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Osmotic pressure is the primary determinant of the distribution of water between the three major compartments. The concentrations of the major solutes in these fluids differ, and each compartment has one solute that is primarily limited to that compartment and therefore determines its osmotic pressure: K + salts in the intracellular fluid (most of the cell Mg 2+ is bound and osmotically inactive), Na + salts in the interstitial fluid, and proteins in the plasma. Regulation of the plasma volume is somewhat more complicated because of the tendency of the plasma proteins to hold water in the vascular space by an oncotic effect which is in part counterbalanced by the hydrostatic pressure in the capillary that is generated by cardiac contraction. The composition of intracellular and extracellular fluids is shown in Table (a).
| Table (a) - Electrolyte Composition of intracellular and extracellular fluids |
| |
Plasma (mmol/L) |
Interstitial fluid (mmol/L) |
Intracellular fluid (mmol/L) |
| Na+ |
142 |
144 |
10 |
| K+ |
4 |
4 |
160 |
| Ca2 |
2.5 |
2.5 |
1.5 |
| Mg2 |
1.0 |
0.5 |
13 |
| Cl- |
102 |
114 |
2 |
| HCO3- |
26 |
30 |
8 |
| PO4 |
1.0 |
1.0 |
57 |
| So4 |
0.5 |
0.5 |
10 |
| Organic Acid |
3 |
4 |
3 |
| Protein |
16 |
0 |
55 |
A characteristic of an osmotically active solute is that it cannot freely leave its compartment. The capillary wall, for example, is relatively impermeable to plasma proteins, and the cell membrane is 'impermeable' to Na + and K + because the Na + -K + -ATPase pump largely restricts Na + to the extracellular fluid and K + to the intracellular fluid. By contrast, Na + freely crosses the capillary wall and achieves similar concentrations in the interstitium and plasma; as a result, it does not contribute to fluid distribution between these compartments. Similarly, urea crosses both the capillary wall and the cell membrane and is osmotically inactive. Thus, the retention of urea in renal failure does not alter the distribution of the total body water.
A conclusion from these observations is that body Na + stores are the primary determinant of the extracellular fluid volume. Thus the extracellular volume - and therefore tissue perfusion - are maintained by appropriate alterations in Na + excretion. For example, if Na + intake is increased, the extra Na + will initially be added to the extracellular fluid. The associated increase in extracellular osmolality will cause water to move out of the cells, leading to extracellular volume expansion. Balance is restored by excretion of the excess Na + in the urine.
Also see Osmotic pressure, Plasma osmolality, Osmoregulation
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