254 PHYSIOLOGY CHAP. 



of sheep, to 4'1 mm. in the brachial vein, and to 114 mm. Hg in 

 the crural vein. 



In the pulmonary circuit the pressure cannot be determined 

 directly without opening the thorax, and giving artificial respira- 

 tion which produces a condition very unlike the normal. Here we 

 can only say that in the left branch of the pulmonary artery 

 Ludwig found a pressure = 29*6 mm. Hg in the dog ; = 17'6 in the 

 cat; =12 mm. Hg in the rabbit. The relation between the 

 pressures in the pulmonary artery and the aorta can be deduced 

 from the highest values which the pressure reaches during systole 

 within the two ventricles. According to Goltz and Gaule the 

 pressure in the pulmonary artery is to that in the aorta as 

 something like 2 : 5. 



Blood pressure may vary very considerably not only in different 

 individuals of the same species, but also in the same individual under 

 different conditions, notwithstanding the regulatory mechanism 

 which tends to keep it constant. These variations depend on 

 those of the three main factors by which pressure itself is normally 

 determined : 



(a) The variations in the energy of the heart, i.e. in the 

 amount of blood driven in the time - unit through the arterial 

 system. 



(6) The variations of resistance encountered by the blood in its 

 passage through the vessels. 



(c) The variations of the total mass of blood contained in the 

 system. 



The energy of the heart, and the work performed in the time- 

 unit, depend both on the frequency of its revolutions, and the 

 mechanical value of each of these. An increase in the frequency 

 of cardiac rhythm can be compensated by a corresponding diminu- 

 tion in the mechanical value of each revolution, and vice versa. 

 This mechanical value is dependent on the degree of systolic 

 evacuation, and the degree of diastolic filling. In brief, whatever 

 the determining conditions, an augmentation or diminution in the 

 quantity of blood driven through from the heart to the arteries in 

 the unit of time produces, ceteris paribus, a proportional increase 

 or decrease in blood pressure. 



Variations in the resistance of the vessels exert the same 

 influence on blood pressure. Given the same energy of cardiac 

 function, a greater or less proportion of the driving force will, on 

 increase or decrease of vascular resistance, be expended on throw- 

 ing the walls into tension, which produces a corresponding rise 

 or fall of pressure. 



Increased vascular resistance can, under physiological con- 

 ditions, be determined only by augmentation of the tonic con- 

 traction of the muscle cells, with which (as we shall see in the 

 next chapter) the middle layer of the vessel walls, particularly in 



