CHAPTER XVI 



THE FACTORS CONCERNED IN MAINTAINING THE 

 BLOOD PRESSURE 



Having become familiar with the principles of the methods by which 

 blood-pressure measurements are made, the next problem is to examine 

 into the causes which operate to maintain the pressure. Two of these 

 causes may be considered as fundamental, since without them no such 

 pressure could exist. These are: (1) the pumping action of the heart, 

 and (2) the peripheral resistance that is, the resistance to outflow of 

 blood from the ends of the arterial system. Less essential though im- 

 portant factors are: (3) the volume of blood in the blood vessels, (4) 

 the viscidity or viscosity of the blood, and (5) the elasticity of the 

 walls of the vessels. We shall now proceed to examine the experimental 

 evidence which indicates the relative importance of each of these factors. 



1. The Pumping' Action of the Heart 



Changes produced in the mean arterial blood pressure by alteration 

 in the pumping action of the heart are most strikingly demonstrated by 

 observing this pressure after cutting or during stimulation of the vagus 

 nerves. As will be explained later (page 217), impulses- conveyed 

 through these nerves to the heart make the beats slower and weaker. 

 These impulses are constantly acting in the heart, so that when both 

 vagus nerves are cut, the beats become more frequent and stronger, 

 with the result that the mean arterial pressure rises considerably. A 

 lesser degree of this effect can usually be obtained by cutting the vagus 

 nerve on one side (Fig. 27). If now the peripheral end of a cut vagus 

 nerve is stimulated, as by applying an electric current to it, the heart will 

 either stop beating altogether or become very much slowed, with the result 

 that the mean arterial blood pressure will fall, in the former case almost to 

 zero and in the latter, to a level Corresponding to the degree of slowing 

 of the heart (Fig. 28). 



2. The Peripheral Resistance 



To demonstrate the influence of peripheral resistance on mean arte- 

 rial blood pressure, the most striking experiment is performed by cut- 

 ting or stimulating the great splanchnic nerve. In this nerve impulses, 



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