980 PHYSIOLOGY 



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 arc 

 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 perforating 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 a negative pressure of 6 mm. Hg. As the 

 chest expands in inspiration.it drags the lungs still more 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 

 inspiration 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 inspiration 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 in 

 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 

 influence 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 

 n< 'native 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 

 factor in itself will tend to raise the arterial blood pressure. The in.spiratory 

 (It-scent of the diaphragm \\ill moreover tend to increase the inflow into the 

 In-art ly raMiiij; 1 1n' positive pressure in the abdomen, so that blood is pressed 

 out of the abdominal veins and sucked into the heart and the thoracic veij. 



Anot her factor which must play some part, is the influence of the respiratory move- 

 ments on the circulation through the lungs. In trying to understand this influence, it 

 must be remembered that the pulmonary capillaries lie in a certain amount of elastic 

 and connective tissue and are separated, on the one side by the alveolar epithelium 

 from air at the ordinary atmospheric pressure, and on the other by the pleural endot he- 

 lium from the pleural cavity, where, the pressure varies from 6 to 30 mm. Hg. below 

 the atmospheric pressure. We may therefore consider the pulmonary capillaries as 

 lying between, and attached to, two concentric elastic bags. Under normal conditions, 



