Carbon dioxide is a by-product of food metabolism and in high amounts has toxic effects including: dyspnea, acidosis and altered consciousness. The primary means for eliminating carbon dioxide form the body is through the lungs and via the alveoli. Normally, alveolar ventilation is regulated so as to keep the partial pressure of carbon dioxide within a very narrow normal range: 40 5 mmHg.
Carbon dioxide elimination is accomplished by bringing fresh air into the lungs; there is no other way to excrete the body's CO2 production.
Carbon dioxide is a by-product of food metabolism. Toxic in large amounts, carbon dioxide is literally a waste product that must be eliminated for the body to function normally. However, carbon dioxide is also a component of the bicarbonate buffer system, the largest of the blood buffers, and hence is an important determinant of acidbase balance. Furthermore, partial arterial pressure of CO2(PaCO2 )is one determinant of arterial oxygen pressure (PaO2) and hence plays a role in oxygenation (shown by the alveolar gas equation). 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.
The total amount of carbon dioxide transported in the body is approximately 49 ml CO2/100 ml for arterial blood and 54 ml CO2/100 ml for venous blood . Carbon dioxide is transported in the blood in three forms: as bicarbonate (the greatest amount), combined with haemoglobin and other proteins (carbamino compounds), dissolved. These three forms of CO2 are in equilibrium with one another, and it is the dissolved fraction in plasma that exerts the partial pressure measured as PaCO2. Normal PaCO2 ranges between 36 and 44 mm Hg; mixed venous partial pressure of carbon dioxide (PvCO2) is approximately 6 mm Hg higher.
At rest, the average sized adult produces approximately 200 ml of carbon dioxide per minute . Carbon dioxide production (VCO2) is of course a continuous process, as is oxygen consumption. VCO2 increases during exercise and declines slightly during sleep. On the average, 288 L of carbon dioxide are produced per day (1440 min x 200 ml CO2/min).
Carbon dioxide elimination is accomplished by bringing fresh air into the lungs; there is no other way to excrete the body's CO production. (A minuscule amount of carbon dioxide, less than 1%, is excreted by the kidneys as bicarbonate.)
In the lungs, fresh air is brought in close proximity to the pulmonary capillaries, where it is separated from the mixed venous blood by a thin, gas-permeable alveolar capillary membrane. Fresh air contains almost no carbon dioxide when it is first inhaled. In the alveoli, the partial arterial pressure of carbon dioxide (PACO2) is practically the same as PaCO2, whatever the latter's value. Carbon dioxide is transferred across the alveolar capillary membrane by virtue of a pressure gradient that exists between mixed venous PCO2 and alveolar PCO2, (normally 46 mm Hg and 40 mm Hg, respectively). 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.