Physiology 5th Ed.

Chapter 7. Acid-Base Physiology

 

pH OF BODY FLUIDS

ACID PRODUCTION IN THE BODY

BUFFERING

RENAL MECHANISMS IN ACID-BASE BALANCE

ACID-BASE DISORDERS

SUMMARY

ent The pH of body fluids is normally maintained at 7.4 in spite of the daily production of large amounts of CO2 (volatile acid) and fixed acids (nonvolatile acids). The mechanisms that maintain a constant pH include buffering, respiratory compensation, and renal compensation.

ent Buffering represents the first line of defense in protecting the pH. A buffered solution is a mixture of a weak acid and its conjugate base. The most effective physiologic buffers have a pK near 7.4. Extracellular buffers include HCO3/CO2 (the most important) and HPO4−2/H2PO4. Intracellular buffers include organic phosphates and proteins (e.g., deoxyhemoglobin).

ent Renal mechanisms in acid-base balance include reabsorption of virtually all of the filtered HCO3 and excretion of H+ as titratable acid and NH4+. For each H+ excreted as titratable acid or NH4+, one new HCO3 is synthesized and reabsorbed.

ent Simple acid-base disorders can be metabolic or respiratory in origin. Metabolic disorders involve a primary disturbance of the [HCO3], caused by gain or loss of fixed H+. When there is a gain of fixed H+, metabolic acidosis occurs; when there is a loss of fixed H+, metabolic alkalosis occurs. Respiratory disorders involve a primary disturbance of PCO2, caused by hypoventilation (respiratory acidosis) or hyperventilation (respiratory alkalosis).

ent Compensation for acid-base disorders is either respiratory or renal. When the primary disorder is metabolic, compensation is respiratory. When the primary disorder is respiratory, compensation is renal (metabolic).


Challenge Yourself

Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are provided at the end of the book.

1 Weak acid “A” has a pK of 5.5 and weak acid “B” has a pK of 7.5. At pH 7, which weak acid is predominantly in its A form?

2 If a person’s arterial blood has a pH of 7.22 and PCO2 of 20 mm Hg, what is the HCO3 concentration?

3 For the person described in question 2, is ventilation increased, decreased, or unchanged (compared with normal)?

4 A person’s arterial blood pH is 7.25, PCO2 is 24 mm Hg, and HCO3 is 10.2 mEq/L. Which of the following might cause this pattern: diarrhea, vomiting, obstructive pulmonary disease, hysterical hyperventilation, salicylate overdose, chronic renal failure?

5 Which class(es) of diuretics cause(s) metabolic alkalosis: carbonic anhydrase inhibitors, loop diuretics, thiazide diuretics, K+-sparing diuretics?

6 A patient is seen in the emergency department with the following blood values: pH, 7.1; HCO3, 10 mEq/L; Na+, 142 mEq/L; and Cl, 103 mEq/L. What is the acid-base disorder, and what is the value of the anion gap?

7 What are the units of osmolar gap?

8 Among patients with the following disorders, which is/are hypoventilating: diarrhea, vomiting, ascent to high altitude, morphine overdose, obstructive lung disease, hyperaldosteronism, ethylene glycol poisoning, salicylate poisoning?

9 What is the correct sequence of these events: Na+-H+ exchange, filtration of HCO3 across glomerular capillaries, facilitated diffusion of HCO3, conversion of H2CO3 to CO2 and H2O, conversion of H2CO3 to H+ and HCO3, conversion of HCO3 to H2CO3?

10 If, in one day, 25 mEq of H+ is excreted as H2PO4 and 45 mEq of H+ is excreted as NH4+, how much new HCO3 is synthesized?

11 Two patients have an elevated arterial PCO2 of 70 mm Hg. One has acute respiratory acidosis and the other has chronic respiratory acidosis. Which patient has the higher blood HCO3concentration? Which patient has the higher pH?

12 A patient has the following blood values: pH, 7.22; HCO3, 18 mEq/L; and PCO2, 45 mm Hg. Are these values consistent with a simple acid-base disorder? If yes, which one? If no, what acid-base disorders are present?

13 In the conversion from acute to chronic respiratory alkalosis, what happens to blood pH?

14 Which is the best indicator of total H+ excreted in the urine: urine pH, filtered load of HPO4−2, filtered load of NH3?

15 Which condition has the highest excretion of NH4+: diabetic ketoacidosis, chronic renal failure, vomiting, hysterical hyperventilation?


SELECTED READINGS

Cohen JJ, Kassirer JP: Acid/Base. Boston, Little, Brown, 1982.

Davenport HW: The ABC of Acid-Base Chemistry, 6th ed. Chicago, University of Chicago Press, 1974.

Rose BD: Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed. New York, McGraw-Hill, 2000.

Valtin H, Gennari FJ: Acid-Base Disorders. Boston, Little, Brown, 1987.