240 



ESSENTIALS OF PHYSIOLOGY. 



THE FORMATION OF LYMPH. 



Although the exchange of material between the blood and the tissues 

 does not necessarily increase the amount of lymph in the tissue spaces, 

 it is found that, in point of fact, lymph is constantly being formed in 

 the body, and, after passing along the lymphatic vessels, is returned 

 to the blood stream along the thoracic duct. The formation of lymph 

 has been attributed by some writers to secretion by the walls of the 

 capillaries, and by others to the action of purely physical processes such 

 as nitration and osmosis. If the latter view is correct, a rise in capillary 

 pressure should lead to an increase in the formation of lymph ; and this 

 is found to be the case. 



The pressure in the capillaries is much more easily altered by a rise 

 in the venous pressure than by a rise in the mean arterial pressure ; 

 a rise in venous pressure, by obstructing the escape of blood from the 

 capillaries, at once raises the capillary pressure. Hence a large rise in 

 capillary pressure can be produced by ligaturing the vena cava or portal 

 vein ; and this is followed by a great increase in the flow of lymph from 

 the thoracic duct. 



Again, when a large quantity of saline solution is injected into the 

 circulation, the blood is not only increased in amount, but becomes 

 more watery, the condition being called hydrcemic plethora. The arterial 

 pressure remains almost unaltered, but the veins are distended to con- 

 tain the greater part of the fluid thus added to the circulation, and the 

 venous pressure rises ; as a result the pressure in the capillaries also 

 rises, and the flow of lymph from the thoracic duct becomes very rapid. 



Hydrsemic plethora may also be produced by injecting into the 

 blood a strong solution of dextrose or other crystalloid body ; this 

 raises the osmotic pressure of the blood, and water passes by osmosis 

 from the tissues into the blood, thereby increasing its volume and 

 raising the capillary pressure. In these circumstances a great increase 

 takes place in the flow of lymph from the thoracic duct, as is seen in the 

 following table : 



HYDR.EMIC PLETHORA. 



