Final answer:
The primary source of carbonic acid in the blood is A) dissolved CO2, which is converted into carbonic acid by the enzyme carbonic anhydrase in red blood cells.
Step-by-step explanation:
The primary source of carbonic acid in the blood is A. Dissolved CO2.
The process by which carbonic acid is formed in the blood begins when carbon dioxide (CO2) diffuses into the red blood cells. Inside these cells, an enzyme called carbonic anhydrase (CA) catalyzes the reaction between carbon dioxide and water to form carbonic acid (H2CO3). This carbonic acid then spontaneously dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+). The bicarbonate ions are a crucial component in maintaining the pH balance in the blood through the bicarbonate buffer system.