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The alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (p A O 2). The equation is used in assessing if the lungs are properly transferring oxygen into the blood. The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms.
The only source of CO 2 is the alveolar space where gas exchange with blood takes place. Thus the alveolar fractional component of CO 2, F A, will always be higher than the average CO 2 content of the expired air because of a non-zero dead space volume V d, thus the above equation will always yield a positive number.
Fraction of inspired oxygen (F I O 2), correctly denoted with a capital I, [1] is the molar or volumetric fraction of oxygen in the inhaled gas. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric F I O 2. Natural air includes 21% oxygen, which is equivalent to F I O 2 ...
Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution, it is possible to measure physiological dead space, employing the concept of mass balance, as expressed by the Bohr equation. [8] [9]
Where PAO2 represents alveolar oxygen pressure, Patm represents atmospheric pressure (at sea level 760 mm Hg), PH2O represents partial pressure of water (approximately 45 mm Hg), FiO2 represents the fraction of inspired oxygen (for room air, 0.21), PaCO2 represents the partial pressure of carbon dioxide in the alveoli (in normal physiological ...
The alveolar oxygen partial pressure is lower than the atmospheric O 2 partial pressure for two reasons.. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.
Since the blood arriving in the alveolar capillaries has a partial pressure of O 2 of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13–14 kPa (100 mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly.
At the end of inspiration, the alveolar pressure returns to atmospheric pressure (zero cmH 2 O). [2] During exhalation, the opposite change occurs. The lung alveoli collapse before air is expelled from them. The alveolar pressure rises to about +1 cmH 2 O. This forces the 500 ml of inspired air out of the lung during the 2–3 seconds of ...