Electrolyte
Mol wt
Valence
Eq wt
Concentrations
Intracellular concentration
mg/dL
mEq/dL
mmol/L
mEq/L
Cations
Na+
23
1
23
326
142
142
14
K+
39
1
39
16
4
4
140
Ca2 + a
40
2
20
10
5
2.5
4
Mg2 +
24
2
12
2.5
2
1.0
35
Total cations
–
–
–
354.5
153
149.5
193
Anions
Cl−
35.5
1
35.5
362
104
104
2
HCO3 −b
44
–
22
55
25
25
8
H2PO4 − HPO4 3−
31
1.8
17
4
2.3
1.3
40
SO4 2−
32
2
16
1.5
0.94
0.47
20
Proteins
–
–
–
7,000
15
0.9
55
Organic acidsc
–
–
–
15
5.76
5.5
68
Total anions
–
–
–
7,437.5
153
137.17
193
Some readers are familiar with the conventional units, whereas others prefer SI units. Table 1.2 summarizes the conversion of conventional units to SI units and vice versa. One needs to multiply the reported value by the conversion factor in order to obtain the required unit .
Table 1.2
Conversion between conventional and SI units for important cations and anions using a conversion factor
Analyte | Expression of conventional units | Conventional to SI units (conversion factor) | SI to conventional units (coversion factor) | Expression of SI units |
|---|---|---|---|---|
Na+ | mEq/L | 1 | 1 | µmol/L |
K+ | mEq/L | 1 | 1 | µmol/L |
Cl− | mEq/L | 1 | 1 | mmol/L |
HCO3 − | mEq/L | 1 | 1 | mmol/L |
Creatinine | mg/dL | 88.4 | 0.01113 | µmol/L |
Urea nitrogen | mg/dL | 0.356 | 2.81 | mmol/L |
Glucose | mg/dL | 0.055 | 18 | mmol/L |
Ca2 + | mg/dL | 0.25 | 4 | mmol/L |
Mg2 + | mg/dL | 0.41 | 2.43 | mmol/L |
Phosphorus | mg/dL | 0.323 | 3.1 | mmol/L |
Albumin | g/dL | 144.9 | 0.0069 | µmol/L |
Osmolarity Versus Osmolality
When two different solutions are separated by a membrane that is permeable to water and not to solutes, water moves through the membrane from a lower to a higher concentrated solution until the two solutions reach equal concentration . This movement is called osmosis. Osmosis does not continue indefinitely, but stops when the solutes on both sides of the membrane exert an equal osmotic force . This force is called osmotic pressure.
The osmotic pressure is the colligative property of a solution. It depends on the number of particles dissolved in a unit volume of solvent and not on the valence, weight, or shape of the particle. For example, an atom of Na+ exerts the same osmotic pressure as an atom of Ca2 + with a valence of 2. Osmotic pressure is expressed as osmoles (Osm). One milliosmole (mOsm) is 1/1,000 of an osmole, which can be calculated for each electrolyte using the following formula:
Osmolarity refers to the number of mOsm in 1 L of solution, whereas osmolality is the number of mOsm in 1 kg of water. However, osmolality is the preferred physiological term because the colligative property depends on the number of particles in a given weight (kg) of water.
The osmolality of plasma is largely a function of Na+ concentration and its anions (mainly Cl− and HCO3 −) with contributions from glucose and urea nitrogen. Since each Na+ is paired with a univalent anion, the contribution from other cations such as K+, Ca2 + and Mg2 + to the osmolality of plasma is generally not considered . Therefore, the plasma osmolality is calculated by doubling Na+ and including the contribution from glucose and urea nitrogen (generally expressed as blood urea nitrogen or BUN), as follows:
![$$ \begin{aligned}& \text{Osmolality}( \text{mOsm/kg }{{\text{H}}_{\text{2}}}\text{O} )=\text{2}[ \text{N}{{\text{a}}^{+}} ]+Glucose+BUN \\& \quad \quad \quad \quad \quad \text{18}\quad \quad \quad \text{2}\text{.8}\end{aligned} $$](https://i0.wp.com/abdominalkey.com/wp-content/uploads/2017/06/A304669_1_En_1_Chapter_Equb.gif?w=960)
