How is the Blood's pH level maintained?
Blood pH level is maintained between 7.35-7.45, so the average blood pH is 7.40, making it slightly alkaline. Blood pH is maintained at this level by bicarbonate-CO2 and phosphate, which neutralize anything too acidic or too alkaline to prevent it from affecting the blood pH. It is essential to keep the blood pH constant as this is the pH that keeps all the organs running. If the pH were to go below 7.35 or above 7.45, the body would shut down.

Acidosis and Alkalosis are two conditions that can help to maintain blood pH. Either one of them can happen when the blood pH goes out of the healthy range.

When the blood pH goes below 7.35, acidosis occurs, which means the blood is too acidic. The body responds to this by increasing breathing (eg. Hyperventilation) to let out more CO2 that decrease the amount of acid within the lungs/blood, and balancing the pH back into the healthy range.

Alkalosis occurs when the blood pH goes above 7.45, which means the blood has become too alkaline. The body responds to this by decreasing the rate of breathing, therefore increasing the amount of acid within the body and balancing it back into the healthy range. The body often decreases breathing by fainting.

In overall, the blood pH within the body is very strongly linked to respiration, and our body has many backup methods to keep the pH constant in case of acidosis or alkalosis.

The link above leads to an image which demonstrates that the blood pH is more or less balanced, but slightly on the alkaline side.
It also demonstrates how the quantity of each solution can be altered by the body's reactions (the faucets).

American Association for Clinical Chemistry (2001-2009), "Acidosis and Alkalosis" [Internet],
[Accessed on: 13th April, 2009]