PHYSIOLOGIC IMPORTANCE OF LYMPH 



IO37 



phatics are modified veins. They are vessels lined by 

 an endothelium which is derived from the veins. 

 They invade the body as do blood vessels and grow 

 into certain constant areas; their invasion of the 

 body is, however, not complete for there are certain 

 structures which never receive them. The lymphatic 

 capillaries have the same relation to tissue spaces as 

 have blood capillaries. None of the cavities of the 

 mesoderm, such as the peritoneal cavity, the various 

 bursae and serous capillaries, forms any part of the 

 lymphatic system. The lymphatic endothelium once 

 formed is specific. Like blood vessels the lymphatics 

 are for the most part closed vessels." 



The lymphatic capillaries may be considered as 

 endothelial tubes resembling blood capillaries but 

 thinner. The medium-size vessels (100-200 n) have 

 muscle fibers, whereas the larger lymphatics are com- 

 posed of an endothelial layer covered by a diffuse 

 connective tissue sheath in which elastic and muscular 

 elements are irregularly scattered. Amyelinated nerves 

 can be traced to the muscle fibers. Valves develop 

 during intra-uterine life in the large vessels and are 

 usually unicuspid or bicuspid. These structures deter- 

 mine the direction of flow toward sites of emptying 

 into the blood stream. 



LYMPH VS. TISSUE FLUID 



It is now generally accepted, primarily from the 

 work of Sabin (191) and MacCallum (132), that the 

 lymphatics form a closed system. "Lymph," therefore, 

 is not synonymous with "tissue fluid," but is the 

 fluid found in lymphatics. This is more than a 

 semantic distinction because, as will be apparent 

 later, the composition of lymph is more particularly 

 determined by the permeability of blood capillaries 

 in a definite area and the consequent pcricapillary 

 filtrate than it is by the metabolism of tissue cells. In 

 this sense, lymph is pericapillary filtrate which has 

 mixed with tissue fluid and has entered the closed 

 lymphatic system. 



Clark & Clark (43) showed that lymphatic capil- 

 laries are sometimes closely associated with small 

 blood vessels, with virtually nothing between the two 

 membranes, while in other cases they bear no rela- 

 tionship to such vessels. In any region, the fluid that 

 enters the lymphatic system to become lymph may be 

 that which is adjacent to the arterial end or to the 

 venular end of a blood capillary, or it may be fluid 

 that is relatively distant from a blood capillary. 

 Mc Master (137) studied the relative pressures within 



the cutaneous lymphatic capillaries and the surround- 

 ing tissues in the mouse's ear. He reported that the 

 mean lymphatic pressure was 1.2 cm water and the 

 interstitial pressure 1.9 cm water. There was always 

 a gradient of pressure from the interstitial tissue to 

 the lumen of the lymphatic even in conditions of 

 increased lymphatic pressure. Presumably, whenever 

 increased amounts of fluid are present in the inter- 

 stitial tissues, the lymphatic vessels are kept open by 

 swelling of connective tissues and increase in the 

 tension of the fibers attached to the lymphatic capil- 

 laries (42, 43, 137, 175). Many more data are needed 

 in other tissues and species to establish firmly the fact 

 that a gradient of pressure is always present between 

 interstitial tissues and the lymphatics, and is an im- 

 portant factor in the formation of lymph. Particularly 

 disturbing in this connection is the recent report of 

 Guyton et al. (93) suggesting the existence of negative 

 pressures in interstitial spaces. 



The close anatomical relationship between the 

 lymphatic and venous systems has raised the question 

 as to the relationship of lymphatic and venous pres- 

 sures. Little definitive information is available, how- 

 ever, in this area, due primarily to the difficulties in 

 measurement of lymphatic pressure. Many of the 

 pressures that have been recorded are end pressures 

 (234) and not particularly representative of the actual 

 pressures under normal conditions of flow. Thus, Lee 

 (124) found the average end thoracic duct pressure 

 in dogs to be 15 cm HjO, whereas Rouviere & 

 Valette (185) found side pressure at the entrance to 

 the subclavian vein to be 6.4 cm H>0. They also 

 found the pressure in the internal jugular vein of the 

 same animal to be 2.4 cm H 2 0, thus demonstrating 

 the existence of a gradient capable of promoting 

 emptying of lymph from the thoracic duct to the 

 jugular vein. Webb & Starzl (222) found side pres- 

 sures of 3.5 to 5.5 cm HjO in the thoracic duct just 

 above the diaphragm in anesthetized dogs. At this 

 point, arterial pulsations affected the lymph pressure, 

 the difference between the pressures during systole 

 and diastole being 2 to 3 cm H 2 0. Although these 

 values are lower than those of Rouviere and Valette, 

 they still permit of a gradient toward the vein. 

 Irisawa & Rushmer (103) recently reported on the 

 relationship between lymphatic and venous pressure 

 in the legs of dogs. Although previous investigators 

 had regarded lymphatic pressure of a resting dog leg 

 as being too low to measure, these authors, working 

 with unanesthetized dogs, found the leg lymphatic 

 pressures to range from 2.5 to 12.0 cm H«0, while 

 the range of pressures in ankle veins was from 5.5 to 



