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This chapter is a review of the homeostatic mechanisms that operate to maintain the osmolality, volume, and ionic composition of the extracellular fluid within normal limits. This includes the concentration of H+, with consideration of respiratory and metabolic acidosis and alkalosis. The material in the chapter should help students to—

Describe how the tonicity (osmolality) of the extracellular fluid is maintained by alterations in water intake and vasopressin secretion.

Describe how the volume of the extracellular fluid is maintained by alterations in renin and aldosterone secretion.

Name the mechanisms that operate to maintain the constancy of plasma concentrations of glucose and Ca2+.

Define acidosis and alkalosis, and give (in meq/L and pH) the normal mean and the range of H+ concentrations in blood that are compatible with health.

List the principal buffers in blood, interstitial fluid, and intracellular fluid, and, using the Henderson-Hasselbalch equation, describe what is unique about the bicarbonate buffer system.

Describe the changes in blood chemistry that occur during the development of metabolic acidosis and metabolic alkalosis, and the respiratory and renal compensations for these conditions.

Describe the changes in blood chemistry that occur during the development of respiratory acidosis and respiratory alkalosis, and the renal compensation for these conditions.

General Questions

1. What are the main sources of the acid loads presented to the body in everyday living? What are some common diseases that cause increased acid loads in the body, and how is the load produced in each?

2. Compare and contrast metabolic acidosis and respiratory acidosis.

3. Describe and explain the alterations in extracellular fluid volume and acid-base balance that occur in patients with chronically elevated plasma aldosterone concentrations due to primary hyperaldosteronism.

4. Describe by means of diagram plotting plasma HCO3- concentration against pH (a Davenport diagram) the immediate and more long-term changes that occur in the acid-base balance of a normal individual who hyperventilates for 5 minutes.

Multiple-Choice Questions

Figure 39-A. Siggaard-Andersen curve nomogram. (Courtesy of O Siggaard-Andersen and Radiometer, Copenhagen, Denmark.)

In questions 1-10, select the single best answer. Questions 1-4 refer to the numbered points on the nomogram in Figure 39-A. Match the numbered point in each question with the lettered condition that is most closely associated with it. Each lettered condition may be selected once, more than once, or not at all.

(A) Values seen in a mountain climber after several weeks at high altitude

(B) Values seen in long-standing severe emphysema

(C) Values seen in diabetic coma

(D) Values seen after 5 minutes of hyperventilation

(E) Values seen after prolonged vomiting

1. Point 1

2. Point 2

3. Point 3

4. Point 4

5. Dehydration increases the plasma concentration of all the following hormones except

(A) vasopressin

(B) angiotensin II

(C) aldosterone

(D) norepinephrine

(E) atrial natriuretic peptide

6. Which of the following is an important buffer in interstitial fluid?

(A) Hemoglobin

(B) Other proteins

(C) Carbonic acid

(D) H2PO4

(E) Compounds containing histidine

7. 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 PO2 of the blood

(D) it decreases the PCO2 in the alveoli

(E) the increased muscle work of increased breathing generates more CO2

8. In uncompensated metabolic alkalosis

(A) the plasma pH, the plasma HCO3- concentration, and the arterial PCO2 are all low

(B) the plasma pH is high and the plasma HCO3- concentration and arterial PCO2 are low

(C) the plasma pH and the plasma HCO3- concentration are low and the arterial PCO2 is normal

(D) the plasma pH and the plasma plasma HCO3- concentration are high and the arterial PCO2 is normal

(E) the plasma pH is low, the plasma HCO3- concentration is high, and the arterial PCO2 is normal

9. In a patient with a plasma pH of 7.10, the [HCO3-]/[H2CO3] ratio in plasma is

(A) 20

(B) 10

(C) 2

(D) 1

(E) 0.1

10. In a patient who has become dehydrated, body water should be replaced by intravenous infusion of

(A) distilled water

(B) 0.9% sodium chloride solution

(C) 5% glucose solution

(D) hyperoncotic albumin

(E) 10% glucose solution

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