THE PULMONARY CIRCULATION 981 



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 expira- 

 tory 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 capil- 

 laries 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 pulmonary 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 enlarge- 

 ment 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 enlarge- 

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

 tory 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 drive 

 more, say 12 c.c. of blood. There is thus more blood entering the left heart, and there- 

 fore a rise of systemic pressure during the last three-quarters of the inspiratory move- 

 ment. Expiration will have exactly the reverse effect. At the beginning of expiration 

 there is a diminution of capacity in the pulmonary vessels from 30 to 25 c.c. Hence 

 during the first second of expiration the outflow of blood from the pulmonary vein into 

 the left heart will be 17 c.c. (12 c.c. + 5 c.c.). After this, the increased resistance in 

 the pulmonary capillaries in consequence of their constriction will come into play, and 

 the flow of blood through them will fall once more from 12 c.c. to 10 c.c. Hence at the 

 beginning of expiration the inflow of blood from the pulmonary vein into the left heart 

 is greater than at any period. The arterial pressure will therefore rise to its greatest 

 height at the beginning of expiration,' and will fall during the last three-quarters of 

 :pi ration, but will attain its minimum only at the beginning of the next inspiration. 





In this way the effect of the respiratory movements on the systemic 

 blood pressure could be entirely explained by the influence they exert on 

 the lung vessels or lesser circulation. On the other hand, Lewis regards the 

 pericardial pressure, i. e. the direct influence of the thoracic movements on the 

 heart, as playing a much more important part than changes in the pulmonary 

 circulation in the production of the respiratory undulations in the blood 

 pressure. He shows moreover that in man the effect of respiration on 

 arterial blood pressure may vary according to the type of respiratory 

 movement, a deep intercostal inspiration (not prolonged) causing a pure 

 fall, while a deep diaphragmatic inspiration gives a pure rise of blood 

 pressure. In expiration the reverse effects hold. He concludes that in 

 man it is not possible to make any general statement as to the nature of 

 the blood pressure response to^a particular respirator}^ act. 



