DIFFUSION AND OSMOSIS. 885 



talloids, such as the ordinary soluble salts, is, as we have seen, very con- 

 siderable, but the ready diffusibility of most of these salts through animal 

 membranes limits very materially their influence upon the flow of water in 

 the body. Thus, if we should inject a strong solution of common salt directly 

 into the blood-vessels, the first effect would be the setting up of an osmotic 

 stream from the tissues to the blood and the production of a condition of 

 hydremic plethora within the blood-vessels. The salt, however, would soon 

 diffuse out into the tissues, and to the degree that this occurred its effect in 

 diluting the blood would tend to diminish because the part of the salt that 

 got into the extravascular lymph spaces would now exert an osmotic press- 

 ure in the opposite direction, drawing water from the blood. This fact, 

 together with the further fact that an excess of salts in the body is soon re- 

 moved by the excretory organs, gives to such substances a smaller influence 

 in directing the water stream than would at first be supposed when the inten- 

 sity of their osmotic action is considered. In addition to the crystalloids the 

 liquids of our bodies contain also a certain amount of proteid, the blood, 

 especially, containing over 6 per cent, of this substance. It has been gen- 

 erally assumed that proteids in solution exert little or no osmotic pressure, 

 but Starling* and others have claimed, on the contrary, that they exert a 

 distinct, although small, osmotic pressure, and it is possible that this fact 

 is of special importance in absorption, because the proteids do not diffuse 

 or diffuse with great difficulty, and their effect remains, therefore, so to 

 speak, as a permanent factor. According to Starling, the osmotic pressure 

 exerted by the proteids of serum is equal to about 30 mms. of mercury. 

 That the osmotic pressure of the serum proteids is so small is not sur- 

 prising if we remember the very high molecular weight of this substance. 

 In serum the proteids are present in a concentration of about 7 per cent., 

 but owing to their large molecular weight comparatively few proteid mole- 

 cules are present in a solution of this concentration; and, assuming that 

 the dissolved proteid follows the laws discovered for crystalloids, its osmotic 

 pressure would depend upon the number of molecules in solution. By 

 means of this weak, but constant, osmotic pressure of the indiffusible pro- 

 teid it is possible to explain theoretically the fact that an isotonic or even 

 a hypertonic solution of diffusible crystalloid may be completely absorbed 

 by the blood from the peritoneal cavity. Reidt has given evidence which 

 indicates that pure proteids exert no osmotic pressure; that as they occur 

 in the body liquids they are combined or mixed with certain substances to 

 which the feeble osmotic pressure formerly attributed to the proteids really 

 belongs. Since these unknown substances are themselves indiffusible, the 

 arguments just used still hold for the conditions in the body. 



Isotonic, Hypertonic, and Hypotonic Solutions. In physiology the 

 osmotic pressures exerted by various solutions are compared usually with 

 that of the blood-serum. In this sense an isotonic or isosmotic solution is 

 one having an osmotic pressure equal to that of serum, a hypertonic or hy- 

 perosmotic solution is one whose osmotic pressure exceeds that of serum, 

 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 more 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 



* "Journal of Physiology," 24, 317, 1899. 

 t Reid, "Journal of Physiology," 1905. 



