CH. xxi.] ARTERIAL PRESSURE. 



is employed, and the surface travels sufficiently fast, these can be 

 recorded (see fig. 280). 



We may now proceed to give some results. The following 

 table gives the probable average height of blood-pressure in 



Fi. 280. Normal arterial tracing obtained with Pick's Kymograph in the dog. 

 (Burdon -Sanderson. ) 



various parts of the vascular system in man. They have been 

 very largely inferred from experiments on animals : 



f + 140 mm. (about 6 inches) 

 Large arteries (e.g. carotid) . 1 



[ mercury. 



Mc'ilium arteries (e.g. radial) . -(-no mm. mercury. 



Capillaries . . . -f- 15 to -j- 20 ,, 



Small veins of arm . . -(- 9 ,, 



Portal vein . . . . -j- 10 



Inferior vena cuva . . -}- 3 ,, 



Large veins of neck from o to 8 ,, 



(Starling.) 



These pressures are, however, subject to considerable varia- 

 tions ; the principal factors that cause variation are the 

 following : 



Increase of arterial blood-pressure is produced by 



1. Increase in the rate and power of the heart-beat. 



2. Increase in the quantity of blood (plethora, after a meal, 



after transfusion). 



3. Increase in the contraction of the arterioles. 

 Decrease in the arterial blood-pressure is produced by 



1. Decrease in the rate and force of the heart-beat. 



2. Decrease in the quantity of blood (e.g. after haemorrhage). 



3. Decrease in the contraction of the arterioles. 



The above is true for general arterial pressure ; but if we are 

 investigating local arterial pressure in any organ, the increase or 

 decrease in the size of the arterioles of other areas may make its 

 effect felt in the special area under investigation. 



Venous pressure varies in the opposite way to arterial pressure, 

 in so far as the first and third factors are concerned. Like 



u 2 



