142 THE CIRCULATION OP THE BLOOD 



the colloids in recovering the blood pressure, the conditions controlling 

 it reveal the mechanism by which the passage of fluid from the blood 

 vessels into the tissues is prevented when solutions of correct composi- 

 tion are injected. Normally the protein content of the blood plasma is 

 higher than that of the tissue lymph, so that there is a continual attrac- 

 tion of water from the tissues to the blood an attraction Avhich is nor- 

 mally balanced by filtration going in the opposite direction. When the 

 filtration pressure in the blood vessels exceeds the difference existing 

 between the osmotic pressure of their contents and that of the tissue 

 fluids, water will pass into the tissue spaces. When the blood is diluted, 

 as by the injection of saline solution, the osmotic pressure of the colloids 

 in a given volume becomes lowered and, the filtration pressure remaining 

 constant, fluid passes into the tissue spaces. Of course these explanations 

 rest on the assumption that the wall of the blood vessels consists of a 

 membrane which is permeable to crystalloids but impermeable or nearly 

 so to colloids. 



Another important property of the transfused saline solution to con- 

 sider is its hydrogen-ion concentration. This value increases in the blood 

 left in the body after hemorrhage, and injection of sodium chloride solu- 

 tion aggravates the acidosis; addition of NaHC0 3 so as to make a 0.2 

 M solution restores the correct P H , and at the same time restores the 

 lost buffer influence (Milroy 7 .) These observations are of interest in the 

 light of the recent discovery of Cannon that a condition of acidosis, as 

 judged by the C0 2 -combining power of the blood, is present in shock, 

 and that the development of this condition can often be guarded against 

 by bicarbonate injections. 



5. Elasticity of Vessel Walls 



The elasticity of the vessel walls is essential to the maintenance of the 

 diastolic pressure. If the walls presented no elasticity but were rigid, 

 blood pressure would fall to zero between the heartbeats. This fact can 

 very readily be shown by a simple physical model consisting of a pump 

 to represent the heart, connected through a T-piece with two tubes, one 

 of \vhich is elastic, the other rigid. The free end of each tube is con- 

 tracted to a narrow aperture representing the peripheral resistance, and 

 either tube may be shut off from the pump by means of a stopcock (see 

 Fig. 30). Each tube should also be connected with a mercury manom- 

 eter. If now the stopcocks are arranged so that the fluid passes into 

 the rigid tube while the pump is in action, it will be found that with 

 each stroke of the pump the pressure in the tube rises considerably, but 

 that it falls to zero between the strokes. If now the stopcocks are turned 

 so that the flow is through the elastic tube, the action of the pump being 



