Chapter 5: Fluids and Electrolytes, Acids and Bases

Question 1

Distribution of Body Fluids

Answer 1

1. The sum of all fluids is the total body water (TBW). Body fluids are distributed among functional compartments. They are classified as
   intracellular fluid (ICF), or within cells, and extracellular fluid (ECF), or outside of cells. ECF includes interstitial, intravascular, and transcellular fluids.
2. TBW varies with age and the amount of body fat. At birth, TBW is 75%–80% of body weight. Through childhood and adolescence, TBW decreases to 60%–65%. Males have a greater percentage because of increased muscle mass. Females have a lesser percentage due to the effects of estrogen, resulting in a higher percentage of body fat.
3. TBW continues to decline in elderly people due to decreases in free fat and muscle mass, less efficient kidney function, and impairments in thirst perception.
4. Water moves between the plasma and interstitial fluid by osmosis (pulling of water) and hydrostatic pressure (pushing of water), which occur across the capillary membrane.
5. Movement across the capillary wall is called net filtration and is described according to Starling law (the balance between hydrostatic and osmotic forces).
6. Water moves between the ICF and ECF compartments principally with osmotic forces.

Question 2

Alterations in Water Movement

Answer 2

1. Edema is a problem of fluid distribution that results in accumulation of fluid within the interstitial spaces.
2. The pathophysiologic process that leads to edema is related to an increase in forces favoring fluid filtration from the capillaries or from lymphatic channels into the tissues.
3. Physiologic conditions that promote edema include: (a) Increased capillary hydrostatic pressure
   (b) Decreased plasma oncotic pressure,
   (c) Increased capillary membrane permeability
   (d) Lymphatic channel obstruction.
4. Edema may be localized or generalized. It is usually is associated with weight gain, swelling and puffiness, tighter-fitting clothes and shoes, and limited movement of the affected area.

Question 3

Sodium, Chloride, and Water Balance

Answer 3

1. There is an integral relationship between the balance of sodium and water levels; chloride levels are generally proportional to changes in sodium levels. Sodium balance is regulated by aldosterone, a
   hormone which increases reabsorption of sodium from the renal filtrate into the blood, at the distal tubule of the kidney.
2. Renin and angiotensin are enzymes that promote secretion of aldosterone and thus regulate sodium and water balance.
3. Natriuretic peptides are hormones involved in decreasing tubular reabsorption and promoting urinary excretion of sodium.
4. Water balance is regulated by the sensation of thirst and by antidiuretic hormone (ADH), which is secreted in response to an increase in plasma osmolality or a decrease in circulating blood volume.

Question 4

Alterations in Sodium, Water, and Chloride Balance

Answer 4

1. Alterations in sodium and water balance may be classified as isotonic, hypertonic, or hypotonic.
2. Isotonic alterations occur when changes in TBW are accompanied by proportional changes in electrolytes. Isotonic fluid loss—hemorrhage, severe wound drainage, excessive sweating, and inadequate fluid intake—causes hypovolemia. Isotonic fluid excess—excessive administration of intravenous fluids, hypersecretion of aldosterone, or the effects of drugs such as cortisone—causes hypervolemia.
3. Hypertonic alterations develop when the osmolality (the concentration of the solution) of the ECF is elevated above normal, usually because of an increased concentration of ECF sodium (hypernatremia) or a deficit of ECF water.
4. Hypernatremia (serum sodium greater than 145 mEq/L) may be caused by an acute increase in sodium level or a loss of water. Hypernatremia can be isovolemic, hypovolemic, or hypervolemic, depending
   on accompanying changes in the level of body water. Hypernatremia, with marked water deficit, is manifested by hypovolemia and dehydration. Hyperchloremia (serum chloride greater than 105 mEq/L) is caused by an excess of sodium or a deficit of bicarbonate. This
   often accompanies hypernatremia because chloride follows sodium.
5. Hypotonic alterations occur when the osmolality of the ECF is less than normal. Hyponatremia is a result of hypotonic alterations.
6. Hyponatremia (serum sodium of less than 135 mEq/L) usually causes movement of water into cells. Hyponatremia may be caused by sodium loss, inadequate sodium intake, or dilution of the body’s
   sodium level with excess water. Hyponatremia can be isovolemic, hypervolemic or hypovolemic, or dilutional depending on accompanying changes in the amount of body water.
7. Hypochloremia (serum chloride less than 97 mEq/L) is usually the result of hyponatremia or elevated bicarbonate concentrations.

Question 5

Alterations in Potassium and Other Electrolytes

Answer 5

1. Potassium is the predominant ICF ion; it regulates ICF osmolality and maintains the resting membrane potential. Potassium is required for deposition of glycogen and glucose in liver and skeletal muscle
   cells, maintenance of normal cardiac rhythm, and contraction of muscles.
2. Potassium balance is regulated by the kidney, by aldosterone and insulin secretion, and by changes in pH.
3. Potassium adaptation allows the body to slowly accommodate increased levels of potassium intake.
4. Hypokalemia (serum potassium less than 3.5 mEq/L) indicates loss of total body potassium. Hypokalemia may result from reduced potassium intake, a shift of potassium into cells, and increased losses of potassium. ECF hypokalemia can develop without losses of total body potassium; plasma potassium levels may be normal or elevated when total body potassium is depleted.
5. Hyperkalemia (serum potassium greater than 5.5 mEq/L) may be caused by increased potassium intake, a shift of potassium from the ICF to the ECF, decreased renal excretion, or drugs that decrease renal potassium excretion.
6. Calcium, phosphate, and magnesium concentrations are rigidly controlled by parathyroid hormone (PTH), vitamin D, and calcitonin.
7. Calcium is an ion necessary for bone and teeth formation, blood coagulation, hormone secretion and cell receptor function, and membrane stability.
8. Hypercalcemia (serum calcium greater than 10–12 mg/dl) can be caused by hyperparathyroidism, bone metastases, sarcoidosis, and excess vitamin D. Hypocalcemia (serum calcium less than 8.5 mg/
   dl) is related to inadequate intestinal absorption, deposition of calcium into bone or soft tissue, blood administration, or decreased PTH and vitamin D levels.
9. Phosphate acts as a buffer in acid–base regulation and provides energy for muscle contraction.
10. Hyperphosphatemia (serum phosphate greater than 4.7 mg/dl) develops with acute or chronic renal failure when there is significant loss of glomerular filtration. Hypophosphatemia (serum phosphateless than 2.0 mg/dl) is usually caused by intestinal malabsorption
    and increased renal excretion of phosphate.
11. Magnesium functions in enzymatic reactions and often interacts with calcium at the cellular level.
12. Hypermagnesemia (serum magnesium greater than 3.0 mEq/L) is usually caused by renal failure. Hypomagnesemia (serum magnesium less than 1.5 mEq/L) may be caused by malnutrition, malabsorption syndromes, and alcoholism.

Question 6

Acid–Base Balance

Answer 6

1. Acid–base balance must be regulated within a narrow range for normal physiologic functioning. This balance maintains membrane integrity and the speed of enzymatic reactions. Many alterations to physiology disturb acid–base balances, producing complications
   that are often more harmful than the disease process itself.
2. Hydrogen ion concentration [H+], is expressed as pH, which represents the negative logarithm (i.e., 10−7) of hydrogen ions in solution (i.e., 0.0000001 mg/L).
3. Different body fluids have different pH values. In biologic fluids, values less than 7.40 are more acidic and values greater than 7.40 are more basic.
4. The renal and respiratory systems, together with the body’s buffer systems, are the principal regulators of acid–base balance.
5. Buffers are substances that can absorb excessive acid or base ions without a significant change in pH, thus maintaining pH within a normal range.
6. Buffers exist as acid–base pairs. The principal plasma buffers are carbonic acid-bicarbonate and the protein hemoglobin. The important intracellular buffers are phosphate and protein.
7. The lungs and kidneys act to compensate for primary changes in pH by increasing or decreasing ventilation and by producing more acidic or more alkaline urine.
8. Acid–base imbalances are caused by changes in the concentration of hydrogen ion in the blood. An increase in hydrogen ions causes acidosis; a decrease causes alkalosis. These changes may be either respiratory or metabolic processes.
9. An abnormal increase or decrease in bicarbonate concentration causes metabolic alkalosis or metabolic acidosis. Changes in the rate of alveolar ventilation and the associated removal of carbon dioxide result in respiratory acidosis or respiratory alkalosis.
10. Metabolic acidosis is caused by an increase in the levels of non–carbonic acids or by the loss of bicarbonate from the extracellular fluid.
11. Metabolic alkalosis occurs with an increase in bicarbonate concentration or from a loss of metabolic acids, such as through vomiting.
12. Respiratory acidosis occurs with decreased alveolar ventilation resulting in hypercapnia (increased carbon dioxide concentration) and increased carbonic acid concentration.
13. Respiratory alkalosis occurs with alveolar hyperventilation resulting in hypocapnia (low carbon dioxide levels) from excessive elimination of carbon dioxide or from decreases in carbonic acid concentration.
14. Renal and respiratory adjustments, in response to primary changes in pH, are known as compensation. Correction occurs when the values for both components of the buffer pair return to normal as the primary disorder is treated or resolves.

Question 7

Question 1

Answer 7

Which is the initial treatment for hypernatremia?

A. Restriction of fluids
B. Administration of a diuretic
C. Isotonic 0.9% normal saline fluid
D. Isotonic salt-free fluid (5% dextrose in water)

Correct Answer: D

Hypernatremia occurs when serum sodium levels exceed 147 mEq/L. Increased serum sodium may be caused by an acute gain in sodium or a net loss of water. Because sodium is largely in the extracellular fluid (ECF), sodium gains cause intracellular dehydration. The treatment of hypernatremia is to give an isotonic salt-free fluid (5% dextrose in water) until the serum sodium level returns to normal. Restricting fluids and administration of a diuretic are used to treat edema, which can occur with hypernatremia, but the initial treatment is to stabilize the sodium level. Isotonic 0.9% normal saline solution is used to treat isotonic fluid loss.