carbon dioxide tension (PCO2), the partial pressure of carbon dioxide, a measure of the relative concentration of the gas in air or in a fluid, such as plasma. It is expressed quantitatively in millimeters of mercury (mm Hg). Alveolar PCO2 directly reflects pulmonary gas exchange in relation to blood flow. Alveolar PCO2 usually decreases as the respiration rate increases. Normal values for arterial and alveolar PCO2 are between 35 and 45 mm Hg. Higher levels occur in conditions of slow blood flow and respiration. Below-normal values are caused by hyperventilation and lead to respiratory alkalosis. Also called carbon dioxide pressure. See also carbon dioxide, hypercapnia, hyperventilation, hypoventilation.
