The correct answer is A. H2CO3 – HCO3-.
The bicarbonate buffer system is the main buffer system of the human blood. It helps to maintain the pH of the blood within a narrow range, which is essential for the proper functioning of the body. The bicarbonate buffer system works by exchanging bicarbonate ions (HCO3-) with hydrogen ions (H+). When the pH of the blood decreases, the bicarbonate ions are converted to carbonic acid (H2CO3), which is then excreted in the urine. When the pH of the blood increases, the carbonic acid is converted back to bicarbonate ions, which are then returned to the blood.
The other options are incorrect because they are not involved in the bicarbonate buffer system. Option B, H2CO3 – CO32-, is the carbonate buffer system. Option C, CH3COOH – CH3COO-, is the acetic acid buffer system. Option D, NH2CONH2 – NH2CONH+, is the urea buffer system.
The bicarbonate buffer system is the most important buffer system in the blood because it is the only buffer system that can effectively buffer changes in pH that occur rapidly. The other buffer systems are not as effective at buffering rapid changes in pH.