RESPIRATORY WAVE IN BLOOD- PRESSURE CURVE. 305 



atmospheric pressure acting on the tributaries of the superior 

 vena cava is unchanged, while the pressure in the abdominal 

 cavity is increased, and the inferior vena cava compressed by the 

 muscular action. The blood then flows more readily during in- 

 spiration into the right heart, and consequently the lungs receive 

 a larger supply of blood during this period. In expiration the 

 negative intrathoracic pressure becomes less negative, the com- 

 pression of the abdominal viscera is relieved, and the flow into 

 the auricle loses somewhat in force. 



It must be carefully borne in mind that the left side of the 

 heart works under different conditions, for the same variations 



FIG. 138 



Blood Pressure and Respiratory Tracings recorded synchronouslyrecording surface 

 moving from right to left showing that the variations in pressure in the arteries (con- 

 tinuous line) and in the thoracic cavity (dotted line) do not exactly correspond, the 

 latter continuing to fall after the blood pressure has commenced to rise. 



of pressure affect both the pulmonary veins and the left auricle 

 equally, since they are both included in the thoracic cavity, and 

 are both subjected to a slightly varying negative pressure. The 

 aid given to the flow into the right heart by the low intrathoracic 

 pressure is quite absent on the left side; so that the thoracic 

 movements do not exert any influence on the flow of blood from 

 the pulmonary veins to the systemic arteries. But while inspira- 

 tion is taking place the lungs receive a larger supply of blood ; 

 and from the relative amounts of blood in the different organs it 

 is probable that this slight excess, having passed the lungs, 

 arrives at the left ventricle at the period of expiration. Thus, 

 26 



