230 CIRCULATION OF THE BLOOD 



(2) The diastole of the heart is favored; 



(3) The flow of the blood in the pulmonary vessels is facilitated because of 

 their dilatation; 



(4) The pressure in the abdominal cavity increases because of the descent 

 of the diaphragm ; and the blood is forced in greater quantity to the right heart. 



The following circumstances tend to lower the aortic pressure: 



(1) The heart systole is rendered more difficult because of the increased suc- 

 tion in the thorax; 



(2) At the beginning of inspiration, while the pulmonary vessels are still 

 dilating, a part of the blood expelled from the right ventricle must remain in 



FIG. 96. Respiratory variations of blood pressure in the rabbit. To be read from right to 

 left. The upper line represents the blood pressure, the middle line the respiratory move- 

 ments (downward stroke, inspiration), the lower line the time record in seconds. 



them, and by this means the mass of blood flowing from the left heart decreases 

 until the pulmonary vessels have been filled, after which the flow is increased. 

 In expiration naturally these mechanisms work in the reverse direction. 



Among these factors the alterations of blood flow to the right heart is of 

 the first importance. The respiratory variations of the aortic pressure wit- 

 nessed in a dog breathing quietly (Fig. 95) could be explained therefore in the 

 following manner. In expiration the right heart has less blood at its disposal, 

 the left heart receives less blood, and the aortic pressure falls. When inspira- 

 tion sets in, and the flow to the right heart becomes greater, it can drive a 

 greater quantity to the left heart, and the aortic pressure rises. But a short 

 time must always elapse before this increased supply to the right heart can 

 be felt in the left heart and in the aorta ; hence at the beginning of inspiration 

 the pressure is still falling. Likewise at the very beginning of the following 

 expiration the right heart still has at its disposal a portion of the increased 

 supply; the rise in pressure in the aorta continues therefore for a moment 

 until the diminished supply to the right heart can be felt on the left, when the 

 aortic pressure begins to fall. 



