170 THE BLOOD-PRESSURE IN THE PULMONARY ARTERY. 



heart ceases as soon as the thoracic cavity is opened, because the elastic traction 

 of the lungs is abolished and the right heart itself is exposed to the full pressure of 

 the air. 



The pressure was found to be in the dog 29.6, in the cat 17.6, and in the 

 rabbit 12 mm. of mercury (in the dog 3 times, rabbit 4 times, and in the cat 5 

 times less than the pressure in the carotid) . 



Faivre and Chauveau, in 1856, introduced a catheter into the right ventricle 

 through the jugular vein and connected it with a manometer. 



Knoll reached the pulmonary artery through the anterior mediastinum, with- 

 out opening the pleural cavities, and introduced a cannula laterally into the trunk 

 of the vessel. By this method he was able to observe the pressure in the artery 

 during spontaneous breathing without restricting the lesser circulation and with- 

 out displacing the heart. He thus found a mean pressure of 12.2 mm. of mercury 

 in the rabbit. 



Indirect estimation can be made by comparing either the muscular walls of 

 the right with those of the left ventricle, or the thickness of the walls of the pul- 

 monary artery and of the aorta, for it must be assumed that there is a definite 

 relation between the thickness of the walls and the pressure within the vessels. 



Beutner and Marey estimate the relation of the pulmonary pressure 

 to the aortic pressure as i : 3 ; Goltz and Gaule, as 2 : 5. Pick and Badoud, 

 in the dog, found the pressure in the pulmonary artery to be 60 mm., 

 and in the carotid in mm. of mercury. According to Knoll the pul- 

 monary pressure in the rabbit is 6.8 times less than the pressure in the 

 carotid. In a child the pressure in the pulmonary artery is relatively 

 greater than in the adult. 



The pulmonary pressure exhibits certain rhythmical variations due to varia- 

 tions in the tone of the heart's action. When the air-pressure in the lung falls, 

 the pressure in the lesser circulation also falls, and conversely. 



The expansion of the lungs in the thoracic cavity is maintained by the nega- 

 tive pressure on their outer pleural surface. When the glottis is open, the inner 

 surface of the lungs and the walls of the alveolar capillaries traversing the lungs 

 are exposed to the full pressure of the air. The heart and the large vascular 

 trunks of the thorax, however, are subject not to the full pressure of the air, but 

 to the pressure of the air minus the pressure corresponding to the elastic traction 

 of the lungs. The trunks of the pulmonary artery and veins are accordingly 

 subject to the same pressure-conditions. The elastic traction of the lungs is 

 proportional to the degree of expansion of the lungs. The blood in the pul- 

 monary capillaries will thus have a tendency to flow from these capillaries into 

 the large vascular trunks. As the elastic traction of the lungs affects chiefly the 

 more delicate pulmonary veins, and as re gurgitation of the blood is prevented by 

 the semilunar valves of the pulmonary artery, as well as by the contraction of 

 the right ventricle, it follows from these pressure-conditions that the capillary 

 blood in the lesser circulation is drained into the pulmonary veins. 



Thin-walle'd tubes embedded within the substance of the walls of an elastic, 

 distensible sac suffer a modification of their lumen, in accordance with the manner 

 in which the sac is distended; for, if the sac is directly inflated so that the air- 

 pressure in its interior increases, the lumen of the tubes is diminished; if, however, 

 the sac is distended by rarefying the air in the closed space surrounding it, the 

 tubes embedded in the wall dilate. When the distention is brought about in the 

 latter way, namely by the negative pressure of aspiration, the two pulmonary 

 sacs within the thoracic cavity are maintained in a state of distention; therefore 

 the vessels of air-containing lung are more dilated than the vessels of collapsed 

 lung. Consequently, more blood flows through the lungs when they are distended 

 within the thorax than when they are collapsed. Inspiratory distention has a 

 similar effect and increases the flow of blood. The negative pressure prevailing 

 in the lungs during inspiration causes a considerable dilatation particularly of 

 the pulmonary veins, into which vessels, therefore, the pulmonary blood readily 

 flows; whereas the blood of the pulmonary artery, flowing through thick-walled 

 trunks under high pressure, undergoes scarcely any alteration. The velocity of 

 the blood in the pulmonary vessels is, therefore, increased during inspiration. 



The blood-pressure in the lesser circulation is higher also when the lungs are 

 in a state of distention. Contraction of the vessels, which causes an increase of 



