482 CIRCULATION OF BLOOD AND LYMPH. 



If one of the venae 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 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 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 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 of variations in intrathoracic 

 pressure upon the pulmonary circulation are referred to in con- 

 nection with respiration. 



