http://www.merck.com/mmhe/sec12/ch159/ch159a.html

An important property of blood is its degree of acidity or alkalinity. Body
acidity increases when the level of acidic compounds in the body rises
(through increased intake or production, or decreased elimination) or when
the level of basic (alkaline) compounds in the body falls (through decreased
intake or production, or increased elimination). Body alkalinity increases
with the reverse of these processes. The body's balance between acidity and
alkalinity is referred to as acid-base balance.

The blood's acid-base balance is precisely controlled, because even a minor
deviation from the normal range can severely affect many organs. The body
uses different mechanisms to control the blood's acid-base balance.

One mechanism the body uses to control blood pH involves the release of
carbon dioxide from the lungs. Carbon dioxide, which is mildly acidic, is a
waste product of the metabolism of oxygen (which all cells need) and, as
such, is constantly produced by cells. As with all waste products, carbon
dioxide gets excreted into the blood. The blood carries carbon dioxide to
the lungs, where it is exhaled. As carbon dioxide accumulates in the blood,
the pH of the blood decreases. The brain regulates the amount of carbon
dioxide that is exhaled by controlling the speed and depth of breathing. The
amount of carbon dioxide exhaled, and consequently the pH of the blood,
increases as breathing becomes faster and deeper. By adjusting the speed and
depth of breathing, the brain and lungs are able to regulate the blood pH
minute by minute.

The kidneys are also able to affect blood pH by excreting excess acids or
bases. The kidneys have some ability to alter the amount of acid or base
that is excreted, but because the kidneys make these adjustments more slowly
than the lungs do, this compensation generally takes several days.

Yet another mechanism for controlling blood pH involves the use of buffer
systems, which guard against sudden shifts in acidity and alkalinity. The pH
buffer systems are combinations of a weak acid and weak base that exist in
balance under normal pH conditions. The pH buffer systems work chemically to
minimize changes in the pH of a solution by adjusting the proportion of acid
and base. The most important pH buffer system in the blood involves carbonic
acid (a weak acid formed from the carbon dioxide dissolved in blood) and
bicarbonate ions (the corresponding weak base).

Acidosis and alkalosis are the two abnormalities of acid-base balance. In
acidosis, the blood has too much acid (or too little base), resulting in a
decrease in blood pH. In alkalosis, the blood has too much base (or too
little acid), resulting in an increase in blood pH. Acidosis and alkalosis
are not diseases, but rather are the result of a wide variety of disorders.
The presence of acidosis or alkalosis provides an important clue to doctors
that a serious problem exists.

Acidosis and alkalosis are categorized as metabolic or respiratory,
depending on their primary cause. Metabolic acidosis and metabolic alkalosis
are caused by an imbalance in the production of acids or bases and their
excretion by the kidneys. Respiratory acidosis and respiratory alkalosis are
caused primarily by lung or breathing disorders.