Interaction of capillary and tissue forces in the cat small intestine

Abstract

The authors measured steady state capillary hydrostatic pressure (P(C,i)), plasma and lymph protein concentrations, lymph and blood flow, and capillary filtration coefficients in an in situ loop of cat small intestine at venous outflow pressures (P(V)) of 0, 5, 10, 15, 20, 25 and 30 mm Hg. The data were used to calculate colloid osmotic pressure of lymph and plasma, interstitial fluid pressure (P(T)), pre and postcapillary resistances, and a tissue pressure volume curve of the intestinal interstitium. When P(V) was elevated from 0 to 30 mm Hg, lymph protein concentration decreased from 3.8 to 1.9 g/100 ml (representing a change in colloid osmotic pressure of 6.2 mm Hg), lymph flow increased 7 fold (or an equivalent imbalance in Starling forces of 4.3 mm Hg), and the calculated P(T) increased from -1.8 to +5.3. Because lymph flow draining the loop decreased during the determination of P(C,i) at venous pressures between 15 and 30 mm Hg, the corresponding calculated P(T) may be in error by 1-2 mm Hg. The tissue pressure volume relationship calculated from the data indicates that the intestinal interstitial volume expands nonlinearly and this expansion is characterized by two distinctly different compliant components: (1) tissue compliance is low at P(V) between 0 and 15 mm Hg (0.4 ml/mmHg), and (2) at P(V) greater than 15 mm Hg the tissue compliance is relatively high (4 ml/mm Hg). We found that when P(V) was elevated from 0 to 15 mm Hg, increases in P(T) are the major tissue adjustments that oppose the increased filtration pressures. Furthermore, at P(V) of 20-30 mm Hg, tissue protein concentration decreases, lymph flow relative to the filtration coefficient (ΔP(DROP)) increases and, to a much lesser extent, P(T) increases. Finally, the combination of these changes in tissue force at high filtration pressures represent a maximum tissue edema 'safety factor' of 10 mm Hg; further increases in filtration pressures result in large volume movements into the intestinal lumen.