228 CIRCULATION OF THE BLOOD 



by their effect on the diameter of the vessels in the lungs. The diameter of these 

 vessels, as d'Arsonval, De Jager, Heger and others have shown, increases with 

 the expansion of the lungs during inspiration, and decreases with the collapse 

 of the lungs during expiration. Since the two factors operate in the same direc- 

 tion, there remains for us to determine to which of them the greater significance 

 is to be ascribed. We must first inquire to what extent the resistance in the 

 pulmonary vessels is changed by the different phases of respiration. 



This question does not admit of a direct answer; but we have certain well- 

 established facts which show very clearly that the resistance in the pulmonary 

 channels is in general so small that only a considerable change in the diameter 

 of the vessels could exercise upon it any very marked influence. 



We should mention first the results which Lichtheim obtained by occluding 

 a large part of the pulmonary vessels. It was shown with dogs which received 

 artificial respiration by rhythmical inflation of the lungs, that about three- 

 fourths of the territory supplied by the pulmonary arteries could be shut out 

 without diminishing in the least the flow of blood to the left ventricle. Again, 

 the left pleural cavity of a rabbit breathing naturally has been opened without 

 interfering with the respiration on the other side, and the entire left lung tied 

 off at the hilus : yet as a rule no fall of blood pressure was observed in the greater 

 circulation. 



We may say, then, that one-half (in curarized animals still less) of the pul- 

 monary blood channel is enough to supply the necessary quantity of blood to 

 the left heart. The explanation might be sought in an increased blood pressure 

 in the lesser circulation and a consequent greater dilatation of the vessels which 

 remain open. But the increase in pressure is so insignificant (it never amounts 

 to more than a few millimeters of Hg.) that it is very doubtful whether it could 

 produce such an effect. Again we might imagine a vasomotor influence upon 

 the pulmonary vessels; but the facts which we have at present on this subject 

 scarcely point to any considerable direct control of these vessels by the central 

 nervous system. Finally, it is possible that under normal circumstances the 

 lungs are never uniformly filled with blood, but that certain regions remain 

 relatively empty, being made accessible to the blood only by unusual opportuni- 

 ties like that just mentioned. 



Be this as it may, it certainly follows from the facts before us that the 

 resistance in the pulmonary vessels is very small. This conclusion is con- 

 firmed also by facts which we possess concerning the velocity of the blood flow 

 in the lungs. Stewart has shown, for example, that a foreign fluid injected 

 into the jugular vein passes the lesser circulation in three to four seconds. 

 When the lungs are inflated by a positive internal pressure sufficient to stop 

 the flow of blood in the pulmonary vessels, and the lungs are then released, 

 the press-ure in the greater circulation returns to its normal height in three to 

 four seconds. 



In view of this low pressure in the pulmonary vessels, one hesitates to sup- 

 pose that the changes in their diameter which take place in spontaneous respira- 

 tion as the direct result of alterations in the lung tissue can play the more 

 important part in determining the variations of blood pressure in the lesser cir- 

 culation. The changes in intrathoracic pressure are much rather to be assigned 

 the place of first importance. The pressure in the right ventricle falls in inspi- 

 ration not chiefly because the vessels in the lungs dilate a little, but because the 

 inspiratory suction distends the heart, and vice versa. 



The blood pressure in the pulmonary arteries is very low on account of the 

 low resistance in the smaller pulmonary vessels. On the average it amounts 



