The amount of CO2 is proportional to the increase of acidity in water.
That is, the more CO2 in the water, more acidic the solution becomes and the lower the pH
That is because CO2 dissolves in water to produce carbonic acid which increasing the pH
In terms of this biologically, the blood is subjected to chances in it's composition, including the addition of acidic substances (such as CO2). The body then has to use homeostasis to counteract the increase in acidity of the blood.
This is achieved through buffers in the blood.