EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



987 



patient with a freely draining fistula of the thoracic 

 duct. Average basal lymph flow during fasting was 

 0.93 ml per min. After a heavy meal, flow from the 

 thoracic duct reached a peak volume of 3.9 ml per 

 min and remained above 1 .0 ml per min for several 

 hours. If allowance is made for the additional and 

 uncollected lymph from the right lymphatic duct by 

 adding an increment of one-fourth to one-third of 

 fasting thoracic duct flow, then total lymph flow in 

 fasting man at rest is approximately 2 liters per 24 

 hours. With allowance made for the effects of meals 

 and activity, it probably approaches 3 or 4 liters per 

 24 hours. 



Figure 5.2 shows schematically the volumes of 

 these several '"circulations" in terms of exchanges in 

 24 hours. With a cardiac output of 6.0 liters per min 

 the first circulation, that of blood itself, amounts to 

 about 8000 liters per 24 hours. From this volume, 

 filtration in the capillary bed removes a minimum of 

 20 liters per 24 hours, a "filtration fraction" of 0.25 

 per cent. This capillary filtrate begins the second 

 circulation, that of interstitial fluid (fig. 5.2, F to IF 

 to A) with capillary absorption, during rest, of 80 to 

 90 per cent or 16 to 18 liters, of the original capillary 



CARDIAC OUTPUT 

 8400 L per 24 HOURS 



DIFFUSION EXCHANGE 

 80,000 L 

 20,000g glucose (400 utilized) 



LYMPH 

 2 to 4 L 



FILTRATION - ABSORPTION 



LYMPH FLOW 



FLUID 20 L+ - 16 to 18 L =C= 2 to 4 L 



PROTEIN 80 to 200g - 5 (?) O 75 to 195 g 



Glucose 20 g (see diffusion exchange) 



fig. 5.2. Diagram of the ''several circulations" with ap- 

 proximate magnitudes of each. For explanation of diffusion 

 exchanges see section 9. For explanation of figures relating to 

 filtration, absorption, and lymph flow see text of this section. 



filtrate. The remaining 2 to 4 liters, including the 

 unabsorbed protein of the original capillary filtrate, 

 then produces the third circulation, that of proteins in 

 lymph. 



The potential magnitude of this protein circulation 

 can be estimated from the observations of Wasser- 

 man & Mayerson (370-372) on the rates at which 

 intravenously injected labeled albumin and globulin 

 disappeared from plasma and appeared in thoracic 

 duct lymph. The faster component of these two- 

 phase disappearance curves indicated a steady disap- 

 pearance of plasma albumin from plasma, and 

 corresponding appearance in lymph, at the rate of 

 approximately 0.1 per cent of the total circulating 

 plasma protein per minute. Allowing for the slightly 

 slower disappearance rate of globulin (372), this 

 amounts to the passage through the capillary wall in 

 24 hours of a mass of plasma protein approximately 

 equal to that in the circulating blood itself. This 

 includes passage from the more permeable hepatic 

 and intestinal capillaries as well as from the less 

 permeable limb capillaries. Courtice (49; 386, p. 87) 

 collected lymph simultaneously from the thoracic, 

 right lymphatic, cervical, foreleg, and hind leg ducts. 

 Expressed as percentage of total intravascular protein, 

 the lymph collected from these several sources con- 

 tained a 24-hour protein mass equaling, respectively, 

 47.5, 3.6, 2.4, 2.2, and 1.8 or, in total, 57.5 per cent 

 of the intravascular protein mass. Again this rate of 

 passage is a basal rate found in resting animals. Intra- 

 venous infusions increased the rate of protein passage 

 severalfold (371) and increased lymph flow from the 

 thoracic duct correspondingly (180), indicating that 

 the interstitial circulation of both protein and fluid 

 can be very rapid indeed. Studies during muscular 

 exercise would be most interesting, but have not been 

 done so far. 



For man the magnitude of this protein circulation 

 can be estimated in two ways. First, the obligatory 

 capillary filtrate of 20 liters per 24 hours, containing 

 0.2 to 0.4 g per cent protein, would carry with it a 

 minimum of 40 to 80 g of protein per 24 hours. This 

 figure is unquestionably too low because it does not 

 include the higher protein content of capillary fil- 

 trates from liver and intestine. Second, collections of 

 thoracic duct lymph with analyses of protein content 

 have been carried out in two patients with accidental 

 fistulae (57, 60) and in patients with terminal neo- 

 plasm (15). As mentioned above, the data of Crandall 

 ct al. (60) justify an estimate of 2 to 4 liters of lymph 

 per 24 hours. Since the protein content of this lymph 

 ranged from 3.19 to 4.88 g per 100 ml the circulation 



