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Acid-Base Balance Tutorial

by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

Acid-Base Balance Physiology

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Physiology IconA good physiological experiment ... should present anywhere, at any time .. the same certain and unequivocal phenomena that can always be confirmed – Johannes Peter Müller

Page Index

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  [ H+ ] x [ HCO3- ] = k x [ CO2 ] x [ H2O ]  

Carbonic Acid.

Carbonic acid (H2CO3) is central to our understanding and evaluation of acid-base disturbances. This is because it is so readily and rapidly changed. The dissociation products and the ionization products are normally in equilibrium:

[ H+ ] x [ HCO3 ] = k1 x H2CO3 = k2 x [ CO2 ] x [ H2O ]

This equation can be simplified because H2CO3 is not of clinical interest, [H2O] is constant in-vivo, and PCO2 is more familiar than [CO2]:

[ H+ ] x [ HCO3 ] = k x PCO2

This is the Modified Henderson Equation, an example of the Law of Mass Action where the products of the concentrations on one side are proportional to the products on the other.

Blue Bar CO2 "A High PCO2 with no Metabolic Compensation."

Pure Respiratory Acidosis (high PCO2) causes molecules of CO2 and water to form carbonic acid which ionizes to increase both [HCO3] and [H+]. The [H+] changes relatively slightly due to buffering of H+, mostly by hemoglobin. At a raised PCO2, the kidney compensates by eliminating [H+]. To maintain chemical equilibrium the [HCO3] rises further, i.e., respiratory acidosis raises the bicarbonate level and metabolic compensation raises it further. Try it (Click):

Respiratory Acidosis Blue Bar

Metabolic Acidosis "Metabolic Acidosis with no Respiratory Compensation." Pure Metabolic Acidosis implies a raised [H+] level with a normal PCO2. To maintain the equilibrium, the high [H+] would merely cause a reciprocal fall in the [HCO3]. In practice respiratory compensation lowers the PCO2 almost at once, which reduces both the [H+] and the [HCO3], i.e., metabolic acidosis lowers the bicarbonate level and respiratory compensation lowers it further. Try it (Click):

Metabolic Acidosis Blue Bar

Bicarbonate "Mistakenly regarded as a measure of Metabolic Acidosis." Bicarbonate measures neither metabolic acidosis nor respiratory acidosis. This is because both the respiratory and the metabolic components affect bicarbonate ion concentration. The exception would be when Blood Chemistry show a bicarbonate change and the patient appears to have normal lungs. This usually indicates a metabolic abnormality.

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The Cell Membrane.

Lipid Conduit, Polar Barrier

Cell "Cell Membrane is really a better description than Cell Wall." The Cell Membrane provides a protected environment for the reactions which sustain life. It limits transfer of various substances, particularly those that are polar or ionized but allows water, lipid soluble substances, and dissolved gases to pass freely. The composition of the cell depends upon the pH for two reasons: first, as the pH changes so will the degree of ionization and, hence, the concentration of ionized substances; second, if the degree of ionization changes greatly, a substance may ionize less and cross the cell membrane more readily.

The pH varies from one part of the cell to another and probably averages close to neutral - which is pH 6.8 at 37oC. This is more acidic than the relatively akaline extracellular fluid. In practice we neither measure, nor directly treat, the pH inside the cell; we treat the extracellular fluid.

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Extracellular Fluid.

The Cell's "Bath Water"

ECF "ECF: Our Internal Swimming Pool." About 20% of the body water is extracellular fluid - typically 14 liters. This is the environment - the "Bath Water" which provides the cell's nutrition, oxygenation, waste removal, temperature, and alkaline environment. Normal extracellular pH (7.4) is slightly alkaline and represents [H+] = 40 nmol/1. This is about 25% of the [H+] inside the cell, 160 nmol/1 (average pH = approximately 6.8). This concentration gradient favors hydrogen ion elimination from the cell but is counterbalanced by the intracellular potential of -60 mV which tends to attract the hydrogen ion into the cell.

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"Treatable" Volume.

The Cell's Larger Bath

Treatment "Bicarbonate actually treats more than just the extracellular fluid." The "Treatable Volume" is, in practice, larger than the extracellular fluid because cell membranes are not completely impermeable – some equilibration occurs between the cell and the extracellular fluid. It is, therefore, customary to make calculations based on a slightly larger volume – 30%.

Thus the treatable space is typically about 21 liters, a useful approximation for emergency therapy. Over a longer period, however, equilibration continues between the intra- and extra- cellular fluid which further increases the apparent size of the treatable space. In addition, there may be other sources of change during a period of therapy, because the body may be either correcting the abnormality or making it worse.

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Intracellular Fluid.

pH 7.0 at 37oC (alkaline !)

ICF "If ECF is the bath for our cells, ICF bathes our metablism." The Intracellular Fluid is a complex environment made up specialized regions with different functions. The pH varies from one part to another. It is commonly assumed that the intracellular pH is approximately neutral (pH 6.8 at body temperature). However, measurements by Sahlin et al (1997) indicated that the average intracellular pH may be closer to pH 7.0 with a bicarbonate concentration of about 10.2 mMol/L.

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Acid Production & Compensation.

Our Fuel makes Acid

Flame "No smoke without fire. Metabolism is our fire, carbon dioxide our smoke." As fire makes smoke, so metabolism makes acid - CO2 and metabolic acids. The body's own regulators of acid-base balance are the lungs, liver and kidneys which are responsible for excreting and metabolizing these acids. In many diseases, there is an imbalance between the quantity of acids produced and the body's ability to respond. The commonest result is an acidosis with a characteristic partial compensation (see Acid Production)

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Acid-Base Tutorial
Alan W. Grogono
Small Logo Copyright Mar 2018.
All Rights Reserved
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