EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



997 



CAPILLARY 



BLOOD 

 PRESSURE 



CM 

 H,0 



10 15 



TIME - MINUTES 



fig. 6.7. Chart indicating the changes of capillary blood 

 pressure in frog's mesentery during capillary stasis produced 

 by applying 25^0 urethan solution (A to C) and during re- 

 covery with resumption of capillary blood flow (C to D). 

 [From Landis (199)-] 



tightly throughout the capillary. After D, arteriolar 

 pressure having been blocked by the packing of 

 erythrocytes up to the arteriocapillary junction, 

 capillary blood pressure fell to the level in the venule. 

 This secondary fall of pressure seems to assist recovery 

 from stasis because, as it occurs, one can observe that 

 the erythrocytes become less tightly packed and slow 

 movement toward the nearest venule begins. At D 

 sluggish flow was being resumed. Hence in injury, 

 capillary stasis and the rapid accumulation of rela- 

 tively large volumes of protein-containing edema fluid 

 or blister fluid depend primarily upon increased per- 

 meability, but also upon increased capillary blood 

 pressure. Recovery from stasis, while assisted by tem- 

 porary lowering of capillary blood pressure, cannot 

 occur until the permeability of the wall to protein 

 returns toward normal. 



TISSUE ASPHYXIA; RELATION OF FILTRATION COEFFI- 

 CIENTS to 0>, C0 2 , and pH. The effects of arrested 

 blood flow and, more specifically, of hypoxia on capil- 

 lary permeability and the filtration-absorption mecha- 

 nism are still uncertain. In general, it appears that 

 arrest of blood flow must be total and prolonged, and 

 that hypoxia must be severe, before changes in 

 permeability become demonstrable. Lazarus-Barlow 

 (213) in 1894 studied the edema of passive congestion, 

 and also the edema which appeared when blood flow 

 was restored after a prior period of complete arterial 

 and venous occlusion. He ascribed the latter edema 



to functional modification of the vessel walls secondary 

 to "starvation of the tissues" and accumulated 

 waste products. More recently Pochin (285) found 

 in the rabbit's ear that occluding the circulation for 2 

 hours led to demonstrable edema which appeared 

 shortly after circulation was re-established. Occlusions 

 of 16 to 18 hours produced enough edema fluid to 

 permit collecting samples in which the protein con- 

 tent approached 5 g per cent. Edema alone might 

 conceivably have been the result of vasodilatation 

 and high capillary blood pressure that probably 

 followed this arrest of the circulation, but the high 

 concentration of protein in the edema fluid indicated 

 that increased permeability was also present. 



Among the factors that might change capillary- 

 permeability under these conditions, the first to be 

 considered are those associated with continued 

 metabolism of tissues in the absence of blood flow, 

 viz. a) reduced oxygen tension, b) increased carbon 

 dioxide tension, and c) local decrease of pH due to 

 accumulation of metabolites such as lactic acid. 

 Table 6.4 summarizes the effects of these variables 

 on the filtration coefficients (k c ) and on the in vivo 

 effective osmotic pressures of the plasma proteins 

 measured in single mesenteric capillaries of the frog 

 (201). They indicate that Ringer's solution, saturated 

 with C0 2 or acidified by HC1 to pH's between 7.0 

 and 5.0, had no significant effect on the permeability 

 of the capillary wall. Only when pH was made 4.0 

 or less, and hence unphysiologically low, was there 

 evidence of increased permeability to fluid and 

 protein. 



Severe and, so far as possible, total oxygen lack 

 made the capillary wall permeable to protein and 

 fluid as indicated by decreased effective osmotic 

 pressure of the plasma proteins and by increased 

 filtration coefficient, respectively. It must be em- 

 phasized that the lowering of 2 tension in these 

 experiments was maximal because not only was 

 blood flow stopped by tightening a loop around the 

 mesenteric artery, but the mesentery was also ir- 

 rigated freely with Ringer's solution previously boiled 

 and kept saturated with nitrogen. The possibility 

 that metabolites from anaerobic metabolism were 

 responsible could not be excluded. The effects on 

 permeability were, however, still reversible because, 

 if the period of severe hypoxia was brief enough, e.g., 

 3 min, resumption of blood flow and irrigation with 

 oxygenated Ringer's solution restored both the 

 filtration coefficient and the in vivo osmotic pressure 

 of the plasma proteins toward normal, as shown in 

 table 6.4. For comparison, at the bottom of table 6.4 



