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Principle of transcapillary exchange (according to Starling)

There are several forces that act in the vascular bed (Fig. 45):

1. Hydrostatic pressure (HP) in the capillary that ensures blood propulsion along the capillary and filtration or reabsorption of fluid. In proportion to blood propulsion HP decreases. In the initial part of the capillary HP is equal to 30 mm Hg, and in the terminal one — 16 mm Hg.

2. Oncotic pressure (OP) is maintained with proteins that retain fluid in the vascular bed. OP is constant along the whole length of the capillary and equal to 25 mm Hg.

3. Hydrodynamic pressure (HDP) is constant in the interstitial space and makes up 8 mm Hg.

4. OP in the interstitial space is also the constant value and equal to 10 mm Hg.

In the initial part of the capillary:

— effective HDP = HDP_cap - HDP_is = 30 - 8 = 22 mm Hg;

— effective OP = OP_cap - OP_js = 25 - 10 = 15 mm Hg;

— a real pressure is equal to a difference of forces of eff. HDP and eff. OP, i.e. P_r = 22 - 15 = +7 mm Hg, that is the percolation pressure.

Fluid leaves the capillary for interstitial space. This part of the capillary with positive percolation pressure is the zone of active filtration or percolation.

In the midpoint of capillary HDP and OP are equal and filtration in this part of the capillary ceases (it is a dead point of filtration).

In proportion to the blood propulsion along the capillary HDP drops, OP remains the same. Now OP will prevail.

Eff. HDP = 16-8 = 8mmHg.

Eff. OP = 25-10= 15mmHg.

The real pressure in the vessel is equal to eff. HDP - eff. OP = 8 - 15 = -7 mm Hg, i.e. a vector of effect of force is being changed. This part of the capillary is called a zone of active resorption, i.e. fluid permeates from the interstitial space into the capillary.

Along with the flow of fluid substances, dissolved in it and necessary for cellular metabolism, enter the cells, and products of their metabolism are eliminated from the cells.

From the above-mentioned follow the conclusions:

— exchange of fluid and substances dissolved in it occurs only at the level of the capillary;

— disturbance of permeability of the capillary endothelium also leads to development of edemas.

Starling's law explains the events observed in practical medicine. Principle trends of therapy of edemas also result from this law:

— to decrease hydrodynamic pressure;

— to increase protein concentration in plasma;

— to strengthen vascular wall.

In imaciated patients with fiypoproteinemia, in patients being in the state of shock VCB can be restored only by means of transfusions of colloid solutions (plasma, albumin).

Histamine, acetylcholine, lactic acid, folliculin, vitamins of B group, acidosis contribute to the dilatation of pores in the capillary endothelium and mustn't be applied in the treatment of shock.

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