THE RESPIRATION 297 



veins and auricles, and thus the flow of blood into them is 

 retarded (Fig. 129), and, less blood passing into the heart, less 

 is pumped into the arteries, and the arterial pressure falls. 



This, however, does not explain the slight fall of pressure 

 at the beginning of inspiration, or the slight rise at the 

 beginning of expiration. To understand these, the action of 

 the pulmonary circulation has to be taken into account. 



As inspiration develops the lungs are dilated, and the 

 capillaries in them are also expanded. These expanding 

 capillaries require more blood to fill them. They are situated 

 on the course of the blood from the right side to the left side 

 of the heart, and thus blood is retained from this stream to 

 fill them, and less blood passes on into the left side of the 

 heart and out into the arteries, and thus at the beginning 

 of inspiration a small fall in the arterial pressure occurs 

 (Fig. 129). 



Similarly, at the beginning of expiration the lungs are 

 compressed and their blood vessels squeezed, and thus the 

 blood is driven out from them. Now, this blood cannot pass 

 back into the right side of the heart, so it must pass on into 

 the left side more blood is driven into the arteries, and thus 

 the pressure rises. As soon, however, as the excess of blood 

 has been squeezed out of the lungs, the contracted state of 

 the vessels further retards the passage of blood to the left 

 side of the heart, and assists in diminishing the arterial 

 pressure. 



Influence of the Action of the Heart on Respiration. 



The heart lies in the thorax surrounded by the elastic 

 lungs. As it contracts and dilates it must alternately pull 

 upon and compress the lungs, and thus tend to cause an inrush 

 and an outrush of air the cardio-pneumatic movement. 



If a simultaneous tracing of the heart-beat and of the 

 movements of the air column be taken, it will be seen that 

 at the beginning of ventricular systole there is a slight out- 

 rush of air from the lungs, probably caused by the blow given 

 to the lungs by the suddenness of the systolic movement. 

 This is followed by a marked inrush of air corresponding to 

 the outflow of blood from the ventricles, and caused by the 

 fact that the contracting ventricles draw on and expand the 



