| 23 Lung and Kidney: The Control of Acid-Base Balance
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After reading this chapter you should be able to:
- Explain the nature of the bicarbonate buffer.
- Describe the gas exchange occurring in the lungs.
- Describe the respiratory and metabolic components of the acid-base balance.
- Define and classify acidosis and alkalosis.
- Comment on clinical conditions associated with disturbances of the acid-base balance.
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The two gases, oxygen and carbon dioxide are key elements of metabolism. While oxygen is used by the body to sustain energy production, metabolism generates carbon dioxide (and hydrogen ion). There is large daily flux of oxygen, carbon dioxide, and hydrogen ion through the human body. Carbon dioxide generated in tissues dissolves in H2O to form carbonic acid, which in turn dissociates releasing hydrogen ion. Metabolism also generates strong acids such as sulfuric acid, and organic acids such as uric acid, lactic acid and others; all are a source of hydrogen ion in the extracellular fluid. Despite large variations in generation of CO2 and acids, for example during exercise, the blood concentration of hydrogen ion (or its negative logarithm, the pH), is surprisingly constant: it remains between 36 and 46 nmol/L (pH 7.36-7.46). Changes in pH profoundly affect the ionization of proteins and, consequently, the activity of many enzymes. Also, a decrease in pH decreases cardiac output and blood pressure, whereas an increase in pH causes constriction of small arteries and may lead to arrhythmia. Also, changes in pH together with the partial pressure of carbon dioxide (pCO2) affect tissue oxygenation by changing the shape of the hemoglobin saturation curve, and thus the ease with which hemoglobin gives up its oxygen to the tissues (see Chapter 4).
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| This chapter describes how the regulation of extracellular and intracellular pH (the acid-base balance) is closely linked with gas exchange, and now problems with gas exchange and acid-base balance underlie many of diseases of the respiratory system and kidney.
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| Maintaining the acid-base balance involves lungs, blood, and kidneys
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| Organs and tissues involved in the maintenance of the acid-base balance are the lungs, the erythrocytes, and the kidneys (Figure 23.1). The lungs control the exchange of carbon dioxide and oxygen between the blood and the external atmosphere; the erythrocytes transport gases between lungs and tissues; and the kidneys control plasma bicarbonate concentration and excrete the hydrogen ion in urine.
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