CH. XXII.] VARIATIONS OF BLOOD-PRESSURE 277 



manometer the inertia is so great that it cannot respond to the very 

 rapid variations in pressure which occur within an artery during each 

 cardiac cycle. If Fick's or Hiirthle's manometer is employed, and 

 the surface travels sufficiently fast, these can be recorded (see fig. 

 245). These instruments, though useful for recording the complete 



FIG. 245. Normal arterial tracing obtained with Fick's Kymograph in the dog. 

 (Burdon-Sanderson . ) 



changes in pressure, require calibration : that is, the extent of move- 

 ment that corresponds to known pressures must be ascertained by 

 actual experiment. They teach us that the highest pressure reached 

 during systole may be twice or thrice the lowest attained during 

 diastole. . 



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 : 



Large arteries (e.g. carotid) ( + 14 mm ' <> bout 6 inches ) 



I 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 



