Question

"Most carbon dioxide is carried in the blood:_______

as dissolved CO2 in the plasma
as bicarbonate in the plasma
as carbonic acid in red blood cells
bound to hemoglobin"

Answer 1

The majority of carbon dioxide in the blood is carried in the form of bicarbonate ions in the plasma, facilitated by the enzyme carbonic anhydrase within red blood cells and exchanged with chloride ions through the chloride shift.

Step-by-step explanation:

The majority of carbon dioxide in the blood is transported as bicarbonate ions in the plasma. When carbon dioxide diffuses into red blood cells, an enzyme called carbonic anhydrase quickly converts it into carbonic acid (H₂CO₃). This unstable intermediate molecule then spontaneously dissociates into bicarbonate (HCO₃⁻) and hydrogen (H⁺) ions. The bicarbonate ions are transported out of the red blood cells in exchange for chloride ions, in a process known as the chloride shift.

Approximately 5 to 7 percent of carbon dioxide is dissolved directly in the plasma, and a smaller proportion, about 10 percent, is carried in the blood bound to hemoglobin as carbaminohemoglobin. Upon reaching the lungs, the bicarbonate ions re-enter red blood cells and convert back into carbon dioxide, which is then exhaled.