CO 2 diffuses into the alveoli and is exhaled. In the capillaries, oxygen binds to hemoglobin in erythrocytes or dissolves into the plasma (oxygenation). The gases diffuse across the barrier following pressure gradients. Gas exchange occurs via simple diffusion across the blood-air barrier. Diseases that affect the perfusion (e.g., pulmonary embolism) or ventilation (e.g., foreign body aspiration) can cause a V/Q mismatch. The Euler-Liljestrand mechanism regulates the perfusion of nonventilated alveoli: if a lung section is perfused but not ventilated, there will be a drop in the oxygen concentration in the blood, resulting in hypoxic vasoconstriction. The ventilation-perfusion ratio is higher in the apex of the lung than at its base. Perfusion of the pulmonary capillaries is closely regulated to match ventilation in order to maximize gas exchange. The physiologic dead space is the volume of inspired air that does not participate in gas exchange. Ventilation is the movement of air through the respiratory tract into (inspiration) and out of (expiration) the respiratory zone ( lungs). Shunt and anatomical deadspace caused some inaccuracy, although they are unlikely to prevent the clinical usefulness of this formula.The main function of the respiratory system is gas exchange (O 2 and CO 2). The relationship between (Pa-E'CO2)/PaCO2 and VDalv/VTalv (best fit: VDalv VTalv = 1.135 x (Pa-E'CO2)/PaCO2-0.005) during normal physiological conditions was approximately linear and less influenced by physiological variation. The relationship between Pa-E'CO2 and VDalv/VTalv was non-linear and was affected significantly by all the factors except anatomical deadspace. The relationships were observed while pulmonary shunt, anatomical deadspace, ventilatory minute volume and metabolic rate were varied. The aim of the study was to examine the relationship between the arterial to end-tidal PCO2 gradient (Pa-E'CO2) and VDalv/VTalv and between (Pa-E'CO2)/PaCO2 and VDalv/VTalv using the Nottingham Physiology Simulator, an original, validated physiology simulation. The alveolar deadspace as a fraction of alveolar ventilation (VDalv/VTalv), while technically difficult to measure, is an objective monitor of pulmonary disease progression and a predictor of successful weaning from mechanical ventilation.
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