Acid Base Balance of the Blood

The pH of blood plasma is maintained within a narrow range of values through the functions of the lungs and kidneys.The lungs regulate the carbon dioxide concentration of the blood, and the kidneys regulate the bicarbonate concentration.

The blood plasma within arteries normally has a pH between 7.35 and 7.45, with an average of 7.40. Using the definition of pH described in chapter 2, this means that arterial blood has a H+ concentration of about 10-7 4 molar. Some of these hydrogen ions are derived from carbonic acid, which is formed in the blood plasma from carbon dioxide and which can ionize, as indicated in these equations:

CO2 + H2O H2CO3

Carbon dioxide is produced by tissue cells through aerobic cell respiration and is transported by the blood to the lungs, where it can be exhaled. As will be described in more detail in chapter 16, carbonic acid can be reconverted to carbon dioxide, which is a gas. Because it can be converted to a gas, carbonic acid is referred to as a volatile acid, and its concentration in the blood is controlled by the lungs through proper ventilation (breathing). All other acids in the blood—including lactic acid, fatty acids, ketone bodies, and so on—are nonvolatile acids.

Under normal conditions, the H+ released by nonvolatile acids does not affect the blood pH because these hydrogen ions are bound to molecules that function as buffers. The major buffer in the plasma is the bicarbonate (HCO3-) ion, and it buffers H+ as described in this equation:

This buffering reaction could not go on forever because the free HCO3- would eventually disappear. If this were to occur, the H+ concentration would increase and the pH of the blood would decrease. Under normal conditions, however, excessive H+ is eliminated in the urine by the kidneys. Through this action, and through their ability to produce bicarbonate, the kidneys are responsible for maintaining a normal concentration of free bicarbonate in the plasma. The role of the kidneys in acid-base balance is described in chapter 17.

A fall in blood pH below 7.35 is called acidosis because the pH is to the acid side of normal. Acidosis does not mean acidic (pH less than 7); a blood pH of 7.2, for example, represents serious acidosis. Similarly, a rise in blood pH above 7.45 is called alkalosis. Both of these conditions are categorized into respiratory and metabolic components of acid-base balance (table 13.6).

Respiratory acidosis is caused by inadequate ventilation (hypoventilation), which results in a rise in the plasma concentration of carbon dioxide, and thus carbonic acid. Respiratory alkalosis, by contrast, is caused by excessive ventilation (hyperventilation). Metabolic acidosis can result from excessive production of nonvolatile acids; for example, it can result from excessive production of ketone bodies in uncontrolled diabetes mellitus (see chapter 19). It can also result from the loss of bicarbonate, in which case there would not be sufficient free bicarbonate to buffer the nonvolatile acids. (This occurs in diarrhea because of the loss of bicarbonate derived from pancreatic juice—see chapter 18.) Metabolic alkalosis, by contrast, can be caused by either too much bicarbonate (perhaps from an

378 Chapter Thirteen

Table 13.7

Plasma CO2

Classification of Metabolic and Respiratory Components of Acidosis and Alkalosis

Plasma HCO3- Condition Causes

Normal

Low

Metabolic acidosis

Increased production of "nonvolatile" acids (lactic acids, ketone bodies, and others), or

loss of HCO3- in diarrhea)

Normal

High

Metabolic alkalosis

Vomiting of gastric acid; hypokalemia; excessive steroid administration

Low

Low

Respiratory alkalosis

Hyperventilation

High

High

Respiratory acidosis

Hypoventilation

intravenous infusion) or inadequate nonvolatile acids (perhaps as a result of excessive vomiting). Excessive vomiting may cause metabolic alkalosis through loss of the acid in gastric juice, which is normally absorbed from the intestine into the blood.

Since the respiratory component of acid-base balance is represented by the plasma carbon dioxide concentration and the metabolic component is represented by the free bicarbonate concentration, the study of acid-base balance can be simplified. A normal arterial blood pH is obtained when there is a proper ratio of bicarbonate to carbon dioxide. Indeed, the pH can be calculated given these values, and a normal pH is obtained when the ratio of these concentrations is 20 to 1. This is given by the Henderson-Hasselbalch equation:

where PCo2 = partial pressure of CO2, which is proportional to its concentration.

Respiratory acidosis or alkalosis occurs when the carbon dioxide concentrations are abnormal. Metabolic acidosis and alkalosis occur when the bicarbonate concentrations are abnormal (table 13.7). Often, however, a primary disturbance in one area (for example, metabolic acidosis) will be accompanied by secondary changes in another area (for example, respiratory alkalosis). It is important for hospital personnel to identify and treat the area of primary disturbance, but such analysis lies outside the scope of this discussion.

A more complete description of the respiratory and metabolic components of acid-base balance requires the study of pulmonary and renal function, and so will be presented with these topics in chapters 16 and 17.

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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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