What are the major transport mechanisms for CO2? Explain.

Carbon dioxide (CO2) is transported in the blood through various mechanisms, each contributing differently to overall CO2 transport. The major transport mechanisms for CO2 include:

1. Dissolved in Plasma: A small fraction of CO2 (~5-10%) dissolves directly in the plasma of blood. CO2 is highly soluble in water, and as blood circulates through the pulmonary capillaries, a portion of the CO2 present in the tissues diffuses into the bloodstream and is carried in solution as dissolved CO2 molecules.

2. Transported as Bicarbonate (HCO3-): The majority of CO2 (~70%) is transported in the form of bicarbonate ions (HCO3-) in the plasma. This process involves a series of reversible chemical reactions known as the bicarbonate buffer system, which occur primarily within red blood cells (RBCs). The reactions are as follows:

   • CO2 + H2O ⇌ H2CO3 (carbonic acid) ⇌ H+ + HCO3-

   In tissues with high CO2 concentration, carbonic anhydrase enzyme catalyzes the hydration of CO2 and water to form carbonic acid (H2CO3). Carbonic acid dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+). Bicarbonate ions are then transported in the plasma to the lungs.

3. Bound to Hemoglobin: A small fraction of CO2 (~20-25%) binds directly to hemoglobin molecules in RBCs. This binding occurs at specific amino acid residues on the globin portion of hemoglobin and forms carbaminohemoglobin. Unlike oxygen, which binds to iron in the heme groups of hemoglobin, CO2 binds to other sites on the globin protein. The binding of CO2 to hemoglobin helps to transport CO2 from tissues to the lungs for elimination.

Overall, the transport of CO2 in the blood involves a combination of these mechanisms, with the majority transported as bicarbonate ions, followed by dissolved CO2 in plasma and a smaller fraction bound to hemoglobin. This efficient transport system ensures that CO2 produced by cellular metabolism is effectively transported from tissues to the lungs for elimination via exhalation, maintaining acid-base balance and respiratory homeostasis in the body.