Body Fluid and Compartments

Body Fluid and Compartments

Total body water in males is approximately 60% of total bodyweight, i.e. 42 L for a 70 kg man. In females, total body water is approximately 10% less, because of the greater proportion of body fat compared with males; fat cells have lower water content than other cells of the body. The water is distributed in various spaces or compartments:

• Intracellular fluid. This is the largest water compartment in the body, representing two-thirds of total body water (approximately 28 L).
• Extracellular fluid. This constitutes the remaining one-third of total body water (14 L) and may be subdivided further into two compartments:
• Intravascular, i.e. within the plasma (3 L)
• Interstitial: this fluid (approximately 11 L) is outside the intravascular compartment.

The composition of the various fluids differs with the requirements of each compartment. Sodium is the main cation in the extracellular compartment, whereas potassium is the principal cation of the intracellular compartment. This is achieved by the different permeability of cell membranes for some cations; the cell membrane is approximately 50 times more permeable to potassium than to sodium. Intracellular protein carries a negative charge which attracts the positively charged potassium ions. Most importantly, however, the sodium pump extrudes sodium actively from the cell in exchange for potassium with the use of the Enzyme Na/K-ATPase.

The water content of plasma is 93%; the remainder comprises proteins, lipids and other high-molecular-weight substances. Therefore, the actual value for sodium concentration in intravascular water should be approximately 153 mmol L~l, which of course is higher than the normal plasma sodium value (135-140mmol L~l). In clinical practice, if the proportion of water is reduced, e.g. by substantial increases in protein, lipid or glucose concentration, the plasma sodium value is spuriously lowered and is termed 'pseudohyponatraemia'. Cell membranes are permeable to water and there is a continual flux of fluid among the different body compartments at different Rates of exchange. Two main mechanisms are responsible for these fluid shifts: osmotic pressure and hydrostatic pressure.

Osmotic pressure is the pressure exerted by the number of particles in a solution. Osmolality, the usual term used in clinical practice, is defined as the number of milliosmoles per kg of water (mosmol kg~l. Ions, being in greater abundance, exert a greater osmotic pressure. Water moves freely across a semi permeable membrane and does so from an area of low osmolality to one of higher osmolality, i.e. the increase in osmotic pressure attracts water.

Hydrostatic Pressure

This is the mechanism for fluid movement across a capillary bed from the intravascular to the interstitial compartment. The above figure shows the various pressures exerted as the hydrostatic pressure decreases from 32 mmHg at the arterial end to 12 mmHg at the venous end of the capillary. The oncotic pressure exerted by plasma proteins represents a constant 'negative' pressure that draws fluid into the capillary. Thus, fluid moves out of the arterial Capillary and is withdrawn at the venous end.