Approximately 70% of carbon dioxide absorbed by the blood will be:

The correct answer is As bicarbonate ions.

Approximately 70% of CO2 transported in the blood is in the form of bicarbonate ions (HCO3-). This process occurs primarily in red blood cells (RBCs) and involves a series of reactions facilitated by the enzyme carbonic anhydrase.

Here's a breakdown of the process:

1. CO2 enters the RBC: Carbon dioxide diffuses from the tissues into the red blood cells.

2. Carbonic anhydrase catalyzes the reaction: Inside the RBC, carbonic anhydrase (CA) catalyzes the reversible reaction between carbon dioxide (CO2) and water (H2O) to form carbonic acid (H2CO3):

CO2 + H2O <—CA—> H2CO3

1. Carbonic acid dissociates: Carbonic acid then rapidly dissociates into a bicarbonate ion (HCO3-) and a hydrogen ion (H+):

H2CO3 <—> HCO3- + H+

1. Bicarbonate ions leave the RBC: The bicarbonate ions (HCO3-) diffuse out of the red blood cell and into the plasma. To maintain electrical neutrality, a chloride ion (Cl-) diffuses into the RBC from the plasma (chloride shift).

2. Hydrogen ions are buffered: The hydrogen ions (H+) produced in the reaction are buffered within the RBC, primarily by hemoglobin. Hemoglobin acts as a buffer by binding to the H+ ions, preventing a significant change in blood pH.

The remaining 30% of CO2 is transported in two other forms:

• Carbaminohemoglobin (approximately 20%): CO2 binds directly to the amino groups of hemoglobin.
• Dissolved CO2 (approximately 10%): A small amount of CO2 is transported dissolved in the plasma.