg), but this drops to 100-105 mm Hg when air is inhaled and enters the lungs. The inhaled air mixes with air already in the alveoli - air that is rich in water vapour and carbon dioxide and both of which contribute to the total pressure there.

    When blood circulates through the tissues it donates its oxygen to the cells that need it and later enters the pulmonary capillaries with its PO2 reduced to 40-45 mm Hg. This is about 60 mm Hg lower than the PO2 in the alveoli. As a consequence, a pressure gradient is set up for oxygen across the respiratory membrane. It is this pressure gradient which drives the oxygen from the alveolar air into the blood to balance the pressure of oxygen on each side of the membrane. The PO2 in the alveoli stays relatively stable at about 105 mm Hg. As the blood moves along the pulmonary capillary it begins to take up oxygen. By the time the venous end of the capillary is reached the partial pressure of oxygen in the blood equals that in the alveolus: 105 mm Hg. Thus the blood leaving the lungs via the pulmonary veins to the heart is rich in oxygen.

    Carbon dioxide (CO2) exchange occurs in a similar way but in the opposite direction across the respiratory membrane. Blood passing through the pulmonary capillaries has a PCO2 of about 45 mm Hg. The alveoli air has a PCO2 of about 40 mm Hg. Although this results in a relatively small pressure gradient of about 5 mm Hg, it is more than adequate. This is because CO2 has a membrane solubility which is 20 times greater than that of O2 and diffuses across the respiratory membrane much more quickly.
    Hope it helps

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