Chapter 36: Physiology and Disorders of Water, Electrolyte, and Acid-Base Metabolism

An individual is brought to the emergency department of a local hospital with signs of narcotic overdose and respiratory depression. What acid-base status would this individual have?

a. Respiratory acidosis

b. Respiratory alkalosis

c. Metabolic acidosis

d. Metabolic alkalosis

ANS: A

Any condition that decreases elimination of carbon dioxide through the lungs results in an increase in PCO2 (hypercapnia) and dCO2 (respiratory acidosis). Thus respiratory acidosis occurs only through decreased elimination of CO2. Causes of decreased CO2 elimination are listed in Box 36-2. These conditions may be separated into those caused by factors that directly depress the respiratory center (such as centrally acting drugs, CNS trauma, or infection).

A person suspected of having chloride responsive metabolic alkalosis caused by prolonged vomiting would exhibit which one of the following?

a. Primary bicarbonate deficit

b. Primary bicarbonate excess

c. Primary cdCO2 deficit

d. Primary cdCO2 excess

ANS: B

Alkalosis occurs when excess base is added to the system, base elimination is decreased, or acid-rich fluids are lost in excess as in prolonged vomiting. Most causes of Cl- responsive metabolic alkalosis occur as a result of hypovolemia (see Box 36-1). When the ECF is severely depleted, the resulting acid-base disorder is often referred to as contraction alkalosis. Common causes of contraction alkalosis include prolonged vomiting.

In regard to respiration, peripheral chemoreceptors located in the carotid arteries and aorta are stimulated by:

a. HCO3 content of arterial blood.

b. PO2 content of blood only.

c. pH only.

d. pH and PO2 content of blood.

ANS: D

Peripheral chemoreceptors are stimulated by a fall in pH caused by accumulation of CO2 or by a decrease in PO2.

An overweight 55-year-old single woman from a rural farming area was brought to the emergency department by her neighbor. The woman had a large abscess on the bottom of her foot; she was irritable and complained of blurred vision and of being thirsty. Her breathing was rapid. The neighbor said that the only medication the woman was using was for blood pressure and sometimes an aspirin. Blood and urine samples were collected. Arterial blood gas results were: pH 7.2; PCO2 47 mm Hg; HCO3 8 mmol/L. Blood glucose was 340 mg/dL and a high anion gap was calculated. Urine glucose and ketones were markedly increased. Based on the laboratory values, what state of acid-base balance is this patient in?

a. Respiratory acidosis

b. Respiratory alkalosis

c. Metabolic acidosis

d. Metabolic alkalosis

ANS: C

Metabolic acidosis is a pathological process that leads to the accumulation of acid that lowers the bicarbonate concentration and decreases the pH. A high anion gap indicates the presence of organic acids, such as acetoacetate and -hydroxybutyrate, which indicate an acidotic state. Urine values confirm the presence of high glucose and ketone bodies.

An overweight 55-year-old single woman from a rural farming area was brought to the emergency department by her neighbor. The woman had a large abscess on the bottom of her foot; she was irritable and complained of blurred vision and of being thirsty. Her breathing was rapid. The neighbor said that the only medication the woman was using was for blood pressure and sometimes an aspirin. Blood and urine samples were collected. Arterial blood gas results were: pH 7.2; PCO2 47 mm Hg; HCO3 8 mmol/L. Blood glucose was 340 mg/dL and a high anion gap was calculated. Urine glucose and ketones were markedly increased. Based on the laboratory values and symptoms of the woman, what is the most likely cause of her acid-base disorder?

a. Abscess formation and infection

b. Diabetic ketoacidosis

c. Pneumonia

d. Hysteria

ANS: B

Because of the elevated blood and urine glucose and elevated anion gap, the symptoms of irritability and thirst, the presence of an abscess on her foot, and the lack of hypoglycemic agents in her pharmacy regimen, it is likely that she has been developing type 2 diabetes mellitus. Uncontrolled DM type 2 leads to overproduction of ketones and increase of intermediate organic acids such as beta-hydroxybutyrate, which are normally metabolized to carbon dioxide and water. In a diabetic, however, they may accumulate to a significant extent resulting in acidemia.

Hypernatremia commonly occurs with:

a. decreased synthesis of antidiuretic hormone (ADH).

b. decreased aldosterone.

c. edema.

d. hyperkalemia.

ANS: A

Tubular permeability to H2O is under the influence of antidiuretic hormone (ADH). When blood volume decreases, ADH keeps water and sodium in the system. Hypernatremia can be caused by decreased or total lack of ADH. Excess (as opposed to decreased) aldosterone also results in excess Na+ and water retention. Edema produces a dilutional hyponatremia.

Hypokalemia may be seen in all of the following except:

a. decreased dietary intake.

b. renal loss.

c. gastrointestinal loss.

d. decreased glucocorticoid concentration in blood.

ANS: D

Renal losses of K+ may occur during the diuretic (recovery) phase of acute tubular necrosis and during states of excess mineralocorticoid (primary or secondary aldosteronism) or glucocorticoid (Cushing syndrome) when the distal tubules increase Na+ reabsorption and K+ excretion.

What is the anion gap given the following serum electrolyte data: Na = 132 mmol/L, Cl- = 90 mmol/L, HCO - = 22 mmol/L, K = 4 mmol/L?

a. 10 mmol/L

b. 18 mmol/L

c. 20 mmol/L

d. 60 mmol/L

ANS: C

Anion gap is measured by subtracting the chloride plus bicarbonate sum from the sodium value. In this case, 132 − (90 + 22) = 20 mmol/L.

Determine the anion gap given the following serum electrolyte data: Na = 132 mmol/L, Cl− = 90 mmol/L, HCO - = 22 mmol/L, K = 4 mmol/L. Is the anion gap you calculated within the healthy reference interval?

a. Yes

b. No

ANS: B

The healthy reference interval for anion gap is 7 to 16 mmol/L.

Which of the following conditions will cause an increased anion gap?

a. Salicylate intoxication

b. Controlled diabetes mellitus

c. Decreased albumin concentration

d. Severe diarrhea

ANS: A

See Table 36-4. Salicylate, itself an unmeasured anion, alters peripheral metabolism, leading to the production of various organic acids without dominance of any specific acid. These processes eventually result in a metabolic acidosis with a high anion gap.

All of the following are causes of hyponatremia with concomitant decreased plasma osmolality and normal volume status except:

a. decreased serum aldosterone.

b. hypothyroidism.

c. diarrhea.

d. decreased serum antidiuretic hormone (ADH).

ANS: C

See Figure 36-1. In hyponatremia with a normal volume status, the most common causes are the syndrome of inappropriate ADH (SIADH), causing increased serum ADH, primary

polydipsia, hypothyroidism, nd adrenal insufficiency (decreased cortisol and/or aldosterone). SIADH is usually a result of ectopic or otherwise "inappropriate" increased ADH production arising from a variety of conditions.

Metabolic organic acidoses typically present with an increased anion gap. In contrast to these, inorganic acidosis with a normal anion gap is typically caused by:

a. uncontrolled diabetes mellitus.

b. loss of bicarbonate-rich fluid via the kidneys or gastrointestinal tract.

c. an acquired defect in enzymes involved in gluconeogenesis.

d. production of glycolic and oxalic acids.

ANS: B

In contrast to high anion gap acidosis, in which bicarbonate is consumed from buffering excess H+, the cause of acidosis in the presence of a normal anion gap is the loss of bicarbonate-rich fluid from the kidney or the gastrointestinal tract.

An individual visits a physician with the complaint of nausea, mental confusion, and needing an excessive amount of salt all the time. Laboratory results indicate decreased serum sodium and low serum osmolality. Physical examination reveals hypovolemia with low orthostatic blood pressure and tachycardia. A urine sodium analysis was suggested by a laboratorian and urine sodium was found to be increased; the physician diagnoses a salt-losing nephropathy (a renal tubule disease). What type of electrolyte disorder is this?

a. Dilutional hyponatremia

b. Depletional hyponatremia

c. Hypernatremia

d. Metabolic acidosis

ANS: B

Typically, when plasma Na+ concentration is low, calculated or measured osmolality will also be low. This type of hyponatremia can be due to excess loss of Na+ (depletional hyponatremia). If urine Na+ is elevated, renal loss of Na+ is likely. Renal loss of Na+ occurs with (1) osmotic diuresis, (2) use of diuretics that inhibit reabsorption of Cl- and Na+ in the ascending loop, (3) adrenal insufficiency (no aldosterone or cortisone prevents distal tubule reabsorption of Na+), or (4) salt wasting nephropathies, as can occur after acute tubular necrosis or obstructive nephropathy.

A hospitalized patient in the ICU has cirrhosis. After a period of time, the heart and kidneys begin to fail and the patient develops edema. What type of electrolyte disorder would develop from this situation?

a. Dilutional hyponatremia

b. Depletional hyponatremia

c. Hypernatremia

d. There would be no electrolyte disorder.

ANS: A

Dilutional hyponatremia is a result of excess H2O retention and often can be detected during the physical examination as edema. In advanced renal failure, water is retained because of decreased filtration and H2O excretion. When ECF is increased but the blood volume is decreased, as occurs in congestive heart failure (CHF), hepatic cirrhosis, or nephrotic syndrome, the kidneys reabsorb Na+ and H2O in response to increased aldosterone and ADH in an attempt to restore the blood volume, but this simply results in further increases in ECF and further dilution of Na+.

How do healthy kidneys compensate for the excess hydrogen ions and concurrent low pH observed in metabolic acidosis?

a. Releasing bicarbonate into urine

b. Stimulating hyperventilation

c. Decreasing urinary excretion of ammonia

d. Increasing excretion of acid

ANS: D

Various compensatory mechanisms attempting to reestablish the normal ratio of cHCO3-/cdCO2 may result in changes in bicarbonate concentration, dissolved CO2 concentration, or both. If possible in metabolic acidosis, the kidneys respond to restore thenormal pH through increased excretion of acid and preservation of base (increased rate of Na+ - H+ exchange, increased ammonia formation, and increased reabsorption of bicarbonate).

How do healthy lungs compensate for the decrease in pH in a state of metabolic acidosis?

a. Releasing bicarbonate into urine

b. Stimulating hyperventilation

c. Decreasing urinary excretion of acid

d. Increasing excretion of acid

ANS: B

Various compensatory mechanisms attempting to reestablish the normal ratio of cHCO3-/cdCO2 may result in changes in bicarbonate concentration, dissolved CO2 concentration, or both. The decrease in pH in metabolic acidosis stimulates hyperventilation(Kussmaul respiration), which results in the elimination of carbonic acid as CO2.

Physiologically important buffers maintaining body pH include all of the following except:

a. bicarbonate.

b. albumin.

c. phosphate.

d. lactate.

ANS: D

The most important buffer of plasma is the bicarbonate/carbonic acid pair, while the total concentration of H2PO4- in both erythrocytes and plasma accounts for about 5% of the nonbicarbonate buffer value of plasma. Proteins, especially albumin, account for the greatest portion (>90%) of the nonbicarbonate buffer value of plasma. Hemoglobin accounts for the major part of the nonbicarbonate buffers in erythrocyte fluid.

At physiological pH of 7.4, which one of the following contributes most to the total serum CO2 (think about the equilibrium of the reaction in the formula)?

a. Dissolved CO2

b. Carbaminohemoglobin

c. HCO3-

d. Carbonic acid

ANS: D

HCO3- -> + H -> + H2CO3 -> CO2 + H2O; in this formula, H+ will react with HCO3 - to form more H2CO3 and subsequently CO2 and H2O

Hypokalemia (<3.0 mmol/L) is considered a serious health threat because:

a. the heart rate increases, leading to weakness, difficulty in breathing, and eventual cardiac arrest.

b. the heart rate slows because of the lowered excitability of cardiac smooth muscle, leading to possible cardiac standstill.

c. the body gains water through the kidney reabsorption, leading to severe edema.

d. arterial pH will decrease (increased H+ concentration), leading to metabolic acidosis.

ANS: A

Plasma K+ concentrations less than 3.0 mmol/L are associated with serious neuromuscular symptoms and indicate a critical degree of intracellular depletion. At lower concentrations, tachycardia and specific cardiac conduction effects are apparent by electrocardiographic examination and can lead to cardiac arrest.

The most important buffer of plasma is the system.

a. bicarbonate/carbonic acid

b. protein/albumin

c. phosphoric acid/phosphate

d. hemoglobin erythrocyte

ANS: A

The most important buffer of plasma is the bicarbonate/carbonic acid pair.

The metabolic component of acid-base regulation is the renal system. Which one of the following statements concerning this component is incorrect?

a. Renal tubule cells reabsorb sodium and bicarbonate to affect bicarbonate concentration.

b. Sodium and bicarbonate are important in the exchange of excess H+ ions, thus altering blood pH.

c. The renal system acts to remove excess H+ ions by combining them with HPO4.

d. In the metabolic component, the renal system responds immediately to a change in acid-base status.

ANS: D

The respiratory component responds quickly to a change in acid-base status, while the metabolic component responds much more slowly.

Which one of the following hormones is an active regulator of water retention/reabsorption in the kidney?

a. Aldosterone

b. Erythropoietin

c. Antidiuretic hormone

d. Renin

ANS: C

Water regulation in the kidney occurs from the distal tubule through the collecting duct, where tubular permeability to H2O is under the influence of antidiuretic hormone (ADH).

Which one of the following hormones is an active regulator of sodium (and passive controller of water) in the kidney?

a. Aldosterone

b. Erythropoietin

c. Antidiuretic hormone

d. Renin

ANS: A

At the level of the distal tubule, the first of the two primary Na+/H2O regulating processes occurs. Here, aldosterone stimulates the cortical collecting ducts to reabsorb Na+ (with water following passively) and secrete K+ (and to a lesser extent, H+) to maintain electrical neutrality. When the kidneys are hypoperfused (as occurs when blood volume decreases, or when the renal arteries are obstructed), the distal tubules, under the influence of aldosterone, reclaim Na+

A 17-year-old woman was brought by her friends to the emergency room in an agitated state. She stated that she had broken up with her boyfriend and he had threatened her. Her temperature was 101° F, and she was breathing rapidly. She claimed that she could not slow her breathing down despite attempts to have her breathe deeply. An arterial blood gas revealed a pH of 7.54, HCO3 of 18 mmol/L, and PCO2 of 28 mm Hg. What is occurring in this patient?

a. Excess retention of acid via the renal route

b. Excess elimination of acid via the renal route

c. Excess elimination of acid via the respiratory route

d. Excess retention of bicarbonate via the respiratory route

ANS: C

The basic cause of respiratory alkalosis is excess elimination of acid via the respiratory route. Excessive elimination of carbon dioxide reduces the PCO2 and causes an increase in the cHCO3-/cdCO2 ratio (due to decrease in cdCO2).

Match the action of acid-base balance regulation with the correct component of acid-base balance.

a. Metabolic component

b. Respiratory component

c. Both components

1. Hydrogen ions are exchanged for sodium or potassium

2. Hydrogen ions are attached to hemoglobin

3. Can alter blood pH

1. A

2. B

3. C