a state of equilibrium between acidity and alkalinity of the body fluids. An acid is a substance capable of giving up a hydrogen ion during a chemical exchange, and a base is a substance that can accept it. The positively charged hydrogen ion (H + ) is the active constituent of all acids.
Most of the body's metabolic processes produce acids as their end products, but a somewhat alkaline body fluid is required as a medium for vital cellular activities. Therefore chemical exchanges of hydrogen ions must take place continuously in order to maintain a state of equilibrium. An optimal pH (hydrogen ion concentration) between 7.35 and 7.45 must be maintained; otherwise, the enzyme systems and other biochemical and metabolic activities will not function normally.
Although the body can tolerate and compensate for slight deviations in acidity and alkalinity, if the pH drops below 7.30, the potentially serious condition of acidosis exists. If the pH goes higher than 7.50, the patient is in a state of alkalosis. In either case the disturbance of the acid-base balance is considered serious, even though there are control mechanisms by which the body can compensate for an upward or downward change in the pH. Shifts in the pH of body fluids are controlled by three major regulatory systems, which may be classified as chemical (the buffer systems), biologic (blood and cellular activity), and physiologic (the lungs and kidneys).
Chemical Controls. The chemical buffer systems are dependent on the capability of certain substances to either combine with or release hydrogen ions. In the plasma and the intracellular and interstitial fluids there are three major buffer systems that regulate hydrogen ion activity: the carbonic acid–bicarbonate system, the protein buffer system, and the phosphate buffer system.
Of these three, the carbonic acid–bicarbonate system is the most important in fluids outside the cell. It is the most extensive and is the first to react to an acid-base imbalance. Carbonic acid and bicarbonate are both derived from water and carbon dioxide and therefore exist in large quantities in the body. Carbonic acid is, however, weakly ionized and needs to coexist with its salt in order to effectively remove excess hydrogen or hydroxyl ions from the extracellular fluids. Hence it is actually the carbonic acid and sodium bicarbonate buffer system that works to maintain normal levels of hydrogen ion concentrations in the extracellular fluids. It is important to remember that these two chemical components must be in the ratio of 1:20; that is, for every one part of carbonic acid (H2 CO3 ) there must be twenty parts of sodium bicarbonate (NaHCO3 ). It is not the absolute amount of each component
that is crucial in the control of acid-base balance, but the ratio of the one substance to the other. The carbonic acid–bicarbonate buffer system is capable of either accepting or releasing hydrogen ions without forcing the pH to dangerous levels.
The protein buffer system is especially remarkable because proteins are powerful buffers that can function as either acid or base, depending on the state of the body fluids. This system is active in the plasma and in intracellular and extracellular fluids.
The phosphate buffer system operates in much the same way as the carbonic acid–bicarbonate system but is more active within the cell than in extracellular fluids.
Although the chemical buffer systems react almost instantaneously to a change in the pH of the body fluids, they cannot provide sustained regulation of the pH because they are absorbed rapidly and cannot be replaced immediately. The hydrogen ions that are not handled by the chemical buffer systems become the responsibility of other regulatory controls which respond less rapidly but are not less important.
Biologic Regulators. This type of control is concerned with the shifting of excess acid or alkali in and out of the cell. As excess ions cross over the cell membrane they must do so in combination with ions of the opposite charge, or in exchange for ions of the same charge. Sodium and potassium are the two cations most often exchanged for the positively charged hydrogen ion.
The hemoglobin-oxyhemoglobin system is another regulatory control. As chloride leaves the oxygenated blood cells and enters the plasma, the bicarbonate moves from the plasma and crosses over into the cellular fluid. This reciprocal exchange between bicarbonate and chloride is a continuous process.
Physiologic Regulators. The lungs begin to compensate for a metabolic acid-base imbalance within minutes of its onset. They do this by regulating the retention or the excretion of carbon dioxide. If acidosis is present, respiratory activity is increased so that the arterial CO2 concentration falls. If alkalosis is present, respiratory activity is automatically decreased, and CO2 is retained, thus producing a compensatory respiratory acidosis.
The kidneys act as regulators by reabsorbing bicarbonate when it is needed to control excess acidity and by excreting it when there is a deficit of acid in the body. The kidneys also facilitate the excretion of excess hydrogen ions in combination with phosphate ions (in the form of phosphoric acid), or in combination with ammonia (excreted in the form of ammonium).
Imbalances of the acid-base ratio are discussed under acidosis and alkalosis. Diagnosis and monitoring of either of these conditions are greatly enhanced by periodic determination of the pH and by blood gas analysis .