BLOOD PRESSURE 361 



this extra amount of blood must become the less, the higher the pressure 

 already established. Concurrently, it may be reasoned that the energy 

 of the heart may be most seriously impaired by forcing it to increase 

 its activity at a time when the tension in the vascular system is high, 

 because the vascular channels cannot then yield so readily to the internal 

 pressure. The veins attain their maximal cubic distention at much 

 lower pressures than the arteries, and their extensibility is much less. 

 They are more easily torn when manipulated, but are more yielding 

 than the arteries. This may well be so, because the pressures which 

 they are called upon to withstand, scarcely exceed 20 mm. Hg even 

 under pathological conditions. 



The Peripheral Resistance. This factor serves as an expression 

 of the size of the "blood-gate" at the arteriocapillary junction. It 

 may be inferred that the resistance placed in the path of the arterial 

 blood, must become the less the larger this orifice. The friction which 

 is responsible for the production of this resistance, is composed in 

 reality of two types of frictions which may be designated respectively 

 as the "external" and the "internal." The former is produced by the 

 blood as a whole as it rubs against the internal surface of the vessel 

 wall and the latter, by the bumping together of the different con- 

 stituents of the blood. The term viscosity is usually applied to this 

 intermolecular friction. It is evident that the hindrance placed in 

 the path of the arterial blood, must increase whenever the "blood- 

 gate" is made smaller and decrease whenever it is made larger. In 

 the first instance, the arterial influx into the capillaries is diminished, 

 and in the second increased. Supposing, therefore, that the other 

 three factors remain the same, the first change must lead to a rise and 

 the second, to a fall in the arterial pressure. 



Special emphasis has been placed upon the conditions existing at 

 the arteriocapillary junction, because the distalmost branches of the 

 arterial system are equipped with especially powerful rings of smooth 

 muscle cells, which enable them to influence the blood stream most 

 decisively. This statement, however, is not meant to imply that the 

 peripheral resistance is formed in the arterioles and not in the capil- 

 laries. A deduction of this kind could not possibly be correct, because 

 it is a well-known fact that no segment of the vascular system pro- 

 duces a greater amount of friction than the capillaries. This must be 

 so, because the column of blood is divided by them into the finest 

 possible streams, many of which are no broader than the diameter of 

 a single red cell. Although generators of the peripheral resistance, it is 

 evident that the capillaries as such are quite unable to vary this 

 resistance, because they are not in possession of an active means for 

 influencing the blood-stream. This function is relegated to the arter- 

 ioles which, as we have j ust seen, act as powerful sphincters, permitting 

 larger and smaller quantities of arterial blood to escape. Conse- 

 quently, the state of filling of the capillaries is determined very largely 

 by the arterioles. In view of their decided vasomotor qualities, it 



