BLOOD-PRESSURE IN THE PULMONARY ARTERY. 131 



( 60). The trunks of the pulmonary artery and veins are subjected to the same 

 conditions of pressure. The elastic traction of the lungs is greater the more they are 

 distended. The blood of the pulmonary capillaries will, therefore, tend to flow 

 towards the large blood-vessels. As the elastic traction of the lungs acts chiefly 

 on the thin-walled pulmonary veins, while the semi-lunar valves of the pulmonary 

 artery, as well as the systole of the right ventricle, prevent the blood from flowing 

 backwards, it follows that the blood in the capillaries of the lesser circulation must 

 flow towards the pulmonary veins. 



If tubes with thin walls be placed in the walls of an elastic distensible bag, the 

 lumen of these tubes changes according to the manner in which the bag enclosing 

 them is distended. If the bag be directly inflated so as to increase the pressure 

 within it, the lumen of the tubes is diminished (Funke and Latscheitberger). If the 

 bag be placed within a closed space, and the tension within this space be diminished 

 so that the bag thereby becomes distended, the tubes in its wall dilate. In the 

 latter case viz., by negative aspiration the lungs are kept distended within the 

 thorax, hence the blood-vessels of the lungs containing air are wider than those of 

 collapsed lungs (Quincke and Pfeiffer, Bowditch and Garland, De JCiger). Hence 

 also, more blood flows through the lungs distended within the thorax than through 

 collapsed lungs. The dilatation which takes place during inspiration acts in a 

 similar manner. The negative pressure that obtains within the lungs during 

 inspiration causes a considerable dilatation of the pulmonary veins, into which 

 the blood of the lungs flows readily, whilst the blood under high pressure in the 

 thick-walled pulmonary artery scarcely undergoes any alteration. The velocity of 

 the blood-stream in the pulmonary vessels is accelerated during inspiration (De 

 Jager, Lalesque). The bloocl-pressure in the pulmonary circuit is raised when the 

 lungs are inflated. Contraction of small arteries, which causes an increase of the 

 blood-pressure in the systemic circulation, also raises the pressure in the pulmonary 

 circuit, because more blood flows to the right side of the heart. 



The vessels of the pulmonary circulation are very distensible and their tonus is 

 slight. [Occlusion of one branch of the pulmonary artery does not raise the 

 pressure within the aorta. Even when one pulmonary artery is plugged with an 

 embolon of paraffin, the pressure within the aortic system is not raised (Lichtheim). 

 When a large branch of the pulmonary artery becomes impervious, the obstruction 

 is rapidly compensated for, and this is not due to the action of the nervous system. 

 The vaso-motor system has much less effect upon the pulmonary blood-vessels than 

 upon those of the systemic circulation. The compensation seems to be due chiefly 

 to the great distensibility and dilatation of the pulmonary vessels (Lichtheitn).] 

 We know little of the effect of physiological conditions upon the pulmonary artery. 

 According to Lichtheim suspension of the respiration causes an increase of the 

 pressure. [In one experiment he found that the pressure within the pulmonary 

 artery was increased, while it was not increased in the carotid, and he regards this 

 experiment as proving the existence of vaso-motor nerves in the lung.] 



During the act of great straining, the blood at first flows rapidly out of the pulmonary veins, 

 and afterwards ceases to flow, because the inflow of blood into the pulmonary vessels is inter- 

 fered with. As soon as the straining ceases, blood flows rapidly into the pulmonary vessels 

 (Lalesque). 



Severini found that the blood-stream through the lungs is greater and more rapid when the 

 lungs are filled with air rich in C0 2 than when the air within them is rich in 0. He supposes 

 that these gases act upon the vascular ganglia within the lung, and thus affect the diameter of 

 the vessels. 



Pathological. Increase of the pressure within the area of the pulmonary artery occurs fre- 

 quently in man, in certain cases of heart disease. In these cases the second pulmonary sound 

 is always accentuated, while the elevation caused thereby in the cardiogram is always more 

 marked and occurs earlier ( 52). Electrical and mechanical stimulation of abdominal organs 

 raises the bloocl-pressure in the pulmonary artery (Morel). 



[The action of drugs on the pulmonary circulation may be tested by Holmgren's apparatus 



