Extracellular fluid, electrolyte content

Except for respiratory and dermal insensible water-vapor losses, all remaining water lost by the body contains electrolytes, mainly sodium and chloride. The normal cation and anion constituent composition of the fluid spaces is given in Table IV. In the extracellular fluid space, sodium is the major cation and chloride the major anion. Those two ions constitute 95 of the extracellular fluid osmolality. Changes in plasma sodium concentration reflect changes in extracellular fluid volume. Potassium is the major cellular cation and phosphates and proteins comprise the major anions. The total cellular osmolality (175 + 135 = 310 mosraol/kg H2O) is equal to the total extracellular osmolality (155 + 155 = 310 mosmol/kg HaO) therefore, equal total osmotic concentrations are maintained between two fluid compartments of widely different ionic contents (Table IV). 

A surgical implant is constantly bathed in extracellular tissue fluid. Basically water, this fluid contains electrolytes, complex compounds, oxygen and carbon dioxide. Electrolytes present in the largest amounts are sodium (Na ) and chloride (Cl ) ions. Most of the fluids existing in the body (such as blood, plasma and lymph) have a chloride content (and pH) somewhat similar to that of sea water (about 5 to 20g/l and pH about 8) . 

The inverse sequence occurs if the primary abnormality is a fall in extracellular potassium concentration the steps are shown in Figure 3.SB. In both A and B, a rise in concentration of one will entrain a rise in the other. For all cells other than those of the renal tubules, this mechanism, since it involves movements to and fro across the cell membrane, results only in movements of ions between compartments of the body without loss of total body content. The renal tubular cells are exceptional because movement of electrolytes from the cytoplasm of these cells into the tubular fluid leads to irretrievable loss of the electrolytes from the body. 

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