THE PULMONARY CIRCULATION 937 



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

 descent of the diaphragm will moreover tend to increase the inflow into the 

 heart by raising the positive pressure in the abdomen, so that blood is pressed 

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

 veins. 



Another important factor is the influence of the respiratory movements 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 endothelium 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, since these bags 

 are always tending to collapse, the inner one must be pulling away from the 

 outer one, and the outer one from the chest wall. Hence there must be a 

 negative pressure in the tissues between these two bags a negative pressure 

 which in the expiratory condition will be something between and - 6 mm. 

 Hg., and in the inspiratory condition between and - 30 mm. Hg. If we 

 regard the average pressure within the pulmonary capillaries as constant, 

 these capillaries must be more dilated in the inspiratory than in the expiratory 

 condition. This dilatation of the pulmonary capillaries will have two 

 effects. Their capacity will be increased and the resistance they present 

 to the flow of blood will be diminished. 



Let us now consider what effect these changes will have on the general 

 arterial blood-pressure. We will assume that during expiration the pul- 

 monary vessels have a capacity of 25 c.c. and that the beat of the right heart 

 is forcing through them 10 c.c. of blood per second. So long as the chest 

 remains in the expiratory condition 10 c.c. of blood will be flowing into the 

 left heart and into the aorta, so that the systemic blood-pressure will remain 

 constant. Now let us suppose that an inspiratory enlargement of the thorax 

 takes place, the negative pressure in the pleura is increased, the two walls 

 of the lungs are pulled farther away from one another, and there is a general 

 enlargement of the pulmonary capillaries. We will assume that this enlarge- 

 ment increases the capacity of the pulmonary capillaries from 25 to 30 c.c. 

 Owing to this increased capacity, the first 5 c.c. of blood which flows into the 

 lungs after the beginning of inspiration will not flow out through the pul- 

 monary vein, but will simply serve to bring the capillaries into the same state 

 of distension as before. Hence at the beginning of inspiration the flow 

 through the pulmonary vein will be diminished ; there will be less blood 

 discharged into the left heart, and therefore a fall in systemic pressure. 

 As soon, however, as the increased capacity of the pulmonary vessels is 

 made up, the dilating effect of the inspiratory movement of these vessels 

 will aid the flow through the lungs, in consequence of the diminution of 

 resistance, so that the same force of the right heart which drove 10 c.c. of 

 blood per second through the former resistance during expiration will now 



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