Chloride Shift in CO2 Transport

Steps:

• CO2 diffuses from tissues into red blood cells (RBCs).
• Inside RBCs, CO2 reacts with water via carbonic anhydrase to form carbonic acid, which dissociates into H+ and HCO3- ions.
• HCO3- ions diffuse out of RBCs into plasma.
• Cl- ions move into RBCs to maintain electrical neutrality.

Why D is correct:

• D accurately describes the chloride shift, where HCO3- exits RBCs into plasma and Cl- enters RBCs, balancing charge as per the ion exchange mechanism in blood transport.

Why the others are wrong:

• A: Reverses the shift; HCO3- moves out of RBCs, not in, and Cl- moves into RBCs, not plasma.
• B: H+ binds to hemoglobin inside RBCs, not moving to plasma; Cl- entry is correct but incomplete.
• C: Cl- enters RBCs but does not bind to hemoglobin, which buffers H+ instead.

Final answer: D