In the equation given above, there is conservation of MASS, CHARGE AND ENERGY.
These three parameters are usually conserved during the course of chemical reactions. When any of these parameter experience a reduction during the course of chemical reaction, such loss is always gained by other elements involved in the same reaction, so that at the end of the day, they are not considered as lost.
27g x 1cm^2/2.7g = answer in cm^2
Grams cancel out
This is due to the solubility of CO₂ in blood is more than the solubility of Oxygen in blood