Acids and Bases

Acids are substances that dissociate to donate H+ (eqn [2]); the stronger the acid, the more readily it dissociates. The dissociation constant (pKa) is the pH

at which 50% of the acid is dissociated. At pH values greater than pKa more H+ will dissociate; the lower the pKa, the stronger the acid. A base is a substance that accepts hydrogen ions. In the following text the term 'fixed acid' is used to describe formed acid, and 'volatile acid' is used to describe the potential acid load imposed by carbon dioxide (CO2). Where 'A' represents an acid, the following applies:

The importance of this relationship in physiological terms is that since the pKa of most organic acids is much lower than the pH of extracellular fluids, most organic acids exist in a dissociated state (as acid anion salts) the free H+ being buffered. In urine, where the minimum achievable pH is around 5, most strong acids (with a pKa below this value) will be in a dissociated state, necessitating the excretion of H+ together with urinary buffers.

Acidosis is the term used to describe conditions where pH is low and those where pH would be low were it not appropriately buffered; similarly, alkalo-sis is the term used for a high pH and for a potentially elevated pH that has been appropriately buffered. Acidemia and alkalemia reflect low or elevated blood pH. It is common to describe acido-sis/alkalosis as respiratory or metabolic depending on their causation.

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