134 REPORTS ON THE STATE OF SCIENCE.—1918. 
osmotic pressure. I showed (1911), by determinations of the osmotic 
pressure of congo-red solutions by a vapour pressure method, that 
the values were the same as those given by the parchment paper 
membrane. Therefore, all the elements present in the solution, 
including the sodium ions, gave their proper contribution to the 
osmotic pressure measured by the osmometer method; otherwise, the 
vapour pressure measurements would have been much higher than 
those with the latter method. Theoretical considerations and cal- 
culations based on them (Bayliss, ‘General Physiology,” p. 649) 
confirm the fact. Owing to the osmotic activity of all the ions 
formed by an electrolytically dissociated colloid comparatively high 
osmotic pressures may be manifested. This is the cause of the 
large rise in osmotic pressure produced by the ‘addition of sodium 
hydroxide to a protein solution. 
In order that we may realize how the osmotic pressure of colloids 
is of importance in problems of the circulation of the blood it is , 
necessary to remember that the pressure in the arteries is something 
over 100 mm. of mercury, falling regularly to about 8 mm. in the 
capillaries, and to zero in the small veins. Since the osmotic pressure 
of the colloids in the blood is only 40 mm. of mercury, and the walls 
of the blood vessels are freely permeable to water and crystalloids, it 
is clear that the osmotic pressure of the colloids, which tends to draw 
water in, is overpowered as far as the commencement of the 
capillaries. Thus a filtration outwards of blood, minus its colloids, 
takes piace in that part of the vascular system in which the pressure 
exceeds 40 mm. The same process occurs ina part of the kidney, 
resulting in the production of what is a very dilute urine, being the 
first stage in the complete process. The blood then is continually 
losing liquid to the tissues up to a certain region in its course. But, 
as we follow the gradual fall in the blood pressure along the 
capillaries, we come to a point where the osmotic pressure of the 
colloids, which has risen somewhat owing to the loss of water, is 
higher than the blood pressure. From this point onwards water is 
taken in again from the tissue spaces by osmosis. This latter process, 
bowever, does not usually suffice to balance the loss completely, and 
the difference is carried away by the lymphatic channels, and finally 
returned to the blood by the thoracic duct. Consider next what 
will happen when a dilute salt solution is introduced into the veins 
in order to replace blood which has been lost by escape from injured 
blood vessels for example. It is clear that the concentration of 
colloids in the blood is lowered, and therefore their osmotic pressure. 
The result is that more rapid loss of liquid by filtration occurs, while 
the region travelled before the blood pressure falls sufficiently to 
permit osmotic inflow is lengthened, leaving a less distance in which 
reabsorption takes place. The net effect is that much more liquid 
escapes to the tissues, while the blood quickly loses that which has 
been putin. In practice this is found to be the case. Simple saline 
solutions are useless. The present writer has shown (1916), however, 
that if a colloid, such as gelatin, or better, gum acacia, be added in 
such amount to the solution injected as to raise its colloidal osmotic 
pressure to that of the blood, then it remains in the .blood vessels 
raising the blood pressure and forming an effective substitute for the 
