RESPIRA TION 269 



pressure curve, so that we ought not to expect them to be abso- 

 lutely constant. 



In artificial respiration oscillations of blood-pressure, synchronous 

 with the movements of the lungs, are also seen. During inflation 

 (inspiration) the arterial pressure rises ; during deflation (expiration) 

 it falls. The waves are not entirely abolished even when the thorax 

 is opened. In the latter case there are, of course, no variations of 

 intrathoracic pressure, and the oscillations must be connected with 

 the changes in the pulmonary circulation, the inflation squeezing 

 blood from the lungs into the left side of the heart, while deflation 

 permits the pulmonary vessels to become filled to their new capacity 

 at the expense of the stream flowing into the left auricle. When 

 artificial respiration is stopped at the height of inflation (Fig. 114), 

 the arterial blood-pressure falls rapidly, and continues low until the 

 rise of asphyxia begins. The fall of pressure when the chest has 

 been previously opened is due to the increased vascular resistance 



FIG. 114. EFFECT ON BLOOD-PRESSURE OF INFLATION OF THE LUNGS : RABBIT. 



Artificial respiration stopped in inflation at i. Interval between 2 and 3 (not 

 reproduced) 51 seconds, during which the curve was almost a straight line. Time 

 tracing shows seconds. 



in the lungs, due to the narrowing of the capillaries by the increased 

 alveolar pressure. With intact chest the increased intrathoracic 

 pressure due to the inflation is also an important factor. When 

 the respiration is stopped in collapse, instead of a fall a steady rise 

 of pressure occurs (as in Fig. 72, p. 172). This ultimately merges 

 in the elevation due to asphyxia, which shows itself sooner than in 

 inflation. When the tracheal cannula is closed in natural respira- 

 tion, no initial fall of pressure takes place (Fig. 115). 



Besides the mechanical effects of the respiratory movements 

 on the circulation, it may be influenced by changes in the cardio- 

 inhibitory and vaso-motor centres synchronous with the rhythm 

 of the respiratory centre. In many animals (the dog, for instance) 

 and in man, it can be very easily made out that the rate of the 

 heart is greater during inspiration, especially towards its end, 

 than in expiration. The phenomenon is especially distinct in 



