THE CIRCULATION OF THE BLOOD 341 



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 in 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; the percentage of the pressure utilized in imparting 



velocity could be determined with the formula , if the actual velocity of the 



blood in the aorta 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 from the distribution of the aortic pressure in the 

 dog in reference to the carotid artery. Thus, if it be assumed that the aver- 



(35) 2 

 age velocity of the blood is 35 cm., the velocity pressure is equal to ---- or 



0.62 centimeters of blood or 0.046 centimeters of mercury, and 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. T 

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

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

 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 

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



