938 PHYSIOLOGY 



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 inspiratory movement. 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 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 peri- 

 cardial 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 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. 



