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The maintenance of a relatively constant internal environment (homeostasis) is one of the major physiologic functions of the organ systems of the body. Body temperature, fluid volume and osmolarity, and electrolytes—including acids and bases—are normally carefully regulated. A thorough knowledge of the mechanisms that control these variables is essential to clinical practice.
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The respiratory system is intimately involved in the maintenance of the balance of acids and bases in the body. It removes approximately 300 times as much acid per day than the excretory system does. This chapter will introduce the major concepts of acid-base balance, particularly with respect to the respiratory system; a more detailed study of this important subject is strongly encouraged.
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THE CHEMISTRY OF ACIDS, BASES, AND BUFFERS
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Although there are several ways to define acids and bases, the most useful is to define an acid as a substance that can donate a hydrogen ion (a proton) to another substance and a base as a substance that can accept a hydrogen ion from another substance. A strong acid is a substance that is completely or almost completely dissociated into a hydrogen ion and its corresponding or conjugate base in dilute aqueous solution; a weak acid is only slightly ionized in aqueous solution. In general, a strong acid has a weak conjugate base and a weak acid has a strong conjugate base. The strength of an acid or a base should not be confused with its concentration.
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A buffer is a mixture of substances in aqueous solution (usually a combination of a weak acid and its conjugate base) that can resist changes in hydrogen ion concentration when strong acids or bases are added. That is, the changes in hydrogen ion concentration that occur when a strong acid or base is added to a buffer system are much smaller than those that would occur if the same amount of acid or base was added to pure water or another nonbuffer solution.
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The Quantification of Acidity
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The acidity of a solution is determined by the activity of the hydrogen ions in the solution. The hydrogen ion activity, which is denoted by the symbol αH+, is closely related to the concentration of hydrogen ions ([H+]) in a solution. In extremely dilute solutions, the hydrogen ion activity is equal to the hydrogen ion concentration; in highly concentrated solutions, the activity is less than the concentration. The hydrogen ion concentration of the blood is low enough that the hydrogen ion activity may be considered to be equal to the hydrogen ion concentration.
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The hydrogen ion activity of pure water is about 1.0 × 10–7 mol/l. By convention, solutions with hydrogen ion activities above 10–7 mol/l are considered to be acid; those with hydrogen ion activities below 10–7 are considered to be ...