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

Acid-Base Physiology

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Physiology

Page Index

Acid Production
Bicarbonate as a Lab Value
Cell Membrane
Extracellular Fluid
Intracellular Fluid
Metabolic Acidosis
Respiratory Acidosis
Treatable Volume

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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. It is an example of the Law of Mass Action: the products of the concentrations on one side are proportional to the products on the other.

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CO2 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 this 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

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Metabolic Acidosis 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 occurs almost at once and lowers the PCO2, 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

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Bicarbonate Bicarbonate

is neither an ideal measure of metabolic acidosis nor a measure of respiratory acidosis. This is because both the respiratory and the metabolic components can affect the concentration of bicarbonate ions. The exception to this rule would be when the only results available are from Blood Chemistry analysis and the patient appears to have normal lungs. Here, a bicarbonate change almost certainly indicates a metabolic abnormality.


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The Cell Membrane.

Lipid Conduit, Polar Barrier

Cell 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. By contrast, water, lipid soluble substances, and dissolved gases 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 cease to be ionized and will, therefore, cross the cell membrane more readily.

The pH varies from one part of the cell to another and probably averages close to 7.0 at 37oC. This is much closer to neutral - which is pH 6.8 at 37oC - than the 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 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 40% of the [H+] inside the cell, 100 nmol/1 (average pH = approximately 7.0). 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 The "Treatable Volume" is larger than the extracellular fluid. The extracellular fluid is the principal part of the body being treated when alkali (or acid) is administered. If the cell membrane were completely impermeable, the extracellular fluid would be the only part of the body treated. Some equilibration, however, occurs between the cell and the extracellular fluid. It is, therefore, customary to treat a slightly larger volume.

The treatable space is about 30% of the body water - typically about 21 liters. This is 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 The intracellular fluid is a complex environment made up specialized regions with different functions. The pH varies from one part to another. It used to be assumed that the intracellular pH was approximately neutral (pH 6.8 at body temperature). However, measurements indicate that the average intracellular pH is slightly on the alkaline side of neutral, about pH 7.0. Sahlin et al (1997) showed that the intracellular pH in muscle in volunteers at rest was about 7.0 with a bicarbonate concentration of about 10.2 mMol/L.


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Acid Production & Compensation.

Our Fuel makes CO2

Flame 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 acid-base disturbances, there is an imbalance between the quantity of acids produced and the body's ability to respond. The usual result is a characteristic, partial compensation (see Acid Production)



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Acid-Base Tutorial
Alan W. Grogono
Small Logo Copyright Oct 2011.
All Rights Reserved
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