EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



979 



apparently between or along connective tissue fibrils 

 (fig. 4.2). Hemorrhage and dehydration, which they 

 used to diminish the volume of interstitial fluid, 

 delayed the appearance of these bristles of dye, but 

 eventually they became clearer than usual, as well as 

 more resistant to displacement by massage. On the 

 other hand, hydremic plethora, and particularly the 

 edema of inflammation, tended to obliterate the 

 bristles and permitted instead a diffuse and more 

 rapid distribution of dye which could easily be dis- 

 placed by pressure from one area into another, pre- 

 sumably because it dissolved in a larger and more 

 freely movable volume of edema fluid. 



The paucity of normal interstitial fluid, its layered 

 distribution, and the disruptive effects of injecting 

 even small volumes of fluid (238) makes it necessary, 

 as with capillary blood pressure, to determine inter- 

 stitial fluid pressure by a "null point" method which 

 provides a balance of pressures with minimal move- 

 ment of fluid into, or out of, the interstitial fluid com- 

 partment. Wells et al. (375) used a capillary tube 

 placed between the manometer and the saline-filled 

 needle that was inserted into the tissue. By observing 

 the meniscus under a microscope, they saw that a 



change of 2 or 3 mm water pressure sufficed to re- 

 verse the flow at the point of balance and hence, after 

 correcting for capillarity in the tube, they measured 

 interstitial fluid pressure with a small volume artifact. 

 Burch & Sodeman (30) and McMaster (238) re- 

 duced the volume change further, but still more re- 

 fined methods are needed to reduce the likelihood of 

 local hemorrhage and mechanical artifacts. 



Table 4. 1 summarizes several representative series 

 of values given in mm Hg for easier comparison with 

 capillary blood pressure and the osmotic pressure of 

 the plasma proteins. In skin, McMaster (238) found 

 it necessary to determine "interstitial resistance" to 

 very slow rates of inflow of fluid because paucity of 

 freely movable fluid prevented determining a true 

 interstitial pressure. Although some of the values in 

 table 4. 1 may be artificially high, their order of 

 magnitude is consistent. P, t in skin and subcutaneous 

 tissues, under resting conditions, ranges from 1 to 5 or 

 6 mm Hg and averages about 2.5 mm Hg. In muscle, 

 P, f tends to be slightly higher, 1 to 9 mm Hg and 

 averages 4.5 mm Hg. In some comparisons P lf was 

 higher in the tightly sheathed muscles, e.g., soleus 

 and anterior tibial, than in the more loosely enclosed 



/ ' yi-BM 



•u^v/- r /',^ Mi _ 





I 



fig. 4.2. Diagrammatic sketch of the extravascular interstitial movement of a 2% solution of 

 pontamine sky blue after its escape from the lymphatics, a: Dye first appears as colored bristles at 

 2-7 min. b: Color becomes more intense and bristles longer at 3-10 min. c: Colored lines become 

 broader at 5-12 min. d: Second phase. Diffuse blue staining between bristles which cannot be dis- 

 lodged by pressure. Bristles disappearing. During a to d color was apparently fixed on tissue ele- 

 ments and not dislodged by pressure, e; Diffuse blue cloud easily displaced with pressure, free fluid. 

 /.■ Dye escaping from ruptured lymphatics, no bristles. [From McMaster & Parsons (240).] 



