BLOOD PRESSURE 357 



remain constant, their influence upon the cardiac energy may be ma- 

 terially modified by the changes in the other three factors. It would 

 lead altogether too far to give a complete analysis of these interactions 

 and hence, it must suffice to illustrate them with the help of a single 

 example, namely the relationship existing between the energy of the 

 heart and the peripheral resistance. It should be stated first of all 

 that the peripheral resistance may be increased or decreased. The 

 former change gives rise to a lessened escape of arterial blood into 

 the capillaries, and the latter to a more copious arterial offlow. Sup- 

 posing now that the cardiac energy is agumented, we would expect to 

 obtain a rise in the arterial blood pressure. This result, however, may 

 be nullified by a vasodilatation, i.e., by a diminution of the peripheral 

 resistance and a greater offlow of the arterial blood. In a similar 

 manner, it may be reasoned that a lessened ventricular discharge 

 must lead to a fall in blood pressure. But this effect is not always 

 obtained, because the diminution in the cardiac output may be com- 

 pensated for by an increase in the peripheral resistance occasioned 

 by a vasoconstriction. The simultaneous appearance of an increased 

 cardiac energy and peripheral resistance would, of course, raise the 

 blood pressure. The opposite result would be obtained after a simul- 

 taneous depression of these two factors. 



The Total Quantity of the Circulating Blood. This factor bears 

 a direct relationship to the blood pressure, because different degrees 

 of pressure may be established very readily by simply varying the 

 volume of the blood, provided, of course, that the other three factors 

 remain unchanged. Conditions of this kind invariably result in the 

 course of hemorrhages, and during the infusion of isotonic solutions and 

 the transfusion of blood. Under normal conditions, the vascular sys- 

 tem possesses the power of adapting itself very quickly to different 

 quantities of blood by (a) varying the size of the bloodbed, (6) forcing 

 the fluid elements of the blood into the lymphatic channels, and (c) 

 transferring the lymph into the bloodstream. Thus, slight losses of 

 blood are quickly compensated for by a vasoconstrictor reaction and a 

 transfer of lymph into the vascular channels. For this reason, a de- 

 cided fall in blood pressure cannot develop under these circumstances, 

 unless the hemorrhage has been sufficiently severe to offset this com- 

 pensation. A similar reaction takes place whenever the amount of the 

 circulating blood is increased. The blood-vessels then relax, and a 

 certain portion of the blood seeks the lymph spaces. 1 These changes 

 are often followed by an extra discharge of water from the body in the 

 excretions. Jt is true, however, that any extraordinary increase in 

 the amount of the circulating blood gives rise to a more decided and 

 more permanent rise in the pressure. It need scarcely be emphasized 

 that these alterations frequently assume a local character and remain 



1 Worm-Muller (Ber. der. sachs. Gesellsch. der Wissensch., 1873), Stolnikow 

 (Arch, fur Anat. und Physiol., 1886), and Johansson and Tigerstedt (Skand. 

 Arch, fur Physiol., ii, 1889). 



