[H+] x [HCO3-] = k1 x H2CO3 = k2 x [CO2] x [H2O]
In vivo, [H2O] is constant and PCO2 is more familiar than [CO2]. The equation can be rewritten:
[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.
Interactive Hypercard Stack: If you are using a Macintosh, you can download an Interactive Hypercard Stack which allows you to experiment and change the values for PCO2 and pH to obtain an analysis and a report for each set of values. There are two versions:
Acid Base Stack (AB.stk). Downloads fast (46 k); requires Hypercard or Hypercard Player.
Acid Base Application (AB.applic). Downloads slowly (1300 k); runs independently.
Pure respiratory acidosis (high PCO2) implies a normal metabolic state and increases both the [HCO3-] and [H+]. The [H+] changes only slightly due to buffering of H+, mostly by hemoglobin. If, at this PCO2, the kidney compensates by reducing [H+], then [HCO3-] rises further, i.e., respiratory acidosis raises the bicarbonate level and metabolic compensation raises it further. Similarly, pure metabolic acidosis implies a normal PCO2; the high [H+] is associated with a reciprocal fall in the [HCO3-]. In practice respiratory compensation promptly 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. In view of this joint effect on the bicarbonate ion concentration, it seems logical to avoid using bicarbonate as a measure of either metabolic or respiratory abnormalities.
The cell wall provides a protected environment for the reactions which sustain life. It limits transfer of various substances, particularly those that are polar, or ionized. 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, escape from the cell. In practice we neither measure, nor directly treat, the pH inside the cell. It is much closer to neutral (pH 6.8 at 37oC) than the extracellular fluid, but it varies from one part of the cell to another.
The extracellular fluid is the environment which provides the cell's nutrition, oxygenation, waste removal, temperature, and alkalinity. The normal pH (7.4) represents [H+] = 40 nmol/1. This is about one quarter of the neutral [H+] inside the cell, 160 nmol/1. This fourfold 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.
Treatable volume: The extracellular fluid is also the principal part of the body being treated when alkali (or acid) is administered. If the cell wall were completely impermeable, then the extracellular fluid would be the only part of the body treated. As some equilibration occurs between the cell and the extracellular fluid, it is customary to treat a slightly larger volume. The extracellular fluid is 20% of the body weight, e.g. 14 liters, but treatable space is estimated to be 30%, e.g., 21 liters. This is a useful approximation for emergency therapy. Over a longer period, however, increasing equilibration occurs between the intra- and extra- cellular fluid; the treatable volume, therefore, appears to be somewhat greater. 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.
Acid elimination and compensation: The body's own regulators of acid-base balance are the lungs and the kidneys which are responsible for excreting the respiratory and metabolic acids respectively. The quantity of respiratory acid produced per day is easily calculated. Two hundred and fifty milliliters of carbon dioxide per minute is 360 liters per day. Since each gram molecule of gas occupies 22.4 liters at STP, approximately 16 moles of carbon dioxide are produced daily. This enormous quantity is matched to a correspondingly effective means of elimination. The power of the lungs to excrete large quantities of carbon dioxide enables them to compensate rapidly. Unless the respiratory system is diseased or depressed, metabolic disturbances promptly stimulate partial respiratory compensation. By contrast the kidney is accustomed to eliminating only 0.1 moles (100 mEq) of acid per day. This smaller quantity corresponds to a relatively slower rate of compensation; a patient can be ventilated at an abnormal PCO2 for days before the typical, partial compensation is achieved.