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Circulation of water between sections

In every water section (intravascular, interstitial and cellular) water is in dynamic equilibrium that lies in the basis of all normal biological processes. The following internal factors retain water in these sections:

— physiology of capillary membrane;

— oncotic pressure;

— difference of osmotic pressure;

— physiology of cellular membrane.

Capillary endothelium separates and at the same time connects a vascular sector with interstitial one playing the role of dialyzing membrane. A capillary membrane is impermeable for proteins and permeable for water, crystalloids, not large organic molecules, urea, glucose, uric acid and creatinine. Presence of proteins, as nondiffusing ions with negative charge, creates a nonuniform distribution of crystalloids with the sign opposite to that of proteins, and there are a greater number of crystalloid ions with the same sign as proteins in the interstice. Thus, a presence of proteins in the intravascular section creates differences of electrolyte level and osmotic pressure (Donnan equilibrium).

A capillary endothelium consists of squamous cells united by a special substance playing a role of intercellular cement. Varied harmful influences (acidosis, anoxia, avitaminoses, endocrine effects and chemical mediators) may cause changes in the intercellular "cement" with dilatation of pores leading to the increase of capillary permeability.

Oncotic pressure — it is a sum of osmotic pressure produced by colloid ions (proteins) and osmotic pressure of the excess of crystalloidal and intravascular ions (colloidal osmotic pressure).

The mechanism of vascular-and-interstitial circulation of water (Fig. 45), discovered by Starling, lies in the antagonism between hydrodynamic and osmotic pressure.

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