Acid Base Status

Because the concentration of carbonic acid is proportional to the concentrar: 3 of dissolved carbon dioxide, we can change the constant and write

Taking logarithms

whence

Because pH is the negative logarithm pH = pKa + log

Because CO2 obeys Henry's law, rhe CO2 concentration (in mmol/liter) can be replaced by (Pco2 x 0.03). The equation then becomes

The value of pK\ is 6.1, and the normal HCO1 concentration in arterial blo c 24 mmol/Uter. Substituting gives

Note that as long as the ratio of bicarbonate concentration to (Pco5 X C:l remains equal to 20, the pH will remain at 7-4- The bicarbonate concern ran : • determined chiefly by the kidney and the Pcoz by the lung.

The relationships between pH, Pco2, and HCO 3 are conveniently shown n a Davenport diagram (Figure 6-8). The two axes show HCO 3 and pH, and lines of equal Pco2 sweep across the diagram. Normal plasma is represented by point A The line CAB shows the relationship between HCO3 and pH as carbonic acic is added to whole blood, that is, it is part of the titration curve for blood and is callr i the buffer line. The slope of this line is steeper than that measured in plasma ser -arated from blood because of the presence of hemoglobin, which has an ad- -tional buffering action. The slope of the line measured on whole blood in vitri is usually a little different from that found in a patient because of the buffering action of the interstitial fluid and other body tissues.  Figure 6-8. Davenport diagram showing the relationships among HCO3, pH, and Pco2- A shows the normal buffer line BAC. B shows the changes occurring in respiratory and metabolic acidosis and alkalosis (see text). The vertical distance between buffer lines is the base excess.

If the plasma bicarbonate concentration is altered by the kidney, the buttc line is displaced. An increase in bicarbonate concentration displaces the buftc line upward as shown, for example, by line DE in Figure 6-8. In this case, the b.: excess is increased and is given by the vertical distance between the two buffer lines DE and BAC. By contrast, a reduced bicarbonate concentration displaces the buffer line downward (line GF), and there is now a negative base excess, r base deficit.

The ratio of bicarbonate to Pco2 can disturbed in four ways: both F'[ ;o m-bicarbonate can be raised or lowered. Each of these four disturbances gives rise t a characteristic acid-base change.

Respiratory Acidosis

This is caused by an increase in Pco, which reduces the HCO^/Pcoi ratio an. thus depresses the pH. This corresponds to a movement from A to B in Figure 68. Whenever the Pco2 r'ses! the bicarbonate must also increase to some extent because of dissociation of the carbonic acid produced. This is reflected by the left upward slope of the blood buffer line in Figure 6-8. However, the ratio HCOi/Pco2 falls- CO; retention can be caused by hypoventilation r ventilation-perfusion inequality.

If respiratory acidosis persists, the kidney responds by conserving HCO->. It i> prompted to do this by the increased Pco, in the renal tubular cells, which then excrete a more acid urine by secreting H+ ions. The H~ ions are excreted js HiPOij or NH4; the HCOl ions are reabsorbed- The resulting increase in plasma HCO 3 then moves the HC07/Pco2 ratio back up toward its normal level. This corresponds to the movement from B to D along the line Pco? = 60 mm Hg in Figure 6-8 and is known as compensated respiratory acidosis, Typical events would be

(Respiratory acid-

(Compensated respiratory acidosis)

The renal compensation is typically not complete, and so the pH does not fully return to its normal level of 7.4. The extent of the renal compensation can be determined from the base excess, that is, the vertical distance between the but:_-lines BA and DE.

Respiratory Alkalosis

This is caused by a decrease in Pco,» which increases the HC07/Pco2 ratio and thus elevates the pH (movement from A to C in Figure 6-8). A decrease in P. is caused by hyperventilation, for example, at high altitude (see Chapter 9). Re nal compensation occurs by an increased excretion of bicarbonate, thus returning the HCOj/Pcoj ratio back toward normal (C to F along die line Pco-, = 20 mm Hg). After a prolonged stay at high altitude, the renal compensation may be nearly complete. There is a negative base excess, or a t'ose deficit.

Metabolic Acidosis

In this context, "metabolic" means a primary change in HCOl, that is, the numerator of the Henderson-Hasselbalch equation. In metabolic acidosis, the ratio ot HCO3 to Pco2 falls, thus depressing the pH. The HCO 3 may be lowered by the accumulation of acids in the blood, as in uncontrolled diabetes mellitus, or after tissue hypoxia, which releases lactic acid. The corresponding change in Figure 6-8 is a movement from A toward G.

In this instance, respiratory compensation occurs by an increase in ventilation that lowers the Pcoz and raises the depressed HCO 3 /Pco2 ratio. The stimulus to raise the ventilation is chiefly the action of H+ ions on the peripheral chemore-ceptors (Chapter 8). In Figure 6-8, the point moves in the direction G to F (although not as far as F). There is a base deficit or negative base excess.

Metabolic Alkalosis

Here an increase in HCO3 raises the HCO 3/'Pco, ratio and, thus, the pH. Excessive ingestion of alkalis and loss of acid gastric secretion by vomiting are causes. In Figure 6-8, the movement is in the direction A to E. Some respiratory compensation sometimes occurs by a reduction in alveolar ventilation that raises the Ptx32- P°inc E then moves in the direction of D (although not all the way). However, respiratory compensation in metabolic alkalosis is often small and may be absent. Base excess is increased.

Note that mixed respiratory and metabolic disturbances often occur, and it may then be difficult to unravel the sequence of events.

Four Types of Acid-Base Disturbances

Primary

Compensation

Acidosis

Respiratory Metabolic

Pco2T HC03 I

HCO 3 t Pco2 i

Alkalosis

Respiratory Metabolic

Pco2 I HCO3 T

HC03 i often none 