2.1. Acid-base balance

A substance that can release or donate hydrogen ions (H⁺) is called an acid and a substance that combines with or accepts hydrogen ions is called a base. When an acid dissociates in a solution, it yields a free H⁺ and its conjugate base (with a negative charge). The acid dissociation constant tells the strength of an acid: the higher the dissociation constant, the more an acid is ionized and the greater is its strength. The concentration of H⁺ in a solution is usually given as pH, which is a negative logarithm of the H⁺ concentration when expressed as moles/L. (Rhoades & Tanner 1995)

Acids in the mammalian body fall into two groups: carbonic acid (H₂CO₃) and all other acids (so-called nonvolatile acids). All can be products of the metabolism. Carbonic acid is in equilibrium with the volatile gas CO₂, which leaves the body via lungs, whereas the other acids in the body are not directly affected by breathing. The nonvolatile acids are buffered in the body and excreted by the kidneys. (Rhoades & Tanner 1995)

Mammalian cells are very sensitive to intracellular changes in the H⁺ concentration and also to that of the extracellular fluid, because the intracellular pH depends in part on the extracellular pH. Nevertheless, they are not identical. The normal pH of the human arterial blood is 7.40. In the venous blood it is slightly lower because of the higher concentration of the carbon dioxide. Intracellular pH values are lower than those of the extracellular fluid and range from 6.8 to 7.3 depending on the tissue and its metabolic rate. When arterial pH is below 7.40, the state is called acidosis and when above it, it is called alkalosis. (Ganong 1981)

Acidosis and alkalosis are both classified as either metabolic or respiratory, depending on whether it is bicarbonate (HCO₃^-) (metabolic) or carbon dioxide (pCO₂) (respiratory) that primarily deviates from the normal range in blood (Martin et al. 1981). In metabolic acidosis HCO₃^- and thus plasma pH (hydrogen concentration) fall, in alkalosis they rise. In respiratory alkalosis pCO₂ value and thus the carbonic acid concentration falls and pH rises (Martin et al. 1981) and vice versa in respiratory acidosis.