90 



THE INDIVIDUAL ORGANISM 



the capillary walls; but many of the white blood cells force their way, 

 amoebalike, between the endothelial cells composing these walls and 

 enter the lymph spaces. Although the tissue fluid at first probably differs 

 little from blood plasma, it is soon modified by the activities of the cells 

 with which it is in contact ; they take from the lymph the food substances 

 that they need, and discharge into it their waste products. Being con- 

 stantly bathed by the tissue fluid, the cells of the body may truthfully 

 be said to be aquatic, the tissue fluid constituting their environment, or 

 internal medium. 



It is evident that there must be some means of preventing stagnation 

 of the lymph in the tissue spaces — of draining off the "used" lymph, 

 waste-charged and low in food, and of replacing it with fresh lymph. 



Fig. 6.4. Diagram of the pressure relations concerned in loss and recovery of fluid by the 

 capillaries. A is a small artery, V, a small vein. At X the hydrostatic blood pressure in the 

 capillaries exceeds the combined opposing effects of the osmotic blood pressure and the out- 

 side lymph pressure, and fluid leaves the capillaries. At Y the pressure relations are reversed, 

 and fluid reenters the capillaries. 



There are two mechanisms by which this is accomplished — the lymphatic 

 system, discussed below, and the blood capillaries themselves. It may 

 seem paradoxical that the same capillaries that lose fluid to the tissue 

 spaces to form lymph should also absorb "used" lymph back into the 

 blood stream, but this can be understood in the light of the following con- 

 siderations (see Fig. 6.4). 



The chief factor in forcing the fluid of the blood into the tissue spaces 

 is the hydrostatic blood pressure in the capillaries, produced by the beat- 

 ing of the heart. Were this not opposed by the pressure of the lymph 

 already in the tissue spaces and by the osmotic pressure of the blood 

 caused by its contained colloidal proteins, most or all of the blood fluid 

 would be forced out through the walls of the capillaries. As it is, wher- 

 ever the hydrostatic blood pressure exceeds the combined effects of the 

 osmotic and lymph pressures, fluid does escape from the capillaries; 

 where the forces are balanced, there is no loss or gain of fluid; and where 

 the osmotic pressure plus the lymph pressure exceeds the hydrostatic 

 blood pressure, fluid enters the capillaries. Hydrostatic blood pressure 



