Production, transport, and excretion of carbon dioxide in the lungs and circulation. Only the dissolved fraction exerts a partial pressure. Venous admixture represents all normal physiologic shunting. These three forms of CO 2 are in equilibrium with one another, and it is the dissolved fraction in plasma that exerts the partial pressure measured as PaCO 2 . VCO 2 increases during exercise and declines slightly during sleep.
Fresh air contains almost no carbon dioxide when it is first inhaled.
Carbon dioxide that enters the alveolar spaces is exhaled during breathing. In a steady state, the amount of carbon dioxide produced from metabolism equals the amount eliminated by the lungs. Because dead space does not take part in gas exchange, all of the expired carbon dioxide comes from the alveolar gas. Because carbon dioxide is a highly diffusible gas, alveolar PCO 2 is assumed to be equal to arterial PCO 2 . Also, in the steady state, carbon dioxide excreted by the lungs equals that produced from metabolism.