In the transport of CO2 from tissues to lungs, what process occurs in venous blood?

In the transport of carbon dioxide (CO2) from the tissues to the lungs, venous blood plays a critical role in the conversion of CO2 and water (H2O) into hydrogen ions (H+) and bicarbonate ions (HCO3-). This process primarily occurs in red blood cells, where CO2, generated as a byproduct of cellular metabolism, diffuses into the blood.

Inside the red blood cells, the enzyme carbonic anhydrase catalyzes the reaction between CO2 and H2O to form carbonic acid (H2CO3), which rapidly dissociates into H+ and HCO3-. This reaction serves as a crucial mechanism for transporting CO2 in the blood, as it allows for more efficient transport by converting CO2 into bicarbonate, which is highly soluble in plasma. The H+ generated contributes to maintaining acid-base balance in the blood.

When the venous blood reaches the lungs, this bicarbonate can be converted back to CO2, allowing for the gas to be expelled during exhalation. This entire process highlights the importance of the bicarbonate buffering system in the circulatory system and its role in gas exchange.