242 ESSENTIALS OF PHYSIOLOGY. 



capillaries alters, and therefore the formation of lymph is increased 

 chiefly by variations in the first and third of these factors. 



ABSORPTION FROM THE TISSUES. 



If a saline solution containing some readily recognisable substance, 

 such as potassium iodide, is injected under the skin or into the pleural 

 or peritoneal cavity, it rapidly disappears, and the presence of 

 potassium iodide can be demonstrated in the blood or urine some 

 time before it appears in the lymph flowing from the thoracic duct. 

 This experiment makes it clear, first, that water and substances in 

 solution can be readily absorbed, and, secondly, that the absorption does 

 not take place into the lymphatic vessels, but through the capillary 

 walls directly into the blood. Similarly, tissue fluid may be absorbed 

 through the capillary walls ; after haemorrhage, for instance, the 

 volume of the blood is rapidly brought back to the normal by the 

 passage of fluid from the tissue spaces into the blood. This process 

 depends upon the fact that proteins exert an osmotic pressure, which, 

 though very small in comparison with that of a solution of crystalloid 

 bodies, is yet appreciable. The osmotic pressure of the crystalloids in 

 blood and lymph is much the same, but, owing to the percentage of 

 protein in blood being higher than that in lyrnph, the blood has a slightly 

 higher osmotic pressure, and fluid tends to pass from the lymph into the 

 blood. At the same time, fluid is being filtered through the capillary 

 wall from the blood into the lyrnph at a rate varying with the capillary 

 pressure. These two processes, namely, filtration and absorption, tend 

 to balance one another and to keep the amount of tissue fluid constant. 

 The balance may be disturbed either by a rise or by a fall of capillary 

 pressure. In the former case, the amount of tissue fluid is increased, 

 whereas, if the capillary pressure falls, for instance after severe 

 haemorrhage, the amount of fluid absorbed exceeds that which is filtered 

 through the capillary walls ; and the volume of the blood increases at 

 the expense of the lymph and tissues. 



The absorption of saline solution, placed under the skin, is brought 

 about partly by simple diffusion of the dissolved substances into the 

 blood, and partly by the osmotic pressure exerted by the proteins in 

 the blood. 



THE FLOW OF LYMPH. 



The tissue fluid is formed under a pressure which is probably rather 

 less than that in the capillaries, and this pressure tends to drive the 

 lymph towards the thoracic duct ; the pressure in the duct where it 



