386 RELATION OF .RESPIRATION TO OTHER PROCESSES [CH. XXVII. 



rise of blood-pressure than during the fall. This difference disappears 

 when the vagi are cut. Bespiratory undulations, however, are still 

 present, though not so marked as before ; hence the cardiac variations 

 are not their sole cause. They are chiefly the result of the mechanical 

 conditions dependent on the lungs and heart with its large vessels 

 being contained within the air-tight thorax. When the capacity of 

 the chest is increased in inspiration, the tension of the lung tissue 

 due to its greater expansion is increased ; hence the difference between 

 the intra-pleural pressure and that in the lungs (which is atmos- 

 pheric) becomes more marked, for the difference of pressure is to be 

 measured by the elastic force of the lung tending to produce its 

 collapse. If the intra-thoracic pressure is measured, it is found that 

 it varies from 5 to 7 mm. of mercury at the end of expiration to 

 30 at the end of a deep inspiration ; that is to say, from 5 to 7 to 

 30 mm. less than the atmospheric pressure (760 mm. of mercury). 

 The pressure outside the heart and large thoracic vessels is corre- 

 spondingly diminished during inspiration to the same extent, and pro- 

 duces its main effect (distension) upon the veins because they are never 

 fully distended, and because the pressure within them is low. This 

 increase in the " pressure gradient " (i.e., the rate of fall of pressure) 

 between the intra- and eatfra-thoracic great veins results in a pro- 

 portionately more rapid flow of blood into the thorax, and therefore 

 into the right side of the heart ; for within certain limits the right 

 heart can be easily expanded more fully if a greater supply of blood 

 is provided. Consequently, the output from the right side of the heart 

 increases, and thus via the pulmonary circuit the inflow into the left 

 side of the heart is increased ; in its turn, therefore, the output from 

 the left ventricle rises, and so the aortic pressure is raised. This 

 effect would be counteracted if the aorta and its branches within the 

 thorax were as easily affected by changes of the intra-thoracic pressure 

 as are the thin-walled and easily distensible veins ; the thick wall of 

 the aorta and its branches, however, prevents them from undergoing 

 much change of this kind during ordinary breathing. The conditions 

 in the veins are reversed when, with the expiratory act, the thorax 

 returns to its former size ; therefore the arterial blood-pressure falls. 



The effect of inspiration on arterial blood-pressure is at first 

 assisted by the pressure of the diaphragm, as it descends, on the 

 abdominal veins, and blood is thus sent upwards into the chest by 

 the vena cava inferior. On the other hand, this is to some extent 

 counterbalanced by the obstruction in the passage of the blood 

 downwards in the abdominal aorta, and upwards from the veins of 

 the lower extremities, but again the veins are the vessels more easily 

 influenced by moderate changes in external pressure. 



We thus see that these various physical conditions produce during 

 inspiration an increased flow of blood into the right heart ; this 



