ro 4 4 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



so that not all of the fluid is irrevocably lost from the 

 body. Large infusions also increase protein leakage 

 but here again the protein is slowly returned to the 

 blood stream and minimizes changes in total circu- 

 lating protein and loss of its oncotic effect. 



It should, perhaps, be emphasized that blood is 

 the chief source of the water of lymph. Benson et al. 

 (15) measured the content of either D 2 or Na'- 4 in 

 intestinal lymph, portal venous blood, and femoral 

 arterial blood of anesthetized hydrated rats after 

 administration of the isotope into the stomach, 

 duodenum, or peripheral or portal vein. Little, if any, 

 water or sodium found its way into lymph after 

 absorption from the small intestine. At least 99 per 

 cent appeared to be carried in portal venous blood. 

 The amount of isotope found in intestinal lymph was 

 proportional to lymph volume whatever the route 

 of administration. Thus, even during absorption of 

 water or sodium ion from the small intestine, blood 

 is the principal source of the water and sodium in 

 lymph. 



TRANSPORT FUNCTION 



Lipids 



There has been considerable interest, particularly 

 during the last decade, in the transport of lipids — the 

 physical state in which they are carried in the blood 

 and their exchange between blood plasma and tissue 

 cells. The availability of isotopes has facilitated the 

 design of experiments concerned with lipid transport 

 by lymph. It is now apparent that the lymphatic 

 system plays an important role in lipid transport as 

 it does in protein transport. This may be because the 

 passage of plasma lipids through the capillary mem- 

 brane depends on lipid-protein complexes rather than 

 on the physical properties of the lipids themselves. 



All the different lipid-protein associations present 

 in the plasma have been identified in thoracic duct 

 lymph (162), as well as in cervical and leg lymph 

 (49). In the dog and cat, alpha-lipoprotein pre- 

 dominates. When rabbits are fed cholesterol, however, 

 the plasma beta-lipoprotein may increase consider- 

 ably with a much smaller rise in alpha-lipoprotein. 

 Lender these circumstances lymph contains beta- 

 lipoprotein. The evidence suggests that beta-lipopro- 

 tein leaves the blood circulation at a slower rate than 

 does alpha-lipoprotein. Not only do alpha- and beta- 

 lipoproteins appear in the lymph in the postabsorptive 

 state, but lymph from the cervical, hepatic, and leg 



ducts — all draining tissues remote from the alimentary 

 tract — also contains chylomicrons (49). As Yoffev & 

 Courtice (234) state: "We can readily understand 

 how the intestinal lymph always contains chylomi- 

 crons even in what we call the postabsorptive state. 

 The presence of chylomicrons in lymph from other 

 tissues, however, suggests that they come either from 

 the blood stream by passing through the capillary 

 membrane or from the fat depots. The evidence 

 indicates that the chylomicron count in the lymph 

 may vary with that in the blood, which suggests that 

 these particles may pass through the capillary mem- 

 brane and so appear in the lymph. For example, the 

 hepatic and cervical lymph ducts were cannulated 

 in a fat-fed cat and chylomicron counts made on 

 lymph and plasma. The thoracic duct which was 

 pouring very fatty chyle into the blood stream was 

 then cannulated and the lymph collected. The 

 chylomicron count in the plasma fell in the next 

 few hours and with this fall the counts in the hepatic 

 and cervical lymph also fell. The fatty chyle which 

 had meantime been collected from the thoracic duct 

 was then injected intravenously making the plasma 

 quite milky. The chylomicron counts in the hepatic 

 and cervical lymph subsequently rose." Geyer et al. 

 (83) attempted to assess the permeability of capillaries 

 to serum cholesterol in humans by measuring the dis- 

 appearance of cholesterol from the blood in the 

 forearm during various degrees of venous congestion. 

 L'nder these circumstances, measurable amounts of 

 cholesterol were filtered and were related to the rate 

 of fluid filtration and the initial level of the serum 

 cholesterol. The results were similar to those of 

 Landis et al. (1 19) for serum proteins. It may also be 

 of interest to mention that the rise of plasma choles- 

 terol occurring in the hypothyroid state does not 

 appear to be due to any decrease in its ability to 

 diffuse out of the plasma (79). Electron microscope 

 studies suggest that chylomicrons can be transferred 

 directly across cell membranes (5, 164) by the active 

 process of pinocytosis. The probability that some 

 active process is concerned in the transfer of macro- 

 molecules from the capillaries to lymph is discussed 

 elsewhere (141). 



When fatty chyle or artificial fat emulsions are 

 injected into the blood stream, they leave the circula- 

 tion very rapidly, but the amounts found in lymph 

 are relatively small (140, 142, 148, 151, 233). The 

 latter investigators injected fatty chyle collected from 

 fat-fed cats into postabsorptive cats and determined 

 the lipid disappearance from plasma and its ap- 

 pearance in hepatic, intestinal, and cervical lymph. 



