1040 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



various parts of the organism. In the mammal, there 

 are no lymph hearts and lymph is moved along the 

 vessel wherever and whenever the vessel is compressed, 

 a situation analogous to that obtaining with veins and 

 venous flow. The presence of smooth muscle fibers 

 in the walls of at least the larger vessels, and nerve 

 fibers running to them, raises the question as to 

 whether lymphatic vessels have contractility or show 

 vasomotion. Of particular interest are their responses 

 to sympathetic and parasympathetic stimulation and 

 to the chemical mediators, epinephrine and acetyl- 

 choline. 



Spontaneous contraction of lymph vessels was 

 described as far back as 1774 by Hewson (99), who 

 reported briefly of having seen actively contracting 

 lacteals in horses and dogs killed immediately after 

 the ingestion of food. Since then, many observers 

 have also reported spontaneous contraction of these 

 and other lymph vessels, but there seem to be species 

 differences (159, 160, 175, 202, 224). Definite spon- 

 taneous contractions have been observed in the 

 peripheral lymphatic vessels of the bat, rat, and 

 guinea pig. No spontaneous contractions have been 

 demonstrated in the cat, dog, rabbit, and squirrel. 

 The results on mice have been equivocal. The few 

 casual observations in man have shown none. The rate 

 of contraction appears to be directly proportional to 

 the rate of formation of lymph and the contractions 

 are apparently initiated by an increase in intra- 

 luminal pressure. They are not dependent on neural 

 control (225). The vasomotion in these vessels seems 

 to be similar to that seen in blood vessels and possibly 

 related to it. Baez and his co-workers (9) observed 

 mesenteric lymphatics during experimental hemor- 

 rhagic shock in rats and reported that the lymphatics 

 undergo pronounced compensatory and decom- 

 pensatory adjustments recalling those seen in met- 

 arterioles and precapillaries of the same region. 

 During the period when the animal is recoverable by 

 transfusion, the lymphatic vessels exhibit progressive 

 enhancement of spontaneous motion and of sensitivity 

 to topically applied epinephrine. Reversal of these 

 features occurs upon the prolongation of drastic 

 hvpotension. Lymph vessel adjustments after sublethal 

 drum trauma are of a compensatory type, compatible 

 with survival, whereas following lethal trauma, the 

 lymphatics invariably appear atonically distended and 

 resemble those seen in irreversible shock. 



Experiments with drugs and faradic and other 

 types of stimulation have also given equivocal results, 

 due probably to differences in experimental proce- 

 dures, in species, and in a failure to distinguish be- 



tween effects on rhythmicity and on caliber of the 

 vessels. There is a suggestion that the response of 

 different lymph vessels may not be uniform. Thus the 

 usual response to sympathetic stimulation or epi- 

 nephrine administration, in general, appears to be a 

 constriction (175, 187-189, 202), whereas the thoracic 

 duct is dilated by the same procedures (1). Much 

 more careful work needs to be done in this area. 



EXCHANGE OF SUBSTANCES BETWEEN 

 PLASMA AND LYMPH 



Much evidence has accumulated during the last 

 decade regarding the exchange of substances between 

 plasma and lymph. As discussed previously, the avail- 

 ability of isotopes has made possible quantitative 

 studies of the disappearance of labeled substances 

 from plasma and their subsequent appearance in 

 lymph. The availability of polyethylene tubing has 

 facilitated the collection of lymph and, with a few 

 exceptions, it has been collected from all areas of the 

 body and its contents more or less completely char- 

 acterized. Beginnings have been made in the study 

 of human lymph (97, 98, 125, 192) under a variety 

 of experimental conditions. The latter studies have 

 been concerned with thoracic duct lymph because 

 of the greater ease of its collection, but will unques- 

 tionably be extended in the near future to the investi- 

 gation of lymph from other areas. 



Concepts of capillary permeability and factors 

 which influence it are discussed in detail in Chapter 

 29. Two concepts have influenced contemporary 

 thinking in the problem of interchange of substances 

 between plasma and lymph, /) the familiar "Starling 

 hypothesis," and 2) the "pore" concept of capillary 

 permeability. 



Starling maintained that the direction and rate of 

 fluid transfer was proportional to the algebraic sum 

 of the effective hydrostatic pressure in the blood 

 capillaries and the osmotic pressure of the plasma 

 proteins. While the capillary membrane was freely 

 permeable to crystalloids, it did not allow larger 

 protein molecules to diffuse readily. The evidence in 

 general confirmation of Starling's hypothesis has 

 recently been reviewed by Yoffey & Courtice (234). 



Although Starling conceived the capillary mem- 

 brane as being only relatively impermeable to protein, 

 there developed a point of view implied or stated in 

 textbooks that capillaries were impermeable to pro- 

 tein if they were healthy and that proteins leaked 

 only when the permeability was abnormal. As will 



