Transvascular fluid exchange

Tissue forces and lymph flow

Figure 1 is a schematic representation of a microvessel, its surrounding interstitium, and a small lymphatic vessel (T,ayloLi9.9.6.). The microcirculation normally filters a small amount of volume into the tissues, which then percolates through them and is removed by the lymphatics without any change in the interstitial volume. When microvessel pressure is increased, more fluid enters the tissues, increasing PT, decreasing pT, and increasing lymph flow, and buffers the increased pressure. However, when these forces can no longer change, observable edema develops. The ability of these forces and lymph flow to change and to oppose changes in microvascular pressure has been defined as edema safety factors (A.uMand..,..a.nd R®.®.dJ9.9.3; T.ay!oL..1..9.9.6). Table 1 shows normal forces measured in a variety of animal tissues. The sum of the forces (Pc - PT) - (pp - pT) in eqn..(1) is small and usually positive, indicating that net transvascular filtration occurs in normal tissues. However, in organs that are always absorbing or filtering fluid, such as those found in the gastrointestinal tract and in the renal glomerular and peritubular capillaries, the imbalance in forces is large.


Fig. 1 Diagram of the capillary-tissue-lymphatic system. (Reproduced with permission from T§yloLi1996.).)


Edema safety factors

Edema forms in tissues when microvascular pressure increases and plasma protein concentration decreases, so that the endothelial barrier is damaged. Figure2 illustrates the formation of edema when microvascular pressure increases. As microvascular pressure increases from about 7 to 25 mmHg, little fluid enters the tissues (curve labeled 'Pressure'). This occurs because, when PT increases by 5 mmHg, the protein osmotic absorptive force s(pp - pT) increases by 10 mmHg and the lymphatic flow factor provides an additional safety factor of 3 mmHg. As microvascular pressure is increased to 30 mmHg, edema develops at a rapid rate which continues unabated at higher microvascular pressures. This rapid rate of fluid accumulation occurs because the safety factors can no longer change to oppose the elevated microvascular pressure. The curve labeled 'Decreased p' was obtained after p was lowered to half normal. Edema develops at lower microvessel pressures because s(pp - pT) cannot increase substantially when pP is small. Observable edema does not develop in damaged microvessels until microvascular hydrostatic pressure is higher than that observed when only pP is low. Obviously, observable edema does occur at lower pressures when the barrier is damaged, but does not occur until microvascular pressure is relatively high (curve labeled 'Permeability'). A recent analysis of this high lymph flow phenomenon concluded that compound(s) are released during the damaging process which cause the lymphatics to increase their pumping capability substantially ( I§y[oLi996).

Fig. 2 Edema formation as a function of capillary pressure: OE, observable edema; SE, slight edema; green solid curve, normal lungs; black broken curve, decreased plasma proteins (pp); green broken curve, damaged capillaries. (Reproduced with permission from TâYloL(199.§.).)

Table..? shows the percentage change in the edema safety factor in a variety of tissues after the microvascular pressure was increased by 20 mmHg, which does not produce observable edema.

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Table 2 Edema safety factors in selected tissues

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