Porth's Essentials of Pathophysiology, 4e

192

Integrative Body Functions

U N I T 2

Carbon Dioxide

U N D E R S TA N D I N G

Body metabolism results in a continuous production of carbon dioxide (CO 2 ). As CO 2 is formed during the metabolic process, it diffuses out of body cells into the tissue spaces and then into the circulation. It is transported in the circulation in three forms: (1) dissolved in the plasma, (2) as bicarbonate, and (3) attached to hemoglobin.

1

Body cell

Plasma. A small portion (about 10%) of the CO 2 that is produced by body cells is transported in the dissolved state to the lungs and then exhaled. The amount of dissolved CO 2 that can be carried in plasma is determined by the partial pressure of the gas (PCO 2 ) and its solubility coefficient (0.03 mL/100 mL plasma for each 1 mm Hg PCO 2 ). Thus, each 100 mL of arterial blood with a PCO 2 of 40 mm Hg would contain 1.2 mL of dissolved CO 2 . It is the carbonic acid (H 2 CO 3 ) formed from hydration of dissolved CO 2 that contributes to the pH of the blood. Bicarbonate. Carbon dioxide in excess of that which can be car- ried in the plasma moves into the red blood cells, where the enzyme carbonic anhydrase (CA) catalyzes its conversion to carbonic acid (H 2 CO 3 ). The H 2 CO 3 , in turn, dis- sociates into hydrogen (H + ) and bicarbonate (HCO 3 – ) ions. The H + combines with hemoglobin and the HCO 3 – diffuses into plasma, where it participates in acid–base regula- tion. The movement of HCO 3 – into the plasma is made possible by a special transport system on the red blood cell membrane in which HCO 3 – ions are exchanged for chlo- ride ions (Cl – ).

CO 2

CO 2

dissolved

in plasma

10% transported as CO 2 dissolved in plasma

CO 2

2

H 2

O

CO 2

dissolved

+

CO 2

H 2

O

in plasma

CA

HCO 3 - (dissolved) Cl - (Chloride shift)

H 2 CO 3 Hb H + + HCO 3 –

HHb

70% transported as HCO 3 - dissolved in plasma