PRESSURE CONDITIONS IN LUNGS AND THORAX. 609 



usually referred to as the aspiratory action of the thorax. At each 

 inspiration blood is " sucked " from the extrathoracic into the intra- 

 thoracic veins. So far as the inferior cava is concerned, this effect 

 is augmented by the simultaneous increase in abdominal pressure. 

 For as the diaphragm descends it raises the pressure in the ab- 

 domen as it lowers the pressure in the thorax. The two fac- 

 tors co-operate in forcing more blood from the abdominal to 

 the thoracic portion of the cava. There should be, of course, a 

 similar effect, but in the opposite direction, upon the flow in the 

 arteries. Each inspiration should retard the arterial outflow from 

 the aorta into its extrathoracic branches. As a matter of fact, this 

 effect probably does not take place. The arteries are thick walled 

 and are distended by a high internal pressure, so that the small 

 change of pressure of three or four millimeters of mercury during 

 inspiration is probably incapable of influencing the caliber of the 

 arteries, while it has a distinct effect upon the thin- walled veins, 

 whose internal pressure is very small. The changes in intra- 

 thoracic pressure during respiration must affect the blood-flow also 

 in the pulmonary circuit, the flow from the right to the left side of 

 the heart. This effect is manifested in the so-called respiratory 

 waves of blood-pressure which may be discussed briefly in this 

 connection. 



Respiratory Waves of Blood-pressure. -^When a record is 

 taken of the blood-pressure the tracing shows waves, unless the 

 respiratory movements are very shallow, which are synchronous with 

 the respiratory movements (see Fig. 250). When the respiration 

 is dyspneic the waves of pressure are very marked. To ascertain the 

 exact relations of these variations to the phases of respiration it is 

 necessary to make simultaneous tracings of blood-pressure and 

 respiration movements with the recording pens properly superposed. 

 In the dog it is found that the blood-pressure falls slightly at the 

 beginning of inspiration, but rises during the rest of the act (Fig. 

 251). At the beginning of expiration the pressure continues to rise 

 for a time and then falls during most of this phase. On the whole, 

 therefore, the effect of inspiration, its final effect, is to cause a rise 

 of arterial pressure, while the effect of expiration is to cause a fall. 

 The relationship of the two curves varies in other animals, depend- 

 ing, among other things, on the rapidity of the respirations; 

 but, since most of the experimental work has been done upon the 

 dog, our attention may be confined to the relationship shown by this 

 animal. Two general explanations may be given for these respira- 

 tory waves: First, that they are due to an activity of the vaso- 

 constrictor center synchronous with that of the respiratory center. 

 Second, that they are due to variations in the amount of blood sent 

 out from the heart into the aorta, this variation, in turn, being due 



