220 



CIRCULATION OF THE BLOOD 



Whenever, other things being equal, an artery is dilated, the lateral pressure 

 in this artery may fall, but a larger amount of blood flows into the capillaries 

 and as a result the pressure in them increases. 



All the complicated mechanisms which help to regulate the blood pressure 

 have for their immediate purpose the maintenance of a normal pressure in 



the aorta, in order that the blood may flow under 

 normal pressure through the capillaries. 



The total quantity of blood in the body is by 

 no means sufficient to supply all the capillaries at 

 once with as much blood as the organs require 

 at their maximum activity. Fortunately all the 

 organs are never at their maximum at the same 

 time, so that their requirements vary. In fact the 

 quantity of blood flowing through the capillaries 

 varies incessantly. An organ upon which devolves an extra quantity of work 

 receives for the time a greater supply of blood than if it were relatively in- 

 active. The arteries belonging to the organ dilate, while the arteries which 

 convey blood to the other organs at the same time constrict. By this means 

 a fall in the aortic pressure is prevented and the blood flows more copiously to 

 the capillary system whose arteries have been enlarged. Pressure and velocity 

 in this region increase together. 



The length of the capillaries is given as 0.4-0.7 mm. (in the liver as much 

 as 1.1) ; their diameter is about 0.009 mm. 



The capillary wall is composed of flattened cells which fit together by their 

 edges. That they are capable of constricting in many places was first demon- 

 strated by Strieker. Rouget and S. Mayer then showed that this constriction is 



FIG. 89. Branched contract- 

 ile cells embracing wall of 

 a capillary vessel in the 

 hyaloid membrane of the 

 frog's eye, after Rouget. 



FIG. 90. A, capillary when not stimulated; B, the same capillary stimulated. The lumen 

 is entirely obliterated, after Steinach. 



brought about by contractile elements situated outside the basement membrane 

 and entirely distinct from it. The nuclei of these cells are arranged parallel to 

 the long axis of the vessel and their cell substance is often divided into little 

 strands which run out at right angles to the nucleus and embrace the capillary 

 vessel like the hoops of a barrel (Fig. 89). The contraction of these elements 

 may entirely obliterate the lumen of the vessel; at the same time fine longi- 



