PROTEINS AND THEIR CLASSIFICATION. 1037 



and a hypotonic or hyposmotic solution is one whose osmotic pressure is 

 less than that of serum. 



Diffusion, or Dialysis, of Soluble Constituents. If two liquids of 

 unequal concentration in a given constituent are separated by a membrane 

 entirely permeable to the dissolved molecules of the substance, a greater 

 number of these molecules will pass over from the mo^e concentrated to the 

 less concentrated side, and in time the composition will be the same on the 

 two sides of the membrane. Diffusion of soluble constituents continually 

 takes place, therefore, from the points of greater concentration to those of 

 less, and this may happen quite independently of the direction of the osmotic 

 stream of water. If, for instance, a 0.9 per cent, solution of sodium chlorid 

 is injected into the peritoneal cavity, it will enter into diffusion relations 

 with the blood in the blood-vessels; its concentration in sodium chlorid 

 being greater than that of the blood, the excess will tend to pass into the 

 blood, while sodium carbonate, urea, sugar, and other soluble crystalloidal 

 substances will pass from the blood into the salt solution in the peritoneal 

 cavity. Through the action of this process of diffusion we can understand 

 how certain constituents of the blood may pass to the tissues of various glands 

 in amounts greater than can be explained if we supposed that the lymph 

 of these tissues is derived solely by nitration from the blood-plasma. An- 

 other important conception in this connection is the possibility that the 

 capillary walls may be permeable in different degrees to the various soluble 

 constituents of the blood, and furthermore the possibility that the permea- 

 bility of the capillary walls may vary in different organs. With regard to 

 the first possibility it has been shown that the blood capillaries are more 

 permeable to the urea molecules than to sugar or Nad. With the aid of 

 these facts it is possible to explain in large measure the transportation of 

 material from the blood to the tissues, and vice versa. For example, to follow 

 a line of reasoning used by Roth, we may suppose that the functional activity 

 of the tissue elements is attended by a consumption of material which in 

 turn is made good by the dissolved molecules in the tissue lymph. The 

 concentration of the latter is thereby lowered, and in consequence a diffu- 

 sion stream of these substances is set up with the more concentrated blood. 

 In this way, by diffusion, a constant supply of dissolved material is kept 

 in motion from the blood to the tissue elements. On the other hand, the 

 functional activity of the tissue elements is accompanied by a breaking down 

 of the complex protein molecule, with the formation of simpler, more stable 

 molecules of crystalloid character, such as the sulphates, phosphates, and 

 urea or some precursor of urea. As these bodies pass into the tissue lymph 

 they tend to increase its concentration, and thus by the greater osmotic 

 pressure developed they serve- to attract water from the blood to the lymph, 

 forming one efficient factor in the production of lymph. On the other hand, 

 as these substances 'accumulate in the lymph to a concentration greater than 

 that possessed by the same substances in the blood, they will diffuse toward 

 the blood. By this means the waste products of activity are drawn off to 

 the blood, from which, in turn, they are removed by the action of the excretory 

 organs. 



Diffusion of Proteins. This simple explanation on purely physical 

 grounds of the flow of material between the blood and the tissues can only 

 be applied, however, at present to the diffusible crystalloids, such as the 

 salts, urea, and sugar. The proteins of the blood, which are supposed to 

 be so important for the nutrition of the tissues, are practically indiffusible 

 through animal membranes, so far as we know. It is difficult to explain their 

 passage from the blood through the capillary walls into the lymph. Pro- 

 visionally it may be assumed that this passage is due to filtration. The blood- 

 plasma in the capillaries is under a slightly higher pressure than the lymph of 

 the tissues, and this higher pressure tends to squeeze the blood constituents, 

 including the protein, through the capillary walls. So far as the protein nutri- 

 tion of the tissues is concerned, the difficulty in regard to the indiffusibility of the 

 proteins is met by the modern conception that the digested protein of the food 

 circulates in the blood as amino-acids, and is supplied to the tissues in this 

 form. The amino-acids have much simpler molecules and may pass through 

 the capillary walls by diffusion. 



