312 



VETERINARY PHYSIOLOGY 



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 a small 

 fall in the arterial pressure occurs 

 at the beginning of inspiration. 



Similarly, at the beginning of ex- 

 piration the lungs are compressed 

 and their blood vessels squeezed, 

 __ B and thus the blood is driven out 

 from them. Now, this blood can- 

 not pass back into the right side 



FIG. 142. -To show relations of of the heftrt g() ifc mugt Qn 



Cavdio-pneu,matic Movements, . . . . , . . 



A, to the Cardiac Cycle, B. In mto the ^ft Side. More blood IS 



A the upstrokes are expiratory, thus driven into the arteries, and 



the downstrokes inspiratory. fa e pressure rises. As SOOn, how- 



ever, 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 eardio pneumatic movements. 



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

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



(1) 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. 



(2) This is followed by a marked inrush of air corresponding 

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



