Dr Rr wrote:
Here's the explaination:
If we review Co2 transport from tissues to venous blood: there is formation of HCO3 ion , which exits from RBC and is followed by entry of Chloride ions (aka Chloride Shift). Presence of Bicabonate and entry of Chloride ions inc the osmotic pressure of venous RBC and water follows inside RBC.
Therefore Vol of Venous RBC > Vol of Arterial RBC
And hence HCt of Venous blood is higher.

Dr_C wrote:
Found something.
Each carbon dioxide molecule added to the red blood cell increases the intracellular osmotic pressure by an increase in either or Cl– ions. Therefore, the red blood cell increases in size and the haematocrit of venous blood is some 3% more than arterial blood. The plasma concentration of chloride ion is lower but bicarbonate ion concentration is greater.