Electrolytes

Normally, the number of anions (negatively charged ions) and cations (positively charged ions) in each fluid compartment are equal. Cell membranes play the critical role of maintaining distinct ICF and ECF spaces, which are biochemically distinct. Serum electrolyte measurements reflect the stores of ECF electrolytes rather than that of ICF electrolytes. Table 27-4 lists the chief cations and anions along with their normal concentrations in the ECF and ICF. The principal cations are sodium, potassium, calcium, and magnesium, while the key anions are chloride, bicarbonate, and phosphate. In the ECF, sodium is the most common cation and chloride is the most abundant anion, while in the ICF, potassium is the primary cation and phosphate is the main anion. Normal serum electrolyte values are listed in Table 27-5.

Osmolality is a measure of the number of osmotically active particles per unit of solution, independent of the weight or nature of the particle. Equimolar concentrations of all substances in the undissociated state exert the same osmotic pressure. Although the normal serum osmolality is 280 to 300 mOsm/kg (280-300 mmol/kg), multiple scenarios exist where this value becomes markedly abnormal. The calculated serum osmolality helps determine deviations in TBW content. As such, it is often useful to calculate the serum osmolality as follows:

Table 27-4 Normal Cation and Anion Concentrations in the ECF and ICF

ECF

ICF

Plasma

Interstitial

mEq/L

(mEq/L or

Fluid (mEq/L

Ion

or

Ion Species

mmol/L}

or mmol/L}

Species

mmol/L

Cations

Cations

Na1

142

144

K*

135

K*

4

A

43

5

2.5

3

1.5

Total

154

Ï52

Total

178

Anions

Anions

ci-

103

T14

PO;-

HCG;

27

30

Protein

70

POf

2

2

SO;-

IS

sot

1

1

Orgar c acid

5

5

Protei n

16

0

Total

154

152

Tatal

178

Table 27-5 Normal Ranges for Serum Electrolyte Concentrations

EC F, extracellular fEuid; 1CF. intracellular fluid

Table 27-5 Normal Ranges for Serum Electrolyte Concentrations

Sodium

136-145 mEq/L or Í3Ó-I45 mrnoVL

Potassium

3.5-5.0 mEq/L or 3.5-5.0 mmol/L

Chloride

9fi-106 mEq/L or 9ñ-]Q6 mrnot/L

Bicarbonate

21- 30 mEq/L or 21-30 mrnol/L

Magnesium

1.4-2,2 mFq/L or 0.7—1.1 mmol/l

Calcium:

Total

4.4-5.2 mEq/L (9-10.5 mg/dL) or 2.23-25 mmol/L

Ionized

2.2-2 Ö mEq/L (4.5-5.6 mg/dL) or hi-1.4 mrnoVL

Phosphorus

3-4.5 mg/dL (1.0 1.4 mmol/L J

Serum osmolality (mOsm/L) = 2 (Na mEq/L) + (glucose [mg/dL])/18 + (BUN [mg/ dL])/2.8.

Note: For glucose, multiply by a factor of 0.055 to convert conventional glucose units (mg/dL) to SI glucose units (mmol/L). To convert SI units of glucose (mmol/L) to conventional glucose units (mg/dL), multiply SI units by a factor of 18.18. For blood urea nitrogen (BUN), multiply by a factor of 0.357 to convert conventional BUN units (mg/dL) to SI BUN units (mmol/L). To convert SI units of BUN (mmol/L) to conventional BUN units (mg/dL), multiply SI units by a factor of 2.8.

Because the body regulates water to maintain osmolality, deviations in serum osmolality are used to estimate TBW stores. Water moves freely across all cell membranes, making serum osmolality an accurate reflection of the osmolality within all body compartments. An increase in osmolality is equated with a loss of water greater than the loss of solute (TBW depletion). A decrease in serum osmolality is seen when water is retained in excess of solute (CHF or hepatic cirrhosis). The difference between the measured serum osmolality and the calculated serum osmolality, using the equation above, is referred to as the osmolar gap. Under normal circumstances the osmolar gap should be 10 mOsm/L or less. An increased osmolar gap suggests the presence of a small, osmotically active agent and is most commonly seen with the ingestion of alcohols (ethanol, methanol, ethylene glycol, or isopropyl alcohol) or medications such as mannitol or lorazepam. Patient Encounter 3 illustrates the utility of serum osmolality in a clinical setting.

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