Hco3

The concentration of undissociated H2CO3 cannot be measured in solution because it rapidly dissociates into CO2 and H2O or to H+ and HCO3-. However, the CO2 dissolved in the blood is directly proportional to the amount of undissociated H2CO3. Therefore, equation 2 can be rewritten as

CO2 HCO3

The dissociation constant (K) for equation 3 is only about 1/400 of the dissociation constant (K') of equation 2 because the proportionality ratio between H2CO3 and CO2 is 1:400.

Equation 3 is written in terms of the total amount of CO2 dissolved in solution. However, most clinical laboratories measure the blood CO2 tension (Pco2) rather than the actual amount of CO2. Fortunately, the amount of CO2 in the blood is a linear function of Pco2 times the solubility coefficient for CO2; under physiologic conditions, the solubility coefficient for CO2 is 0.03 mmol/mm Hg at body temperature.This means that 0.03 millimole of H2CO3 is present in the blood for each millimeter of mercury Pco2 measured. Therefore, equation 3 can be rewritten as

toward alkalosis. An increase in Pco2 causes the pH to decrease, shifting the acid-base balance toward acidosis.

The Henderson-Hasselbalch equation, in addition to defining the determinants of normal pH regulation and acid-base balance in the extracellular fluid, provides insight into the physiologic control of acid and base composition of the extracellular fluid. As discussed later, the bicarbonate concentration is regulated mainly by the kidneys, whereas the Pco2 in extracellular fluid is controlled by the rate of respiration. By increasing the rate of respiration, the lungs remove CO2 from the plasma, and by decreasing respiration, the lungs elevate Pco2. Normal physiologic acid-base homeostasis results from the coordinated efforts of both of these organs, the lungs and the kidneys, and acid-base disorders occur when one or both of these control mechanisms are impaired, thus altering either the bicarbonate concentration or the Pco2 of extracellular fluid.

When disturbances of acid-base balance result from a primary change in extracellular fluid bicarbonate concentration, they are referred to as metabolic acid-base disorders. Therefore, acidosis caused by a primary decrease in bicarbonate concentration is termed metabolic acidosis, whereas alkalosis caused by a primary increase in bicarbonate concentration is called metabolic alkalosis. Acidosis caused by an increase in Pco2 is called respiratory acidosis, whereas alkalosis caused by a decrease in Pco2 is termed respiratory alkalosis.

Henderson-Hasselbalch Equation. As discussed earlier, it is customary to express H+ concentration in pH units rather than in actual concentrations. Recall that pH is defined as pH = -log H+.

The dissociation constant can be expressed in a similar manner.

Therefore, we can express the H+ concentration in equation 4 in pH units by taking the negative logarithm of that equation, which yields

Rather than work with a negative logarithm, we can change the sign of the logarithm and invert the numerator and denominator in the last term, using the law of logarithms to yield pH = pK + log

HCO3

For the bicarbonate buffer system, the pK is 6.1, and equation 7 can be written as pH = 6.l + log

HCO3

O.O3 x Pco2

Equation 8 is the Henderson-Hasselbalch equation, and with it, one can calculate the pH of a solution if the molar concentration of HCO3- and the Pco2 are known.

From the Henderson-Hasselbalch equation, it is apparent that an increase in HCO3- concentration causes the pH to rise, shifting the acid-base balance

Bicarbonate Buffer System Titration Curve. Figure 30-1 shows the changes in pH of the extracellular fluid when the ratio of HCO3- to CO2 in extracellular fluid is altered. When the concentrations of these two components are equal, the right-hand portion of equation 8 becomes the log of 1, which is equal to 0. Therefore, when the two components of the buffer system are equal, the pH of the solution is the same as the pK (6.1) of the bicarbonate buffer system. When base is added to the system, part of the dissolved CO2 is converted into HCO3-, causing an increase in the ratio of HCO3- to CO2 and increasing the pH, as is evident from the Henderson-Hasselbalch equation. When acid is added, it is buffered

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Essentials of Human Physiology

Essentials of Human Physiology

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