516 CIRCULATION OF BLOOD AND LYMPH. 



It will be seen, therefore, that the mean pressure is not more 

 than one-seventh to one-eighth of that prevaihng in the aorta. 

 The thinner walls and smaller muscular power of the right ventricle 

 as compared with the left are an indication of the fact that less force 

 is necessary to keep up the circulation through the pulmonary 

 circuit. 



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 

 systemic pressure, as taken in the carotid, varying from 144 to 222 

 mms., that in the pulmonary artery changes correspondingly 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 pressure 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 therefore the amount of flow 

 and the pressure in the pulmonary artery, depend mainly on the 

 amount of blood received through the vense cavse by the right auricle. 

 If one of the vense cavse 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 such means, therefore, the variations in blood-flow in 

 the systemic circulation indirectly influence and control the pressure 

 relations in the pulmonar}^ 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 sufficiently, 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 moi^e 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 of variations in intrathoracic 

 pressure upon the pulmonary circulation are referred to in con- 

 nection with respiration. 



