The Controller and Acid-Base Physiology: An Introduction to a Complex Process
Learning Objectives
- To describe the role of the respiratory system as a cause of primary acid–base disturbances and as a critical component of the body’s ability to compensate for metabolic disturbances in acid–base balance.
- To demonstrate the importance of the carbon dioxide–carbonic acid equilibrium in the body’s acid–base balance.
- To describe the principles underlying the Henderson-Hasselbalch equation and their application to acid–base physiology.
- To enumerate the four basic acid–base disorders and the use of the arterial blood gas (ABG) test in recognizing the disturbances.
- To introduce a few simple elements of renal physiology that are important for the understanding of acid–base physiology.
The normal function of enzymatic activity of the body requires that the concentration of hydrogen ion in the blood, and consequently the cells, be closely regulated. Hydrogen ion concentration in the blood is very low, approximately 40 neq/L, and these ions are small and highly reactive, that is, they bind more strongly to negatively charged molecules than do other cations such as sodium and potassium. The range of hydrogen ion concentration in the blood that is compatible with human life is 16 to 126 neq/L, which translates into a blood pH between 7.80 and 6.90. Major problems occur, however, in multiple physiologic processes when pH deviates below 7.20 or above 7.55. Proteins necessary for many of the chemical reactions in the body are sensitive to relatively small changes in blood pH. The binding of oxygen to hemoglobin, as you recall from Chapter 5, for example, is diminished in an acid environment.
Carbon dioxide dissolved in blood combines with water to form carbonic acid, which is in equilibrium with hydrogen ions and bicarbonate.
Regulation of PaCO2 is, therefore, critical to the maintenance of an acceptable pH level in the blood. From this relationship, you can also see that derangements of the respiratory system, including any of the three components—controller, pump, and gas exchanger—we have discussed thus far that lead to an elevated or reduced PaCO2, can have significant implications for the health of the organism. In Chapter 6, we discussed how changes in pH and PaCO2 affect the peripheral and central chemoreceptors. The controller is designed, in part, to respond to alterations in blood pH and to prevent significant changes in hydrogen ion concentration associated with elevations in carbon dioxide production.
A full understanding of acid–base physiology requires integrated study of the respiratory and renal systems. Because all important journeys must begin with one step, we will start your investigation of this complex topic by examining the important role of the respiratory system in acid–base physiology.