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Compensation

Patients are often encountered who significantly deviate from the normal values given above, yet nonetheless maintain blood pH within a range compatible with life. Such patients have compensated for circumstances that would normally produce acidosis or alkalosis.

The table below lists some of the changes observable in these different states.

	respiratory					metabolic
	acidosis		alkalosis			acidosis		alkalosis
	U	C	U	C		U	C	U	C
pH
HCO3-:CO2 ratio
[HCO3-]
pCO2
total CO2

= increased = decreased = no major change

U = uncompensated C = compensated = Red arrows indicate the primary defect. = Green arrows indicate compensation mechanisms.

Notice that pH is nearly normal in compensated cases. The chart above highlights in red the "primary" indicator for respiratory problems, which is pCO₂, and the "primary" indicator for metabolic problems, with is [HCO₃^-].

Because respiration is the main way we expel CO₂, the fact that pCO₂ is the primary indicator of a respiratory problem should be no surprise. However, the fact that [HCO₃^-] is the "primary" indicator for metabolic problems perhaps requires some explanation.

Suppose your patient has ingested an acid. The [H⁺] will obviously go up, pushing the bicarbonate equation to the left:

CO2

+

H2O

[H2CO3]

H+

+

HCO3-

Notice that, as the equilibrium shifts to the left, HCO₃^- levels are also affected (consumed, in this case). One could argue that pH (since it directly relates to [H⁺]) is the "primary" defect, but pH can be adjusted by compensatory mechanisms and so is not usually the most reliable measure. HCO₃^-, by its fluctuation from normal in response to metabolic (or exogenous) insults, is really what gives you the most reliable indicator of a "metabolic" problem.

The various compensatory mechanisms (some of which are shown in green above) are the subject of the last four questions in this problem set.