1020 



PHYSIOLOGY 



More difficulty is presented by the question of the mechanism of absorp- 

 tion by the blood-vessels of the normal tissue fluids such an absorption as 

 we have seen to occur after loss of blood by haemorrhage. It seems probable 

 that this absorption depends on the small proportion of protein contained in 

 the tissue fluid as compared with the blood-plasma, and is due to the osmotic 

 pressure of the protein. If blood-serum be placed in a bell-shaped vessel 

 (the mouth of which is closed by a gelatinous membrane which does not 

 permit the passage of protein), and suspended in normal salt solution, it is 

 found that the serum absorbs the salt solution until the manometer attached 

 to the bell-jar indicates a pressure of 25-30 mm. Hg. Thus we may con- 

 ceive that there is normally a balance in the capillaries 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 there- 

 fore 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. Apparently 

 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. 



