1056 PHYSIOLOGY 



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 inspira- 

 tion 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 therefore 

 a rise of systemic pressure during the last three-quarters of the inspira- 

 tory movement. Expiration will have exactly the reverse effect. At the 

 beginning of expiration there is a diminution of capacity in the pulmo- 

 nary vessels from 30 to 25 c.c. Hence during the first second of expira- 

 tion 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 expiration, 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 can 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 respira- 

 tory 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 of pressure, while a 

 deep diaphragmatic inspiration gives a pure rise of blood pressure. 

 In expiration the reverse effects hold. He concludes that it is not 

 possible to make any general statement as to the nature of the blood- 

 pressure response to a particular respiratory act. 



