DISORDERS OF ACID-BASE PHYSIOLOGY

Chapters 1 and 2 have provided the necessary background to consider disturbances of acid-base physiology and the body s responses, which forms the subject matter of this chapter. 

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As will be explained later, the respiratory system contributes more than mere maintenance of a constant [CO2]. When the pH is low as in the present examine, hyperventilation contributes to the physiological response by changing the PCO2 to a subnormal value, this lowers [CO2] and thereby raises the pH to above 7.1. Such is the efliciency of the body in its response to disorders of acid-base physiology. 

The pH values of arterial plasma measured for all the individuals in a crowd of normal healthy people would fall mostly in the range 7.35 to 7.45, with an average of 7.4 (Robinson, 1962, p. 3). The plasma pH values measured on patients admitted to the metabolic ward of a hospital and suffering from untreated disorders of acid-base physiology would probably range between 7.1 and 7.7, this being the range compatible with life. For short periods of time, it is possible for the pH to go even further from normal particularly on the acid side, but these are useful limits to remember and are shown in Table 2.1. 

Range in disorders of acid-base physiology compatible with life ... 

Disorders of acid-base physiology which are not of respiratory origin are called metabolic disorders. This nomenclature derives from the fact that such disorders result from abnormal metabolism. Metabolic disorders of metabolism may also be due to excessive intake of acid or alkali or to failure of renal function, when the tubular mechanisms for formation of acid or alkaline urine are impaired. 

Respiratory disorders of acid-base physiology have the common feature that the primary abnormality is a deviation from normal of the amount of carbon dioxide in the body. Metabolic disorders are caused by a large variety of primary abnormalities resulting from an excess of non-respiratory acid or alkali in the body. It is useful to classify the causes thus ... 

In order to understand metabolic disorders of acid-base physiology, blood buffers must be considered in a little more detail. It is useful to subdivide... 

Disorders of acid- base physiology not of respiratory origin are called metabolic... 

In general, if the primary disturbance causes an excess of unwanted add in the body, the compensation involves addition to the body of extra alkali to neutralize the acid. A primary acidosis calls up in response a physiological alkalosis. The disorder of acid-base physiology is then a mixture of these two components which operate in opposite directions. A primary addosis is accompanied by a secondary (compensatory) alkalosis and vice versa. It is clearly important to unravel the different components of an add-base disorder from an examination of a sample of arterial blood. 

In any disorder of acid-base physiology, there are liable to be two components, respiratory and metabolic, and the contribution of each must be identified. For the respiratory component, the arterial PCO2 immediately indicates any deviation from normality (Table 4.1 A). If this is within normal limits, there is no respiratory component to the acid-base disorder. 

In previous chapters, respiratory and non-respiratory disorders of acid-base physiology have been described. In this chapter, methods of measuring the respiratory and non-respiratory components of a disturbance of acid-base physiology have been considered. We now consider these two groups of ideas together. 

Figure 43. Similar to Figure 4.4, with indications of the regions of the plane corresponding to particular disorders of acid-base physiology. 

Disorders of acid base balance are usually a consequence of disease such as diabetes mellitus. The main thrust of treatment is reversing the primary disease process and then the regulatory mechanisms of the body automatically reverse the acid- base abnormalities. There are, however, conditions in which the acid-base disorder must be treated directly because otherwise the patient dies of the acid base upset before the primary illness can be treated. Acidaemia in particular needs treatment. In most pathological conditions involving metabolic disorders of acid-base physiology, the result is acidaemia, alkalaemia being much less common. 

INTRACELLULAR HYDROGEN ION CONCENTRATION IN DISORDERS OF ACID-BASE PHYSIOLOGY... 

In metabolic acidotic disorders of acid-base physiology, it is the peripheral chemoreceptors which sense the pH of the extracellular fluid and cause the hyperventilatory compensatory response. 

Metabolic disorder of acid-base physiology A disorder of acid-base balance not caused by a respiratory disorder. An example is the excess of acid which occurs in uncontrolled diabetes mellitus. 

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