Ganong's Review of Medical Physiology, 23rd Edition

686 SECTION VIII Renal Physiology
Additional graduations on the upper curved scale of the
nomogram (Figure 40–7) are provided for measuring buffer
base content; the point where the CO 2 calibration line of the
arterial blood sample intersects this scale shows the mEq/L of
buffer base in the sample. The buffer base is equal to the total
number of buffer anions (principally Prot – , HCO 3 – , and Hb – )
that can accept hydrogen ions in the blood. The normal value
in an individual with 15 g of hemoglobin per deciliter of
blood is 48 mEq/L.
The point at which the CO 2 calibration line intersects the
lower curved scale on the nomogram indicates the base
excess. This value, which is positive in alkalosis and negative
in acidosis, is the amount of acid or base that would restore 1
L of blood to normal acid–base composition at a PCO 2 of 40
mm Hg. It should be noted that a base deficiency cannot be
completely corrected simply by calculating the difference
between the normal standard bicarbonate (24 mEq/L) and the
actual standard bicarbonate and administering this amount of
NaHCO 3 per liter of blood; some of the added HCO 3 – is converted
to CO 2 and H 2 O, and the CO 2 is lost in the lungs. The
actual amount that must be added is roughly 1.2 times the
standard bicarbonate deficit, but the lower curved scale on the
nomogram, which has been developed empirically by analyzing
many blood samples, is more accurate.
In treating acid–base disturbances, one must, of course, consider
not only the blood but also all the body fluid compartments.
The other fluid compartments have markedly different
concentrations of buffers. It has been determined empirically
that administration of an amount of acid (in alkalosis) or base
(in acidosis) equal to 50% of the body weight in kilograms
times the blood base excess per liter will correct the acid–base
disturbance in the whole body. At least when the abnormality is
severe, however, it is unwise to attempt such a large correction
in a single step; instead, about half the indicated amount should
be given and the arterial blood acid–base values determined
again. The amount required for final correction can then be
calculated and administered. It is also worth noting that, at least
in lactic acidosis, NaHCO 3 decreases cardiac output and lowers
blood pressure, so it should be used with caution.
CHAPTER SUMMARY
■ The cells of the proximal and distal tubules secrete hydrogen
ions. Acidification also occurs in the collecting ducts. The reaction
that is primarily responsible for H + secretion in the proximal
tubules is Na + –H + exchange. Na is absorbed from the
lumen of the tubule and H is excreted.
■ The maximal H + gradient against which the transport mechanisms
can secrete in humans corresponds to a urine pH of about
4.5. However, three important reactions in the tubular fluid
remove free H + , permitting more acid to be secreted. These are
the reactions with HCO 3 – to form CO2 and H 2 O, with HPO 4 2–
to form H 2 PO 4 – , and with NH3 to form NH 4 + .
■ Carbonic anhydrase catalyzes the formation of H 2 CO 3 , and
drugs that inhibit carbonic anhydrase depress secretion of acid
by the proximal tubules.
■ Renal acid secretion is altered by changes in the intracellular
PCO 2, K + concentration, carbonic anhydrase level, and adrenocortical
hormone concentration.
MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.
1. Which of the following is the principal buffer in interstitial fluid?
A) hemoglobin
B) other proteins
C) carbonic acid
D) H 2 PO 4
E) compounds containing histidine
2. Increasing alveolar ventilation increases the blood pH because
A) it activates neural mechanisms that remove acid from the
blood.
B) it makes hemoglobin a stronger acid.
C) it increases the PO 2 of the blood.
D) it decreases the PCO 2 in the alveoli.
E) the increased muscle work of increased breathing generates
more CO 2.
3. In uncompensated metabolic alkalosis
A) the plasma pH, the plasma HCO 3 – concentration, and the
arterial PCO 2 are all low.
B) the plasma pH is high and the plasma HCO 3 – concentration
and arterial PCO 2 are low.
C) the plasma pH and the plasma HCO 3 – concentration are low
and the arterial PCO 2 is normal.
D) the plasma pH and the plasma HCO 3 – concentration are
high and the arterial PCO 2 is normal.
E) the plasma pH is low, the plasma HCO 3 – concentration is
high, and the arterial PCO 2 is normal.
4. In a patient with a plasma pH of 7.10, the [HCO 3 – ]/[H2 CO 3 ]
ratio in plasma is
A) 20.
B) 10.
C) 2.
D) 1.
E) 0.1.
CHAPTER RESOURCES
Adrogué HJ, Madius NE: Management of life-threatening acid–base
disorders. N Engl J Med 1998;338:26.
Brenner BM, Rector FC Jr. (editors): The Kidney, 6th ed. 2 vols.
Saunders, 1999.
Davenport HW: The ABC of Acid–Base Chemistry, 6th ed. University
of Chicago Press, 1974.
Halperin ML: Fluid, Electrolyte, and Acid–Base Physiology, 3rd ed.
Saunders, 1998.
Lemann J Jr., Bushinsky DA, Hamm LL: Bone buffering of acid and
base in humans. Am J Physiol Renal Physiol 2003;285:F811.
Review.
Vize PD, Wolff AS, Bard JBL (editors): The Kidney: From Normal
Development to Congenital Disease. Academic Press, 2003.