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HANDBOOK OF 1'HYSIOLOGY 



CIRCULATION II 



lected during exercise (379) contains more erythro- 

 cytes than control lymph does. This finding suggests 

 mechanical rupture of some capillaries, probably 

 when compressed between adjacent contracting 

 fibers. If this occurs, undetermined amounts of whole 

 plasma may accompany the erythrocytes and con- 

 tribute to the protein found in lymph. The possibility 

 of osmotic shifts of fluid produced by small molecules, 

 e.g., lactic acid, from contracting muscle has also 

 been proposed (207). 



Finally, in lymphedema, the effect of obliterating 

 lymph flow by obstructive fibrosis of the larger 

 lymphatic vessels (77) is an accumulation of extra- 

 vascular fluid with abnormally high concentrations 

 of protein in both the edema fluid as well as in the 

 stagnant lymph. The protein content of capillary 

 filtrate is unknown and may be quite variable be- 

 cause of the tendency in lymph stasis toward inter- 

 mittent infection and consequent injury to capillaries 

 in severely lymphedematous extremities (77). It is 

 clear that more information is needed in all these 

 conditions. 



C. Circulation of Interstitial Fluid; 

 Circulation of Protein 



It has been customary in the past to say that capil- 

 laries '"leak" protein as if this were a useless defect of 

 the capillary wall. However, many lines of evidence 

 indicate that passage of plasma proteins through the 

 capillary wall is quite as important for cellular 

 metabolism and for defense against infection as the 

 retention of plasma protein is for normal fluid balance. 

 Whipple & Madden (376) showed that the circulating 

 plasma proteins within the blood vessels form a 

 "medium of exchange" which is an important part 

 of a larger nutritional pool. For example, dogs were 

 maintained in full nitrogen equilibrium by intra- 

 venous administration of dog plasma only. Drinker 

 (75) called attention to the benefits derived, during 

 infection, from the passage of globulins, including 

 antibodies, through the capillary wall into the inter- 

 stitial fluid around the cells and thence to the lym- 

 phatics. Still more recently several reviews have de- 

 scribed the binding of hormones (63, 304), fatty 

 acids (iog), and drugs (121) to plasma proteins. It is 

 significant, too, that the greatest passage of protein 

 through the capillary walls occurs in the liver, where 

 metabolic requirements are greatest and most varied, 

 and where albumin is synthesized. 



Two paracapillary circulations (i.e., beside and 

 beyond the capillaries) can be identified. The first is a 



filtration-absorption circulation which includes the 

 total capillary filtrate, the total interstitial fluid, and 

 finally that part of the interstitial fluid which passes 

 back into the capillary blood by the process of absorp- 

 tion. The second paracapillary circulation begins also 

 with capillary filtrate but then reduces to the unab- 

 sorbed fraction of interstitial fluid and its contained 

 protein, both of which, after bathing the tissue cells, 

 enter the finest lymphatic capillaries and are con- 

 ducted, via the major lymphatic trunks, back to 

 venous blood (see Chapter 30). Enough information 

 is available now to justify approximate calculations of 

 the magnitudes of these two circulations. Because both 

 depend upon the total volume of capillary filtrate 

 this figure can be considered first. 



Continued blood flow through the resistance of the 

 capillaries requires, even at resting flow rates, a sig- 

 nificant pressure gradient in the capillary bed itself. 

 As indicated in table 2.1 and figure 2.3 this average 

 gradient lies above the osmotic pressure of the plasma 

 proteins in the first half of the capillary network. It 

 follows that, secondary to the basal pressure head 

 which is necessary for this resting blood flow, there is 

 necessarily a "basal filtration" of fluid under resting 

 conditions. Most of this filtrate is absorbed and the 

 low rates of lymph production in resting extremities 

 can give no indication of the rate at which the original 

 capillary filtrate is formed. A simple calculation sug- 

 gests, however, that in the resting animal capillary 

 filtrate is continuously produced at an average rate 

 which is at least five to ten times greater than average 

 resting lymph flow. 



Landis & Gibbon (209) found in the human fore- 

 arm at 34 to 35 C that elevating venous pressure by 

 1 cm H 2 increased filtrate by .0033 ml per 100 ml 

 forearm tissue per min. Assuming that 80 per cent of 

 a rise in venous pressure is transmitted to the capil- 

 laries, this becomes .0040 ml per 100 ml forearm 

 tissue per min for a 1 cm water increase of capillary 

 pressure. From capillary pressure measurements in 

 human skin, mean resting filtering pressure is (32 - 

 25 mm Hg)/2 or 3.5 mm Hg, or 4.8 cm H2O. Assum- 

 ing, for the purpose of obtaining a minimum figure, 

 that the unit increment of filtration given above 

 applies to the whole body, the total resting capillary 

 filtrate for a 75-kg human being is approximately 20 

 liters per 24 hours. To the extent that filtration 

 coefficients in liver and intestine may be greater than 

 in the forearm the volume of filtrate formed per 24 

 hours will be somewhat larger still. 



For total lymph flow in man the most helpful data 

 are those of Crandall et al. (60) obtained from a 



