1036 APPENDIX. 



gram-molecular solution of a non-electrolyte is known to lower the freezing 

 point 1.87 C. Thus, if blood-serum gives A = 0.56 C., its concentration in 



terms of a gram-molecular solution will be ^p or 0.3. In other words, 



blood-serum has 0.3 of the osmotic pressure exerted by a gram-molecular 

 solution of a non-electrolyte, that is, 22.32 X 0.3, or 6.696 atmospheres. 



Remarks upon the Application of the Foregoing Facts in Physiol- 

 ogy. In the body water and substances in solution are continually pass- 

 ing through membranes, for example, in the production of lymph, in the 

 absorption of water and digested foodstuffs from the alimentary canal, in 

 the nutritive exchanges between the tissue elements and the blood or lymph, 

 in the production of the various secretions, and so on. In these cases it is a 

 matter of the greatest difficulty to give a satisfactory explanation of the 

 forces controlling the flow to and fro of the water and dissolved substances, 

 but there can be little doubt that in all of them the physical forces of nitration 

 and osmotic pressure take an important part. Whatever can be learned, 

 therefore, concerning these processes must in 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 appli- 

 cations that are made of these processes in explaining physiological phenomena. 



Osmotic Pressure of Proteins. The osmotic pressure exerted by crys- 

 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 protein, the blood, 

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

 erally assumed that proteins 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 proteins 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 proteins of serum is equal to about 30 mms. of mercury. That 

 the osmotic pressure of the serum proteins is so small is not surprising if we 

 remember the very high "molecular weight of this substance. In serum the 

 proteins are present in a concentration of about 7 per cent., but owing to 

 their large molecular weight comparatively few protein molecules are present 

 in a solution of this concentration ; and, assuming that the dissolved protein 

 follows the laws discovered for crystalloids, its osmotic pressure would depend 

 upon the number of molecules in solution. By means of this weak but con- 

 stant osmotic pressure of the indiffusible protein it is possible to explain 

 theoretically the fact that an isotonic or even a hypertonic solution of diffusi- 

 ble 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 hy- 

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



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



