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If I understand the question correctly, you are asking what happens when pH of the blood fluctuates too much from the norm?

If the pH fluctuates too much (becomes more acidic or even more basic than it is), you would have denaturation of proteins found in the blood. Changes in pH affect conformation of proteins...denaturation of the proteins means that you have a loss of protein shape. If proteins are not shaped properly, they cannot function properly either! So, one prime example of protein carried in the red blood cells is hemoglobin. Hemoglobin is responsible for carrying oxygen to places throughout the body and also taking up CO2 to get rid of it from the body. If this protein is misshaped due to fluctuations in pH, one would have a troublesome time getting oxygen to needed areas and getting rid of CO2 waste.

Enzymes have a very narrow range they can perform in. If blood pH is too acidic or basic they do not work and can denature. They have an optimum pH near 7ish.