Neutral is the pH at which there are equal numbers of [H+] ions and [OH-] ions. Water is more ionized at body temperature than at room temperature; neutral is pH 6.8 rather than 7.0. This is also the average pH inside the cell. The body preserves neutrality (pH 6.8) inside our cells, where most of the body's chemistry occurs, and maintains the blood at pH 7.4, which is 0.6 pH units on the alkaline side of neutral (Reeves and Rahn, 1979).
pH is the negative logarithm of the hydrogen ion concentration. A complete definition requires that the logarithm is defined as being to the base ten and the concentration be measured as activity in moles per liter. The pH notation is one source of confusion because, as the acidity increases, the pH decreases. To avoid confusion when discussing acid-base balance, it is best to avoid "increase" and "decrease" and use "acid change" and "alkaline change" instead.
Logarithm. It is helpful to think of "power." Thus 103 = 1000 and log (1000) = 3. This is another source of confusion in acid base balance and is responsible for the mistaken impression that the body maintains remarkably tight control over its hydrogen ion concentration. (Blood pressure or pulse measured with a logarithmic notation would also appear remarkably stable). When the pH changes by 0.3 units, e.g., from 7.4 to 7.1 the hydrogen ion concentration doubles (from 40 to 80 nmol/1.)
Respiratory Acid and Respiratory Acidosis. Carbon dioxide is respiratory acid - it is the only acid which can be exhaled. Strictly speaking carbon dioxide is a gas, not an acid. Carbonic acid is only formed when combined with water. Nevertheless, clinicians customarily regard carbon dioxide and respiratory acid as synonymous. Respiratory acidosis is a high PCO2.
Metabolic Acids and Metabolic Acidosis. The term "metabolic acids" includes all of the body's acids except carbon dioxide. Metabolic acids are not respirable; they have to be neutralized, metabolized, or excreted via the kidney. Metabolic acidosis is a pH which is more acid than appropriate for the PCO2. This definition emphasizes the importance of the respiratory component to the overall pH. The pH is always a product of the two components, respiratory and metabolic, and the metabolic component is judged, calculated, or computed by allowing for the effect of the PCO2, i.e., any change in the pH unexplained by the PCO2 indicates a metabolic abnormality.
Acidosis and Alkalosis. Acidosis is an abnormality which tends to produce an acid pH unless there is a dominating, opposing alkalosis. Alkalosis is the opposite and tends to produce an alkaline pH unless there is a dominating, opposing acidosis.
Bicarbonate. In acid-base determinations the concentration (in milliequivalents per liter) of the bicarbonate ion (HCO3-) is calculated from the PCO2 and pH. Because it is also altered by both the respiratory and the metabolic components (see below under Physiology), it cannot be an ideal measure of either.
Base Excess (BE) is a measure of metabolic acid level, and is normally zero. The blood base (total base) is about 48 mmol/l depending on the hemoglobin concentration. Changes are termed excess or deficit. It is helpful to remember that the phrase "this patient has a base excess of minus ten" means "this patient has a metabolic acid excess (acidosis) of 10 mEq/1." The base excess may be used to estimate the amount of treatment (neutralization) required to overcome the metabolic acidosis (or alkalosis).