278 SCIENCE PROGRESS 



the volume of the blood as far as possible. What is the nature 

 of this fluid ? It was shown by Scott that, although the 

 lymph outside the blood-vessels contains proteins, that which 

 enters the blood is devoid of them, consisting only of a solu- 

 tion of the salts and diffusible substances of small molecular 

 size, such as glucose and amino-acids. In other words, th** 

 normal capillary wall is impermeable to proteins. If, then 

 the proteins of the blood possess an osmotic pressure, this 

 will be manifest in a tendency to absorb water from the out- 

 side. Starling showed that they have such an osmotic pres- 

 sure, and that it reaches the value of about 35 mm. of mercury. 

 This may be regarded as the force tending to draw water into 

 the blood. But on the arterial side of the circulation, the 

 pressure inside the blood-vessels is over 150 mm. of mercury. 

 This is opposed to the osmotic pressure, and the excess pressure 

 causes the filtration of fluid (= lymph) outwards. Following 

 the blood-pressure as it falls in its course from the arterioles 

 to the capillaries, we find that the pressure inside the latter 

 only amounts to some 10 mm. of mercury. This is less than 

 the osmotic pressure of the proteins, and water is absorbed 

 from the outside. The two processes, however, do not com- 

 pletely compensate one another, and a certain quantity of the 

 lymph is drained off by the lymphatic vessels, being ultimately 

 returned to the blood by the thoracic duct. When we inject 

 into the veins a solution containing salts or glucose only, we 

 decrease the osmotic pressure of the proteins by dilution, more 

 pressure is available for filtration, while the place where re- 

 absorption begins is pushed farther towards the veins, so that 

 a diminished area is in activity. The result is that the fluid 

 injected leaves the circulation rapidly, and the blood returns 

 to its diminished volume. This agrees with clinical experi- 

 ence. If, however, we add to our injection fluid a colloid 

 possessing an osmotic pressure of the correct value, there is 

 no tendency to loss by increased filtration, and if the permea- 

 bility of the capillaries is normal the increased blood volume 

 is permanent. The present writer has shown that the addition 

 of 6 or 7 per cent, of gum arabic to the 0*9 per cent, sodium 

 chloride serves this purpose, and such a solution was in 

 extensive use during the later period of the war. 



Incidentally, it may be remarked that the filtration of 

 protein-free liquid from the blood-vessels owing to the pres- 



