80 THE CIRCULATION OF THE BLOOD 



plate are made air-tight by greasing. The bag is con- 

 nected with a pump and manometer. At a certain pres- 

 sure the blue colour of the vein disappears. A similar 

 apparatus of smaller size is used for subcutaneous capil- 

 laries. This method does not give accurate results, since 

 the resistance of the skin is unknown. The same objection 

 apphes to von Kries's method for measuring capillary 

 pressure. In this method a glass plate of a certain area 

 is pressed upon the skin and weighted until the skin is 

 blanched. On dividing the weight by the area of the 

 plate the pressure upon unit area of skin is obtained. 



The Regulation of Blood-pressure 



In young adults the systohc pressure in the brachial 

 artery is about 110 mm. Hg., the diastolic 70, giving a 

 mean pressure of 90. In the horizontal position the blood- 

 press vire is almost uniform in large and small arteries. In 

 the arterioles the blood meets with considerable resistance 

 owing to the narrow cahbre of the vessels. The consequence 

 is that between the small arteries and the capillaries there 

 is a considerable drop in pressure, from 90 in the former, 

 to anything between 40 and 15 in the latter. The pressure 

 in the veins is lower again than that in the capillaries. It 

 varies between 10 and mm. Hg., and in the great veins 

 entering the heart may even have a negative sign. It 

 will be seen that as the blood flows through the systemic 

 circulation the pressure which it exerts upon the vessel 

 walls does not fall uniformly. The greatest resistance to 

 the flow of blood is met at the junction of the arterioles 

 with the capillaries. In overcoming this resistance the 

 blood falls from a region of high pressure in the arteries to 

 a region of low pressure in the capillaries and veins. On 

 this account we may regard the arterial system as a kind of 

 reservoir. The purpose which such a reservoir serves will 

 become clear when we consider under what conditions and 

 by what mechanism the pressure of blood within it is liable 

 to alteration. For the moment it will suffice to point out 



