Carbon dioxide can react with water to produce bicarbonate and hydrogen ions through 2 reversible reactions as follows:
CO₂+ H₂O "H₂CO₃"H⁺+ HCO₃^-
Blood flows from arteries into capillaries and then into veins. As the blood moves through the capillaries, carbon dioxide produced by the cells enters the blood. Therefore, venous blood has a higher pH than arterial blood.

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Venous blood has a lower pH than arterial blood because it has a higher concentration of carbon dioxide, which forms carbonic acid when dissolved in blood, lowering the pH.

Carbon Dioxide Can React with Water to Produce Bicarbonate and Hydrogen