When the blood pH rises above 7.8 what will happen?

Living things require specific conditions of pH to survive. An acid is a substance that adds Hydrogen ions (H+) to solution while a base reduces the H+ ion concentration. 

The pH scale is the negative logarithm of the hydrogen ion concentration which is 10-7 M in a neutral solution while the OH- ions of a base in a neutral solution are also 10-7 M. These balance each other in a neutral solution of pH 7. 

Any value below 7 is the increasing range for acid on the pH scale and any value above 7 is the represents increasing alkalinity.  A small change in pH makes a large difference in concentration of H+ ions and OH- ions in solution. If the pH decreases from 6 to 5, the solution will be 10 times more acidic, because each unit represents a 10 fold change in the H+ ion and OH- ion concentrations.

Most body fluids have a specific pH range of between 6-8--slightly acidic, to neutral to slightly alkaline.

Blood pH is usually about 7.4 and if it drops below this or above this for more than a few minutes,  the results can be devastating. A pH of 7.8 can result in a person's death if it doesn't drop back down to its normal range. 

Buffers are special chemicals in the blood which can accept hydrogen ions from solution when the pH is too low and can donate hydrogen ions when the pH is too high. The buffer consists of a weak acid and its associated base. The equilibrium between the buffer carbonic acid and its base bicarbonate in solution help to maintain pH. Carbonic acid forms in the blood when carbon dioxide in the blood reacts with water from the liquid part of blood called plasma.

In the blood, the buffer carbonic acid (H2CO3)  dissociates to form bicarbonate ion (HCO3-) and a hydrogen ion (H+). This is a reversible reaction that can proceed in either direction depending on the amount of H ions in solution.

Various chemical reactions of metabolism can add to or remove H+ ions from solution and the body needs to constantly adjust to these changes. The equilibrium of the carbonic acid and bicarbonate ion reaction can shift to the left or to the right to help remove or add H+ions to keep blood pH in the optimal range.

In your example, if the pH is too high, the dissociation of carbonic acid will add H+ions to solution when the reaction shifts to the right and this will help lower the pH to an acceptable range.