Читать книгу Biochemistry For Dummies - John Moore T., Richard Langley H., John T. Moore - Страница 27

In the human body, the pH of various body fluids is important. The pH of blood is 7.4, the pH of stomach acid is , and the pH in the intestinal tract is . If the pH of blood is more than 0.2 pH units lower than normal, a condition known as acidosis results; a corresponding increase in pH of about the same magnitude is alkalosis. Acidosis and alkalosis, which may lead to serious health problems, each have two general causes:

 Respiratory acidosis is the result of many diseases that impair respiration, including pneumonia, emphysema, and asthma. These diseases are marked by inefficient expulsion of carbon dioxide, leading to an increase in the concentration of carbonic acid, H2CO3.

 Metabolic acidosis is due to a decrease in the concentration of (the bicarbonate ion). This decrease may be the result of certain kidney diseases, uncontrolled diabetes, and cases of vomiting involving nonacid fluids. Poisoning by an acid salt may also lead to metabolic acidosis.

 Respiratory alkalosis may result from hyperventilation because this excessive removal of carbon dioxide can lead to a decrease in the H2CO3 concentration. Immediate treatment includes breathing into a paper bag, which increases the carbon dioxide concentration in the inhaled air and, therefore, in the blood.

 Metabolic alkalosis may result from excessive vomiting of stomach acid.

To resist these pH problems, the blood has a number of buffer systems — systems that resist a change in pH by reacting with either added acids or bases. In general, buffers may be amphiprotic substances or mixtures of weak acids and weak bases. In the body, these include several proteins in blood plasma and the bicarbonate buffer system.

The bicarbonate buffer system is the main extracellular buffer system. This system also provides a means of eliminating carbon dioxide. The dissolution of carbon dioxide in aqueous systems sets up the following equilibrium:

The presence of the conjugate acid-base pair (H₂CO₃ and ) means that this is a buffer system. The conjugate acid-base ratio is about 20:1 at a pH of 7.4 in the bloodstream. This buffer system is coupled with the following equilibrium (instrumental in the removal of carbon dioxide in the lungs):

The second ionization of phosphoric acid, K_a2, is the primary intracellular buffer system. The pH of this conjugate acid-base pair ( and ) is 7.21 for a solution with equal concentrations of these two species.