Disturbances of Acid-Base Balance

Abstract

In everyday practice the term acid-base balance refers to the state of the arterial blood. The term is quite different when the term balance relates to water, sodium or potassium; in this sense balance means the difference between the intake in food or drink and the output in the urine, faeces, sweat or expired air. The acid-base balance of the blood is the only measure of acid-base balance available during intensive care and resuscitation. The measurements are used as a guide to the acid-base state of the internal medium or ECF. Severe changes in the acid-base balance of the internal medium disrupts the functioning of cells and organs and the patient dies. The more rapidly the change occurs the greater the disruption of cell function, since there is then less time for the body to defend the change by compensatory mechanisms. Before analysing the acid-base balance of the blood, that of the body as a whole is described, to enable the reader to make comparisons with the balances of fluid and nitrogen (Chapter 5). In Figure 4.1 is shown at ‘A’ the normal balance for sodium; a daily intake of food or beverages of 100 mmol, and the same amount lost in the urine, faeces, etc.; none is made in the body! At ‘B’ is the normal acid balance. A conventional western diet contains a few mmol of acid, and about 60 mmol of acid is excreted in the urine. But a healthy subject at rest produces about 15000 mmol of carbon dioxide a day. Carbon dioxide is not an acid, but a potential acid; that is when dissolved in water or the body fluids it forms carbonic acid. Unless removed from the body at the same rate as produced, the carbon dioxide can lead to an intolerable positive balance of acid. It will be recalled (Chapter 2) that the carbon dioxide in the arterial blood and ECF represents the balance between the production of carbon dioxide and its elimination by the lungs. The output of carbon dioxide (B) below the horizontal base line is therefore proportional to alveolar ventilation. So much for the body balance of acid. What is the state of affairs for base? The intake in food is again negligible and in health that lost in the urine is small.

The condition known as acidosis is today thardly less familiar than tanaemia. L. J. Henderson, 1928