SPECIAL PHYSIOLOGY 



V. THE LAWS OF BLOOD PRESSURE DETERMINED FROM AN 

 ARTIFICIAL CIRCULATORY SYSTEM. 



Having tested by experiment some of the laws of governing the 

 flow of liquid through tubes under the influence of intermittent 

 pressure, we come to the point where we may attempt to reproduce 

 experimentally a set of physical conditions so nearly like those which 

 exist in the animal body that we shall be able to draw conclusions from 

 our experiments that shall hold good for the animal circulatory system. 



The last preceding exercise demonstrated (1) that the contin- 

 uous and even flow of liquid through the capillaries is made possible 

 by the elasticity of the arterial walls; (2) that the pulse is caused by 

 a varying pressure within the elastic artery; (3) that the varying 

 pressure is due to the alteration of systole and diastole of the heart; 

 and (4) that the pressure within the arteries is largely influenced by 

 the size of the capillary through which the fluid must pass i. e., by 

 the peripheral resistance. 



Blood pressure is then the product of two factors: Cardiac 

 force X peripheral resistance ( P = H X R) ; but cardiac force 

 is in turn due to the product of two factors: Rate X strength; 

 (H = r X s); therefore: Pressure is the product of heart rate X heart 

 strength X peripheral resistance (P=r X s X R). 



We have here to deal with these three variables. Applying a 

 principle set forth in a previous exercise (to the effect that "when 

 a value which is being tested by experiment is affected by two or 

 more variable factors only one of these must be allowed to vary in 

 any one experiment") one will so arrange his experiment that these 

 three factors of pressure will vary one at a time. 



1. Appliances. An artificial circulatory system may be con- 

 structed as follows: A rubber bulb such as used in the preceding 

 exercise, to which is attached a capacious entrance tube. To the 

 exit tube attach the 100-cm. soft-rubber tube used before. This will 

 serve as the main artery, at the end of which a T-tube may be inserted, 

 one limb passing to the arterial manometer. Beyond the T-tube is 

 another rubber tube leading to a Y-tube. From each limb of the 

 Y-tube lead off a smaller elastic tube, one branch being a small, thin- 

 walled tube supplied with a screw clamp, while the other passes to 

 a large calcium chloride tube which has been filled with sponge to 

 represent the capillary system of minute tubes. (See Fig. 48.) 



After traversing the capillary system the liquid is collected at a Y 

 and returns to the heart through a tube which is nearly twice as 

 large as the artery. In this vena cava is inserted a T-tube, to which 

 is attached the venous manometer. 



Between the Y-tubes the blood may be opposed by high resist- 

 ance or, with the screw clamp open, by the low resistance. 



