656 PHYSIOLOGY OF RESPIRATION. 



effect of the difference in pressure is felt, however, upon the flow 

 of lymph and blood, especially the latter. The large veins in the 

 neck and axilla are under the pressure of an atmosphere exerted 

 through the skin, and the same is true for the inferior cava in the 

 abdomen. But the superior and inferior cava^ and the right auricle 

 are under a pressure less than one atmosphere. This difference in 

 pressure must act as a constant favoring condition to the flow of 

 blood to the heart. The difference is markedly increased at each 

 inspiration; so that at each such act there is an increase in the 

 ^velocity and volume of the flow to the heart, — an effect which is 

 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. This aspiratory effect upon the ven- 

 ous flow to the heart is made more important by the arrangement 

 of the valves in the jugular, subclavian, and femoral veins, which, as 

 explained on p. 514 facilitate the emptying of the venous cis- 

 tern toward the heart. There should be, of course, a similar 

 effect, but in the opposite direction, upon the flow in the arter- 

 ies. 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 pulmonar}' 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, wliich are synchronous with 

 the respiratory movements (see Fig. 271). 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 usually stated that the blood-pressure falls slightly 



