BLOOD PRESSURE 139 



and immediately after a hemorrhage. The specific gravity of the post- 

 hemorrhagic blood is also decidedly below normal, indicating that the 

 diluting fluid contains a lower concentration of dissolved substances than 

 the blood plasma. The dilution of the blood is indeed often so great that 

 hemolysis occurs, the plasma being distinctly tinted red. 



Hemorrhage also slightly raises the hydrogen-ion concentration of the 

 blood plasma, and diminishes the store of reserve alkali, so that the ad- 

 dition of a certain amount of acid to the blood (e.g., carbon dioxide) 

 causes a greater rise in the hydrogen-ion concentration. 



The deficiency in the blood elements produced by the dilution is recti- 

 fied by the manufacture of new corpuscles in the bone marrow, etc., but 

 this process in a liberally fed animal takes several days for accomplish- 

 ment, and while it is going on microscopic examination of the blood will 

 reveal the presence of immature corpuscles. 



Careful studies of blood regeneration following the removal on two 

 successive days, of 25 per cent of the blood, have shown that even in 

 starving animals the total amount of hemoglobin (percentage of hemo- 

 globin multiplied by the volume of blood) slowly recovers (Whipple 

 and Hooper). Recovery is greatly hastened by feeding with flesh or 

 even with gelatin. Removal of the spleen or the establishment of a bili- 

 ary fistula does not interfere with the recovery. 



Incidentally it will be advantageous to consider here the effects of 

 transfusion. These are very different according to the nature of the fluid 

 used for transfusion. Three transfusion fluids have been investigated: 

 (1) blood itself, (2) physiological saline solution (see page 95), and (3) 

 physiological saline solution containing viscid substances such as gelatin. 

 The effects are also very different according to whether the solutions are 

 injected into animals with normal blood pressure or into those Avhose 

 blood pressure has been lowered by preceding hemorrhage. 



When blood is injected into animals with normal blood pressure, it 

 will very soon cause the pressure to rise, and as the injection is main- 

 tained the rise may continue until the pressure is perhaps 50 per cent 

 or more above its normal level. If the injection is long continued, how- 

 ever, a sudden fall of pressure occurs, on account of engorgement of the 

 right side of the heart. If the injection is not pushed so far, the increased 

 blood pressure after being maintained for a short time returns to its old 

 level. 



Injection of saline into a normal animal, if made slowly, has no effect 

 at all on the blood pressure; if more rapidly injected, the pressure will 

 rise slightly, but to a much less extent than that observed when blood 

 itself is injected. Much larger quantities of the saline than of the blood 

 can be tolerated before cardiac embarrassment ensues. After the dis- 



