DIFFUSION AND OSMOSIS. 69 



the end have an important bearing upon the explanation of the nutritive exchanges 

 between the blood and tissues. Some additional facts may be mentioned to indicate 

 the applications that are made of these processes in explaining physiological phenomena. 



Osmotic Pressure of Proteids. — The osmotic pressure exerted by crystalloids, such as 

 the ordinary soluble salts, is, as we have seen, very considerable, but the ready diffusi- 

 bility of most of these salts through animal membranes limits very materially their influ- 

 ence 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 tis- 

 sues, and to the degree that this occurred its effect in diluting the blood would tend to dimin- 

 ish because the part of the salt that got into the extra-vascular lymph-spaces would now 

 exert an osmotic pressure 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 removed by 

 the excreting organs, gives to such substances a smaller influence in directing the water 

 stream than would at first be supposed when the intensity of their osmotic action is con- 

 sidered. In addition to the crystalloids the liquids of our bodies contain also a certaiu 

 amount of proteid, the blood, especially, containing over 6 per cent, of this substance. It 

 has been generally assumed that proteids in solution exert little or no osmotic pressure, 

 but Starling 1 and others have claimed, on the contrary, that proteids in solution exert a 

 distinct although small osmotic pressure, and it is probable 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. Accord- 

 ing to Starling, the osmotic pressure exerted by the proteids of serum is equal to about 

 30 mm. of mercury. That the osmotic pressure of the serum proteids is so small is not 

 surprising 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 molecules 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 solu- 

 tion. By means of this weak but constant osmotic pressure of the indiffusible proteid 

 it is possible to explain the fact that an isotonic or even a hypertonic solution of diffus- 

 ible crystalloid may be completely absorbed by the blood from the peritoneal cavity. 



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 hyperosmotic 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 ofless, and this may bap- 

 pen quite independently of the direction of the osmotic stream of water. I f. for instance, a 

 0.9 per cent, solution of sodium chloride is injected into the peritoneal cavity, it will enter 

 into diffusion relations with the blood in the blood-vessels; its concentration in sodium 

 chloride 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 



1 Journal of Physiology, L899, vol. 24, |>. .".IT. 



