Basic Concepts of Fluid and Electrolyte Therapy

The intravascular and extravascular components of the ECF are separated
by the capillary membrane, with its micropores, which allow
only a slow escape rate of albumin (5%/hr), which is then returned to
the circulation via the lymphatics at the same rate, thereby maintaining
a steady state of equilibrium (Fig. 2). While the hydrostatic pressure
within the circulation tends to drive fluid out, the oncotic pressure
of the plasma proteins, e.g. albumin, draws fluid in and maintains
the relative constancy of the plasma volume as a proportion of the
ECF (Starling effect).
Transcapillary
escape rate
of Albumin
4-5% per hour
IVS
capillary membrane
ISS
Flux 10x the rate of
Albumin synthesis
Albumin
40 g/l Lymph
Albumin 35 g/l
ISS Albumin exceeds the
IVS Albumin by 30%
ISS = Interstitial space
IVS = Intravascular space
Thoracic duct
Figure 2: Transcapillary escape of albumin in health.
There is also a clinically important flux of fluid and electrolytes
between the ECF and the gastrointestinal (GI) tract involving active
secretion and reabsorption of digestive juices (Fig. 3). In health there
is a constant flux between these various spaces and important physiological
mechanisms ensure a constant relationship between them,
which we may term the internal fluid balance.
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