362 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



may also be concluded that they are the chief factor regulating the 

 peripheral resistance. 



Reference has repeatedly been made to the close functional relation- 

 ship existing between the peripheral resistance and the energy of the 

 heart. Thus, it has been said that a high blood pressure resulting from 

 vasoconstriction, is commonly associated with a decrease in the fre- 

 quency of the heart, and vice versa. Although not wishing to over- 

 emphasize this reflex compensation, the foregoing facts will go far to 

 show that the blood pressure is more closely dependent upon the inter- 

 action of the two factors just mentioned than upon the quantity of the 

 circulating blood or the elasticity of the blood-vessels. No doubt, the 

 former are subject to more frequent changes than the latter, i.e., under 

 normal conditions the quantity of the blood and the elasticity remain 

 the same for much longer periods of time. 



THE DIRECT AND INDIRECT METHODS OF RECORDING BLOOD 



PRESSURE 



Methods for Determining the Arterial Blood Pressure. — The pro- 

 cedures employed to ascertain the pressures in the different parts of the 



Fig. 188. — Diagram Illustrating the Indirect Method of Measuring Blood- 

 pressure. 

 A, arm surrounded by a flat rubber pouch, R\ by means of a rubber bulb, B, a pressure 

 is set up in this system of tubing sufficient to compress the artery. This moment is 

 indicated by the manometer (M). 



vascular system, differ somewhat in accordance with the nature o 

 the blood-vessel. If the direct method is resorted to, the vascular chan- 

 nel is opened and the blood brought into immediate contact with 

 the recording instrument. If, on the other hand, the indirect method 

 is employed, the blood-vessel is left intact, while the pressure existing 

 therein is accurately balanced by a known pressure set up in an arti- 

 ficial system immediately adjoining it (Fig. 188). Obviously, there- 

 fore, the direct procedure is applicable only to animals and to blood- 

 vessels of larger caliber, whereas the indirect or bloodless method 

 may be practised upon animals as well as upon man. 



The first attempt to ascertain the pressure of the blood, was made 

 in 1732 by the Rev. Stephen Hales, ^ an English clergyman. A long 



1 Statical Essays, 1733. 



