208 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



Osmotic Pressure of Proteids. — It has been generally assumed that proteids, 

 the most abundant and important constituents of the blood, exert little or no 

 osmotic pressure. Starling, however, has claimed that they have a small 

 osmotic pressure ; if this is so, it is of importance, for proteids, unlike salt, do 

 not diffuse readily, and their effect therefore remains as an almost permanent 

 factor in the blood. Starling gives the osmotic pressure of the proteids of 

 the blood-plasma as equal to 30 mm. of mercury. By others this is attributed 

 to the inorganic salts with which proteids are always closely associated. 

 Moore, for instance, finds that the purer a proteid is, the less is its osmotic 

 pressure ; the same is true for other colloidal substances. It really does not 

 matter much, if the osmotic force exists, whether it is due to the proteid 

 itself, or to the saline constituents which are almost an integral part of a 

 proteid. It is merely interesting from the theoretical point of view. We 

 should from the theoretical standpoint find it difl&cult to imagine that a pure 

 proteid can exert more than a minimal osmotic pressure. It is made up of 

 such huge molecules that, even when the proteids are present to the extent 

 of 7 or 8 per cent., as they are in blood-plasma, there are comparatively few 

 proteid molecules in solution. Still, by means of this weak but constant 

 pressure it is possible to explain the fact that an isotonic or even a hypertonic 

 ablution of a diffusible crystalloid may be completely absorbed from the 

 peritoneal cavity into the blood. 



The functional activity of the tissue elements is accompanied by the 

 breaking down of their proteid constituents into such simple materials as urea 

 (and its precursors), sulphates, and phosphates. These materials pass into 

 the lymph, and increase its molecular concentration and its osmotic pressure ; 

 thus water is attracted (to use the older way of putting it) from the blood to 

 the lymph, and so the volume of the lymph rises and its flow increases. On 

 the other hand, as these substances accumulate in the lymph they will in 

 time attain there a greater concentration than in the blood, and so they wiU 

 diffuse towards the blood, by which they are carried to the organs of 

 excretion. 



But, again, we have a difficulty with the proteids ; they are most important 

 for the nutrition of the tissues, but they are practically indiffusible. We 

 must provisionally assume that their presence in the lymph is due to filtration 

 from the blood. The plasma in the capillaries is under a somewhat higher 

 pressure than the lymph in the tissues, and this tends to squeeze he 

 constituents of the blood, including the proteids, through the capillary walls. 

 I have, however, already indicated that the question of lymph -formation is 

 one of the many physiological problems which await solution by the 

 physiologists of the futmre. 



The whole subject of osmosis and its applications to physiology is attracting 

 considerable attention at the present time. I have not attempted to discuss 

 the question at all fully, but merely to indicate the lines research has followed 

 and will have to follow in the future. In the preparation of this short sketch 

 I have had of necessity to consult much of the current literature, particularly 

 the writings of AVaymouth Reid and Starling. I am also indebted for much 

 help to the very excellent article on the subject in Howell's ' Text-Book of 

 Physiology,' and Dr. Koeppe's ' Physikalische Chemie.' 



