THE REGULATION OF BLOOD-PRESSURE 81 



that the maintenance of the normal blood -pressure is of 

 the greatest importance, and that the body possesses an 

 elaborate mechanism for maintaining a constant blood- 

 pressure in the face of any tendency to disturb it. 



It will be convenient here to consider in a general way 

 the factors upon which arterial blood-pressure depends. 



For a proper understanding of this question it is neces- 

 sary not to lose sight of the fact that the blood is circulating 

 at a considerable rate — that we are dealing with a dynamic 

 and not a static condition. Blood-pressure is caused by 

 the heart-beat, and is supported by the resistance in the 

 arterioles. 



Blood-pressure depends upon four primary factors — 



1. The output of the heart. 



2. The peripheral resistance. 



3. The volume of the circulating blood. 



4. The relative distribution of the blood, at any given 



moment, between the heart, arteries, capillaries 

 and veins. 



1 . The Output of the Heart. — If the peripheral resistance 

 is unaltered the arterial pressure will vary directly with the 

 output of the heart. If the latter is increased the blood- 

 pressure will rise. 



2. The Peripheral Resistance. — This is the resultant of 

 two factors — the viscosity of the blood and the calibre of 

 the arterioles. Of these the latter is the more important. 

 The output of the heart being constant, the blood-pressure 

 varies directly with the resistance. 



3. The Volume of the Circulating Blood. — The pressure 

 will vary with the volume of the blood, provided that the 

 distribution of the blood between the several parts of the 

 circulation is undisturbed. 



4. The Distribution of the Blood. — The capillaries and 

 veins are, as we shall see, capable of considerable alteration 

 in capacity at low pressures. A change in the capacity of 

 the capillaries does not constitute a change in the peri- 



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