Basic Physiology

At the end of inspiration, assuming that there is no rebreathing, the airway and the lungs are filled with CO₂-free gases. Carbon dioxide diffuses into the alveoli and equilibrates with the end-alveolar capillary blood (PACO₂ = PcCO₂ = 40 mm Hg). The actual concentration of CO₂ in the alveoli is determined by the extent of ventilation and perfusion into the alveoli (V/Q ratio). The alveoli with higher ventilation in relation to perfusion (high V/Q alveoli) have lower CO₂ compared to alveoli with low V/Q ratio that would have higher CO₂. As one moves proximally in the respiratory tract, the concentration of CO₂ decreases gradually to zero at some point. The volume of CO₂-free gas is termed respiratory dead space and here there is no exchange of oxygen (O₂) and CO₂ between the inspired gases and the blood. As the patient exhales, a CO₂ sensor at the mouth will detect no CO₂ as the initial gas sampled will be the CO₂-free gas from the dead space. As exhalation continues, CO₂ concentration rises gradually and reaches a peak as the CO₂ rich gases from the alveoli make their way to the CO₂ sensing point at the mouth. At the end of exhalation, the CO₂concentration decreases to zero (base line) as the patient commences inhalation of CO₂ free gases. The evolution of CO₂ from the alveoli to the mouth during exhalation, and inhalation of CO₂ free gases during inspiration gives the characteristic shape to the CO₂ curve which is identical in all humans with healthy lungs.1 Any deviation from this identical shape should be investigated to determine a physiological or a pathological cause producing the abnormality.