Partial pressure of dissolved CO2 in arteries, directly measured by an ABG Water vapor adds to total inhaled air pressure, and must be accounted for Partial pressure of water (47 mmHg): Inspired air is humidified once inside airways. (Air pressure generated by CO2 – molecules that are produced byīarometric Pressure Total air pressure inhaled into lungs (760mmHg at sea level, and less at higher altitudes) (Total pressure of O2 inhaled into alveoli) Partial Pressure of O2 in systemic arteries Measured directly via Arterial Blood Gas (ABG) Partial Pressure of O2 within the alveoli: Why an A-a gradient exists, please consult: “The Alveolar-arterial pO2 gradient: Why it exists, and why we care”įraction of inhaled air made up of O2 (21%, if atmospheric) Published September 4, 2013, updated Octoon The Alveolar-arterial pO2 gradient: Explainingtheformula A-aGradient=PO –POįor an explanation of the physiology behind Normal A-a Gradient: 15 mmHg) is always a sign of pathology (see relevant slides) Note: some respiratory pathologies can present with a normal A-a gradient as well (see relevant slides) Note: Abnormally high A- a gradients indicate issues with gas diffusion between alveoli & the pulmonary capillaries. PAO2 – PaO2 = the pO2 “gradient” between the alveoli and the systemic arteries Thus, O2 content of blood when it finally reaches the systemic arteries (PaO2) is less than the O2 content in the alveoli (PAO2) Some veins in the coronary circulation drain into the left ventricle, instead of into the coronary sinus/R atrium Venous drainage from the bronchial artery directly mixes with the pulmonary veins (oxygenated blood) Less oxygenated blood from systemic veins are mixed into oxygenated blood from the lungs (“Venous admixture”): Less well-oxygenated blood from lung bases reduces the blood’s overall pO2 Since gravity causes more blood to settle to the lung bases, there is too much blood there for it all to be fully oxygenated by the alveoli. Should diffuse from alveoli into lung capillaries, and no O2 should be lost from the blood until the blood reaches the systemic arteries – so the PaO2 should equal PAO2.īlood in the lung capillaries are not fully oxygenated to begin with: Theoretically, in lung capillaries adjacent to alveoli: O2 (Can’t be measured directly, must be calculated). Agrawal, Ciara Hanly, Xiumei Deng (邓秀梅), Zesheng Ye (叶泽生), Yonglin Mai (麦泳琳)*, Juri Janovcik* * MD at time of publication Author: Yan Yu Reviewers: Steven Liu, Amogh K.
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