by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology
|[ H+ ] x [ HCO3- ] = k x [ CO2 ] x [ H2O ]|
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:
This equation can be simplified because H2CO3 is not of clinical interest, [H2O] is constant in-vivo, and PCO2 is more familiar than [CO2]:
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.
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):
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):
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.
|Lipid Conduit, Polar Barrier|
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 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.
|The Cell's "Bath Water"|
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.
|The Cell's Larger Bath|
In practice, the "Treatable Volume" is 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.
|pH 7.0 at 37oC (alkaline !)|
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.
|Our Fuel makes Acid|
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)
Alan W. Grogono
|Copyright Mar 2018.|
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