468 CIRCULATION OF BLOOD AND LYMPH. 



The Variations in Pressure in the Pulmonary Circuit. 



Experimental results indicate that the pressures in the pulmonary 

 circuit do not undergo as marked changes as in the systemic circu- 

 lation; the flow is characterized by a greater steadiness. With a sys- 

 temic pressure, as taken in the carotid, varying from 144 to 222 mms., 

 that in the pulmonary artery changes only from 20 to 26 mms., and 

 extreme variations of pressure in the pulmonary artery probably 

 do not exceed, as a rule, 15 to 20 mms. The regulations of the pres- 

 sure and flow of blood in the small circulation do not seem to be 

 so direct or complex as in the aortic system. The part taken by the 

 vasomotor nerves is referred to in the chapter upon the innervation 

 of the blood-vessels, and attention may be called here only to the 

 mechanical factors, which, indeed, for this circulation are probably 

 the most important. The output from the right ventricle, and there- 

 fore the amount of flow and the pressure in the pulmonary artery, 

 depends mainly on the amount of blood received through the 

 venae cavse by the right auricle. If one of the venae cavee is closed 

 the pulmonary pressure sinks; pressure upon the abdomen, on the 

 other hand, by squeezing more blood toward the right heart may 

 raise the pressure in the pulmonary artery. By this means, there- 

 fore, the variations in blood-flow in the systemic circulation in- 

 directly influence and control the pressure relations in the pulmonary 

 circuit. But the changes in the systemic circulation may affect 

 the blood-flow through the lungs in still another way, namely, by 

 a back effect through the left auricle. When for any reason the 

 blood-pressure in the aorta is driven much above the normal level 

 the left ventricle may not be able to empty itself completely, and if 

 this happens the pressure in the left auricle will rise and the flow 

 through the lungs from right ventricle to left auricle will be more or 

 less impeded. On the whole, it would seem that the pulmonary 

 circulation is subject to less changes than in the case of the organs 

 supplied by the aorta. The mechanical conditions, especially in the 

 capillary region, are such that the blood is sent through the lungs 

 with a relatively high velocity, although under small actual 

 pressure. The special effects of the respiratory movements and 

 variations in intrathoracic pressure upon the pulmonary circulation 

 are described in connection with respiration. 



