282 RESPIRATION 



increase of blood pressure produced by the effect of inspiration upon the 

 volume of discharge of the veins of the chest, and the balance of the whole 

 action would be in favor of an increase of blood pressure during the inspira- 

 tory period. When a blood-pressure tracing is taken at the same time that 

 the respiratory movements are being recorded, it will be found that, although, 

 speaking generally, the arterial tension is increased during inspiration, the 

 maximum of arterial tension does not correspond with the acme of inspira- 

 tion, figure 241. In fact, at the beginning of inspiration the pressure con- 

 tinues to fall for a brief moment, then gradually rises until the end of 

 inspiration, and continues to do so for a moment after expiration has com- 

 menced. For explanation of the influence of heart rate- in this variation of 

 blood pressure, associated with the respiratory movement, see page 181. 



In ordinary expiration all this would be reversed, but if the abdominal 

 muscles are violently contracted, as in extraordinary expiration, the same 



FIG 241. Comparison of Blood -Pressure Curve with Curve of Intrathoracic Pressure (To 

 be read from left to right.) a is the curve of blood pressure with its respiratory undulations, the 

 slower beats on the descent being very marked; b is the curve of intrathoracic pressure obtained 

 by connecting one limb of a manometer with the pleura! cavity. Inspiration begins at * and expira- 

 tion at e. ihe intrathoracic pressure rises very rapidly after the cessation of the inspiratory 

 ettort, and then slowly falls as the air issues from the chest; at the beginning of the inspiratory 

 effort the fall becomes more rapid. (M. Foster.) 



relative effect would be produced as by inspiration. The immediate effect 

 during inspiration of the diminished intrathoracic pressure upon the pul- 

 monary vessels is to produce an initial dilatation of both artery and veins, 

 and this delays for a moment the passage of blood toward the left side of the 

 heart, resulting in an initial fall in the arterial pressure, but the fall of blood 

 pressure is immediately followed by a steady rise, since the flow is increased 

 by the initial dilatation of the vessels. The converse is the case with ex- 

 piration. As, however, the pulmonary veins are more easily dilatable than 

 the pulmonary artery, their greater distensibility increases the flow of blood 

 as inspiration proceeds, while during expiration, except at its beginning, 

 this property of theirs acts in the opposite direction, and diminishes the flow. 

 Thus, at the beginning of inspiration the diminution of blood pressure, which 



