1142 PHYSIOLOGY 



attached to the bell-jar indicates a pressure of 25-30 mm. Hg. Thus 

 we may conceive that there is normally a balance in the capil- 

 laries between the processes of exudation and of absorption, the 

 former being conditioned by the capillary blood pressure and the 

 latter by the difference in protein content, and therefore of osmotic 

 pressure between the blood-plasma and tissue-lymph. A rise of 

 capillary pressure will upset this balance in favour of transudation 

 and the blood will become more concentrated, whereas a fall of pressure 

 will turn the scale in favour of absorption and the volume of blood 

 will be increased at the expense of the tissue fluids. 



THE PART PLAYED BY THE LYMPH IN THE NUTRITION 



OF THE TISSUES 



The fact that the tissue-cells are separated by the lymph and the 

 capillary wall from the blood shows that in all interchanges between the 

 blood and tissues the lymph must act as the medium of communication. 

 The lymph-flow plays very little part in this process. The muscles 

 of a resting limb are taking up nourishment as well as oxygen 

 from the blood and giving off their waste products carbonic acid 

 and ammonia, though not a drop of lymph may flow from a cannula 

 placed in a lymphatic trunk of the limb. In fact the interchange of 

 material between tissue-cell and blood through the mediation of the 

 lymph is carried out in the same way as are the gaseous interchanges, 

 viz. by a process of diffusion. This explanation, however, holds good 

 only for the diffusible constituents of the blood and will not account 

 for the supply of the indiffusible protein molecules to the cell. Ap- 

 parently the only way in which the tissues can obtain their supply 

 of protein is from the small proportion of this substance which has 

 filtered through the vessel wall into the lymph. The increased 

 exudation of concentrated lymph to the tissues which occurs in 

 inflammatory conditions or as the result of injury is therefore of 

 advantage, since it furnishes an abundant supply of protein food to 

 be used up in the regeneration of the damaged cells. 



