GENERAL VARIATIONS IN BLOOD PRESSURE 



217 



General Variations in Blood Pressure. The arterial blood pressure 

 may be made to vary by alterations in either of the chief factors upon which 

 the pressure in the vessels depends, but primarily by the cardiac contrac- 

 tions and the peripheral resistance. Thus, increase of blood pressure may 

 be brought about by either, i, a more frequent or more forcible action of 

 the heart, or, 2, by an increase of the peripheral resistance. On the other 

 hand, diminution of the blood pressure may be produced, either by a, a 

 diminished force or frequency of the contractions of the heart, or by b, a 

 diminished peripheral resistance. These different factors, however, al- 

 though varying constantly, are so combined that the general arterial pressure 



FIG. 191. Schema Showing the Relation between Blood Pressure, Velocity of Flow, 

 and Vascular Area, in the Arteries, Capillaries, and Veins. Ordinates represent height 

 of pressure and speed of flow. The abscissa, b-c, represents zero pressure and speed. 

 Space between lines a-b and d-e represents arterial system; between d-e and/-g, capillary 

 system, and between f-g and h-i, the venous system. Line A-B equals pressure; line C-D, 

 speed of flow; and line E-F, vascular area. (Modified from Gad.) 



remains fairly constant. For example, the heart may, by increased force or 

 frequency of its contractions, distinctly increase the blood pressure, but this 

 increased action is almost certainly followed by diminished peripheral re- 

 sistance, and thus the two altered conditions may balance, with the result 

 of bringing back the blood pressure to what it was before the heart began 

 to beat more rapidly or more forcibly. 



It will be clearly seen that the circulation of the blood within the blood 

 vessels must depend upon the diminution of the pressure from the heart 

 to the capillaries, and from the capillaries to the veins, the blood flowing in 

 the direction of least resistance. We shall presently see further that the 

 local flow also depends upon the relations between the heart's action and 

 the peripheral resistance both general and local. 



