Question

the size of red blood cells (rbc) in venous blood is greater than that of arterial blood. this increased size of red blood cell in the venous blood is due to

Answer 1

says that CO2 taken up in the venous red blood cell after having exchanged with O2 with water due to carbo-anhydrase will form H2CO3, which will dissociate into H+ and HCO3-, HCO3- is exchanged for extracellular Cl-, the chloride shift, while H+ is bound to hemoglobin. There still are extra unbound ions causing the osmotic pressure inside the venous red blood cell to rise, thus attracting water into the RBC and thus it will become a little big later (higher MCV).