The bicarbonate, or carbonate, buffer system is one of the most important buffering systems in nature. Like any buffering system, a bicarbonate buffer resists change in pH, so it helps stabilize the pH of solutions like blood and ocean water. Ocean acidification and the effects of exercise on the body are both examples of how bicarbonate buffering works in practice.
When carbon dioxide gas is dissolved in water, it can react with water to form carbonic acid. Carbonic acid can in turn give up a hydrogen ion to become bicarbonate, which can give up another hydrogen ion to become carbonate. All these reactions are reversible. This means they work both forward and in reverse. Carbonate, for example, can pick up a hydrogen ion to become bicarbonate.
The series of reactions that leads from dissolved carbon dioxide to carbonate quickly reaches a dynamic equilibrium, a state in which the forward and reverse processes of this reaction happen at equal rates. Adding acid will increase the rate of the reverse reaction and of carbon dioxide formation, causing more carbon dioxide to diffuse out of the solution. Adding base, on the other hand, will increase the rate of the forward reaction, causing more bicarbonate and carbonate to form. Any pressure on this system, in other words, causes a compensating shift in a direction that restores equilibrium. The buffering system continues to work as long as its concentration is large in comparison to the amount of acid or base added to the solution.
In humans and other animals, the carbonate buffering system helps maintain a constant pH in the bloodstream. The pH of blood depends on the ratio of carbon dioxide to bicarbonate. The concentrations of both components are very large compared to the concentrations of acid added to the blood during normal activities or moderate exercise. During strenuous exercise, for example, rapid breathing helps to compensate for the increased rate at which CO2 is added to your blood. Other mechanisms are involved as well. The hemoglobin molecule in your red blood cells, which also helps to buffer blood pH, is one such mechanism.
In the ocean, dissolved CO2 from the atmosphere is in equilibrium with seawater concentrations of carbonic acid and bicarbonate. According to NASA, however, increased CO2 emissions through human activity have increased atmospheric CO2 levels, causing an increase in dissolved CO2. As the concentration of dissolved CO2 increases, the rate of the forward reaction of the buffering system increases until the system reaches a new equilibrium. This means that an increase in dissolved CO2 causes a slight decrease in pH. The ocean's buffering capacity -- its ability to soak up acid or base -- is very large, but gradual changes of this kind can have serious ramifications for many kinds of life in the ocean. Animals that make their shells from calcium carbonate, for example, might find their shell-making capabilities reduced by significant changes in the acid-base equilibrium of ocean water.