Furthermore, PaCO 2 states directly, with one number, the relationship of VA to carbon dioxide production, at least at the time the sample is taken. Toxic in large amounts, carbon dioxide is literally a waste product that must be eliminated for the body to function normally. Not only must carbon dioxide from metabolism be continually eliminated, but the body must also maintain a certain level of carbon dioxide in the blood. 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.