CHAPTER XXVIII 

 CIRCULATION IN SPECIAL PARTS 



The Pulmonary Circulation. The pulmonary artery, carrying 

 venous blood, divides and subdivides, and the smallest branches end 

 in a plexus of capillaries on the walls of the air cells of the lung. From 

 this plexus the blood is drained by the radicles of the four pulmonary 

 veins which open into the left auricle. The pressure in the pulmonary 

 artery has been found to be 12 to 30 mm. Hg that is, from 

 one-third to one-sixth of the aortic pressure; the blood also 

 takes only one -third of the time to complete the pulmonary circuit 

 that it takes to make the systemic. The four chief factors which 

 influence the pulmonary circulation are (1) the force and output of 

 the right ventricle; (2) the diastolic filling action of the left auricle 

 and ventricle; (3) the diameter of the pulmonary capillaries, which 

 varies with the respiratory expansion of the lungs; (4) the intra- 

 thoracic pressure. 



In inspiration, the lungs are distended in consequence of the greater 

 positive pressure on the inner surfaces being greater than the negative 

 pressure on their outer pleural surfaces. The negative pressure in 

 the intrathoracic cavity results from the enlargement of the thorax 

 by the inspiratory muscles. When the elastic lungs are distended by 

 a full inspiration, they exert an elastic traction amounting to about 

 15 mm. Hg. The heart and vessels within the thorax are submitted 

 to this traction that is, to the pressure of the atmosphere . minus 

 15 mm. Hg while the vascular system of the rest of the body bears 

 the full atmospheric pressure. The thin-walled auricles and veins 

 yield more to this elastic traction than the thick-walled ventricles 

 and arteries. Thus, inspiration exerts a suction action which furthers 

 the filling of the veins and auricles. This action is assisted by the 

 positive pressure exerted by the descending diaphragm on the con- 

 tents of the abdomen. Blood is thus both pushed and sucked into 

 the heart in increased amount during inspiration. 



Experiment has shown that the bloodvessels of the lungs when 

 distended are wider than those of collapsed lungs. Suppose an elastic 

 bag having minute tubes in its walls be dilated by blowing into it, 

 the lumina of the tubes will be lessened, and the same occurs in the 

 lungs if they are artificially inflated with air ; but if the bag be placed 

 in a glass bottle, and the pressure on its outer surface be diminished 

 by removing air from the space between the bag and the side of the 

 bottle, the bag will distend and the lumina of the tubes be increased. 

 Thus, it seems that inspiration, by increasing the calibre of the pul- 

 monary vessels, draws blood into the lungs, and the movements of 

 the lungs become an effective force in carrying on the pulmonary 



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