274 THE CIRCULATION IN THE BLOOD-VESSELS [oil. XXI. 



actual experiment. They teach us that the highest pressure reached 

 during systole may be twice or thrice the lowest attained during 

 diastole. 



FIG. 27(5. Normal arterial tracing obtained with Pick's Kymograph in the dog. 

 (Burdon-Sanderson . ) 



The following table gives the probable average height of blood- 

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

 been very largely inferred from experiments on animals : 



T / ..j\ f + HO mm. (about 6 inches") 



Large arteries (e.g. carotid) 



mercury. 



Medium arteries (e.g. radial) . +110 mm. mercury. 

 Capillaries . . . + 15 to + 20 

 Small veins of arm . . + 9 



Portal vein . . . . + 10 

 Inferior vena cava . . + 3 ,, 



Large veins of neck from to - 8 



(Starling.) 



These pressures are, however, subject to considerable variations ; 

 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 contraction of the arterioles. 



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



after transfusion). 

 Decrease in the arterial blood-pressure is produced by 



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



2. Decrease in the contraction of the arterioles. 



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



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 directly with the volume of the blood ; in 

 the arteries the effect of increase of fluid is slight and temporary, 

 owing to the rapid adaptability of the peripheral resistance; the 

 excess of fluid collects in and distends the easily dilatable venous 

 reservoir. With regard to the first and second factors in the foregoing 

 table, venous pressure varies in the opposite way to arterial pressure. 



