178 BLOOD-PRESSURE IN THE PULMONARY ARTERY. 



to the pressure which corresponds to the atmospheric pressure minus 

 the pressure exerted by the elastic traction of the lungs ( 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 disten- 

 sible 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 Latschenberger). 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 Pfeifler, Bowditch and Garland, De Jager). 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 blood-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 (v. Openchowski). 



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 (Beutner). Even when 

 one pulmonary artery is plugged with an embolon of paraffin, the 

 pressure within the aortic system is not raised (Lichtheim). Thus, 

 when a large branch of the pulmonary artery becomes impervious, the 

 obstruction is rapidly compensated, and this is not due to the action of 



