342 TEXT-BOOK OF PHYSIOLOGY 



That the high pressure in the arteries is largely due to this physiologic factor 

 is shown by the rapid and pronounced fall of pressure that occurs when this 

 muscle suddenly relaxes as it does when the spinal cord is transversely 

 divided in the cervical region, thus cutting off from the arteriole muscle 

 those nerve influences that largely determine its contraction. Under 

 such- circumstances the pressure hi the dog may fall from approximately 

 140 mm. to 40 mm. of mercury or even less. Stimulation of the distal 

 extremity of the spinal cord will be followed by the temporary contraction of 

 the arteriole muscle and a rise of pressure to its former value. 



The Distribution of the Intra-ventricular Pressure. The pressure 

 developed during the ventricular contraction is thus expended in imparting 

 velocity to the blood and overcoming the cohesion and friction of its mole- 

 cules. The percentage of the pressure utilized in overcoming the resistance 

 could be approximately determined from the pressure in the aorta if this were 

 accurately known; it may be assumed to be about 150 mm Hg.; then the 

 percentage of the pressure utilized in imparting velocity could be deter- 



v 2 

 mined with the formula , if the actual velocity of the blood in the aorta 



2g 



could be experimentally determined. On account of the difficulty in 

 obtaining this latter factor at least, the results must be only approximative. 

 An idea of the ratio between the velocity pressure and the resistance 

 pressure, however, may be obtained if it be assumed that the average 

 velocity of the blood in the aorta is 35 cm.; then the velocity pressure is 



equal to -^ or to the pressure of a column of blood 0.62 cm. high or 

 1960 



to a column of mercury 0.046 cm. high; then if the average aortic pressure 

 is 150 mm. of mercury, the ratio of the velocity pressure to the resistance 

 pressure is as i to 326. 



The Pulse Wave. Inasmuch as the heart's action is intermittent and 

 the walls of the arteries are elastic, there is a temporary increase and decrease 

 of the pressure with each beat of the heart, attended by a corresponding 

 expansion and recoil of the arteries, giving rise to a wave on the surface of 

 the arteries which is propagated with more or less rapidity though with 

 decreasing amplitude from the beginning of the aorta to the capillaries, 

 and causing in each successive section a corresponding expansion and recoil, 

 and which is known as the expansion wave or pulse wave. 



The general statements regarding the phenomena attending the flow of 

 blood through the systemic vascular apparatus contained in the foregoing 

 paragraphs will be further elaborated in the following pages. For special 

 reasons it is convenient to consider the pressure first. 



BLOOD-PRESSURE 



Blood-pressure may be defined as the pressure exerted radially or later- 

 ally by the moving blood-stream against the sides of the vessels. This pres- 

 sure is the result of (i) the driving power of the heart, and (2) of the resist- 

 ance offered to the forward movement of the blood a resistance due to the 

 cohesion and friction of the molecules of the blood, of the blood corpuscles, 

 and the adhesion of the blood to the sides of the blood-vessels. That there is 

 such a pressure within the arteries, capillaries, and veins, different in amount 



