392 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



such a way that a greater quantity of blood is drawn into the central 

 venous channels during inspiration than during expiration. Conse- 

 quently, as the heart receives more blood during the former period, it 

 is in a position to pump more blood into the arteries. 1 This explains 

 the inspiratory rise in arterial pressure. This mechanical explanation 

 of the respiratory variations finds support in the following conditions : 



(a) It has already been stated that the heart increases its frequency during 

 inspiration. This phenomenon may be explained in two ways. Thus, it may be 

 assumed that it is a reflex elicited within the heart in consequence of the influx 

 of a greater quantity of blood, or that it is due to accelerator impulses generated 

 by the cardiac center. The latter explanation has been submitted by Fredericq, 

 who has found that this acceleration takes place even after the mechanical in- 

 fluence of respiration upon the heart has been removed by opening the chest. 

 The fact that the division of the vagi nerves destroys the acceleration immediately 

 proves that this system is directly concerned with the production of this phe- 

 nomenon. It is also interesting to note that this acceleration is more marked in 

 persons whose nervous system is in a state of hyperirritability. 



(6) The transfer of blood from the right into the left side of the heart is greatly 

 facilitated by inspiration, because this movement permits of a greater distention 

 of the pulmonary blood-vessels, thereby lessening the resistance in this circuit. 

 During expiration, on the other hand, .the elastic pull upon these vessels is dimin- 

 ished and the resistance within them increased. 



(c) The inspiratory descent of the diaphragm favors the venous return from 

 the abdominal organs, .because it tends to increase the pressure in the abdominal 

 cavity and to lessen the resistance in the thorax. 2 



(d) The fact that these changes may be rendered more conspicuous by in- 

 creasing the amplitude of the respiratory movements is another point in favor 

 of this explanation. Last of all, it should be taken into account that these varia- 

 tions are completely reversed during artificial respiration. 3 This need not cause 

 surprise, because the artificial inflation of the lungs induces conditions practically 

 the reverse of those prevailing during normal respiration, when this organ is ex- 

 panded by a force resting upon its external surface. As the air is forced into the 

 pulmonary passage, the capillaries of the lungs are subjected to a certain pressure 

 which tends to increase the resistance within them. This implies that the venous 

 pressure is increased during the period of inflation, whereas the influx of blood is 

 diminished. The deflation of the lungs, on the other hand, relieves this com- 

 pression of the pulmonary capillaries and permits a more unhindered through- 

 flow in consequence of the diminution in the resistance. 



As has been emphasized by Wiggers, 4 the respiratory variations in 

 pressure may be explained without difficulty upon the basis of 

 the circulatory changes in the lesser circuit just enumerated. Lewis, 5 

 on the other hand, believes that the respiratory motions affect the 

 heart in a direct way, and that the effect upon the arterial blood 

 pressure varies with the type of respiration. Thus, diaphragmatic res- 

 piration is said to give an inspiratory rise and expiratory fall in ar- 

 terial pressure, while a pronounced costal movement induces an inspira- 

 tory fall and expiratory rise. This result, however, is easily explained 



1 Burton-Opitz, Am. Jour, of Physiol., vii, 1902, 435. 



2 Burton-Opitz, ibid., xxxv, 1914, 64. 

 J Burton-Opitz, ibid., ix, 1903, 198. 



* Ibid., xxxv, 1914. 

 6 Ibid., xvi, 1906. 



