936 



PHYSIOLOGY 



beginning of inspiration. During the fall which accompanies expiration the 

 heart-beats, as shown in the diagram (Fig. 420), become less frequent, and 

 an obvious explanation of the fall of pressure would be to ascribe it to a 

 reflex inhibition of the heart. On dividing both vagi, this difference in 

 the pulse-rate during inspiration and expiration disappears, but the main 

 features of the blood-pressure curve remain the same ; so that we must look 

 for some mechanical explanation of the respiratory undulations. 



We have already seen that under normal conditions the lungs are in a 

 state of over- distension, and that in consequence of this condition they are 

 constantly tending to collapse, and are therefore exerting a pull on the chest 



wall. As soon as we admit air 

 into the pleural cavity by per- 

 forating the chest wall the lungs 

 collapse. The force with which 

 the lungs tend to collapse amounts 

 to 6 mm. Hg. at the end of a quiet 

 expiration, so we say that in the 

 pleural cavity there is normally 



Arterial 

 Moodpre. 



Reyxra&yfaceuy. 



F I0 . 420. Diagram of blood-pressure curve, negative pressure of 6 mm. Hg. 

 showing effects of the respiratory movements As the chest expands in inspira- 



open. As these become more 

 distended their pull on the chest wall becomes greater, and hence the 

 negative pressure in the pleura may be increased during forcible inspira- 

 tion to 30 mm. Hg. It must be remembered that the heart and great 

 veins and arteries are in the thorax separated from the pleural cavity only 

 by a thin yielding membrane, so that they are practically exposed to any 

 pressure, positive or negative, which may exist in the pleural cavity. Hence 

 even at the end of expiration the heart and large vessels are subjected to a 

 negative pressure of 6 mm. Hg. Outside the thorax all the vessels are 

 exposed to a positive pressure, conditioned in the neck by the elasticity of 

 the tissues and in the abdomen by the contractions of the diaphragm and 

 abdominal muscles. 



Blood, like any other fluid, will always flow from a point of higher to 

 a point of lower pressure. There must thus be a constant aspiration of blood 

 from peripheral parts into the thorax. This aspiratory force will not in- 

 fluence arteries and veins alike. The arteries, having thick, comparatively 

 non-distensible walls, will be very little affected by the negative pressure 

 obtaining in the thoracic cavity, whereas the thin-walled distensible veins 

 will be largely influenced by the same factor. The total result then of the 

 negative pressure in the pleural cavities is to increase the flow of blood from 

 the veins into the heart without affecting to any appreciable degree the 

 outflow of blood from the heart into the arteries. The more pronounced the 

 negative pressure in the thorax, the greater will be the amount of blood 

 sucked into the heart from the veins. During inspiration therefore the 

 heart will be better supplied with blood than during expiration, and this 



