Most of the carbon dioxide is transported by

a. hemoglobin in red blood cells

b. the blood plasma as bicarbonate

c. proteins in the blood plasma

d. dissolving it in the red blood cells

a. hemoglobin in red blood cells

Explanation:

Carbon dioxide is constantly produced by cells during cellular metabolism (cellular respiration), generating a concentration difference between the interior of the cell and its exterior (intercellular or interstitial space), and a consequent diffusion of this carbon dioxide into the interstitial fluid. Interstitial fluid now has a higher concentration of carbon dioxide than blood plasma, which causes a diffusion into the capillary. Carbon dioxide is carried by hemoglobin or dissolved in plasma.