RESPIRATORY VARIATIONS OP BLOOD PRESSURE 229 



in the dog to about 20 mm. Hg., in the cat to about 18 and in the rabbit to 

 about 15 (cf. page 170). 



As mentioned above (page 208), the pressure in the greater circulation 

 in the same individual may exhibit great variations from one moment to 

 another. The pulmonary circulation is entirely different in this respect, the 

 pressure variations there being on the whole very small scarcely ever more 

 than 10-15 mm. under normal circumstances. 



It follows that the work of the left ventricle must vary in amount much 

 more than that of the right, and this is borne out also by the fact that the 

 left ventricle becomes more or less fatigued, when the right is still entirely 

 capable. 



The lesser circulation is dependent upon the greater not only because it 

 draws its supply from the venae cavae, but also because it must deliver its blood 

 to the left ventricle and so is affected by the conditions of the blood flow from 

 that chamber. If, for example, the left ventricle is unable, because of high 

 resistance, to discharge all the blood coming to it, so that a certain quantity 

 accumulates within this chamber, a state of affairs will finally be reached where 

 the flow from the right heart is hindered, which in fact has been experimentally 

 demonstrated (Waller). 



In general, however, this reverse effect of the left heart on the right is only 

 slight, a fact associated with the great capacity of the pulmonary vessels. The 

 lungs serve the same purpose with respect to the flow of blood into the left heart 

 as does the liver with respect to the flow of blood to the right (cf. page 227). 



Moreover, the great capacity of the pulmonary vessels has this advantage, 

 that in great respiratory distress where the vessels of the greater circulation are 

 powerfully contracted, the greatest possible quantity of blood is exposed to the 

 alveolar air and the greatest possible quantity of oxygen is therefore absorbed. 

 By this means the blood is relieved of the products of combustion, and the influ- 

 ence which these exert through their stimulation of the vasomotor centers is 

 diminished to some extent. 



B. RESPIRATORY VARIATIONS OF BLOOD PRESSURE 



Just as the lesser circulation is influenced in several respects ~by the greater, 

 it in turn exerts no less an influence upon the greater. Consequently there 



FIG. 95. Variations of the aortic blood pressure in the dog, due to normal respiratory move- 

 ments, after De Jager. /, inspiration; E, expiration. To be read from right to left. 



appear in the aorta variations of blood pressure which are synchronous with 

 the respiratory movements and are doubtless dependent upon these and upon 

 the variations in the pulmonary circulation. The mechanism by which these 

 influences are brought to bear are rather complicated, and we have to take 

 into account the following conditions. 



The following circumstances tend during inspiration to increase the blood 

 pressure in the aorta: 



(1) The aspiration of the blood to the right heart increases; 



