264 THE CIRCULATION OF THE BLOOD 



CIRCULATION THROUGH THE LUNGS 



The pulmonary or lesser circulation, as it is called, is quite different 

 from the systemic circulation. In the first place, because the pressure 

 in the pulmonary arteries does not amount to more than about 20 mm. 

 Hg, or about one-sixth of that of the systemic arteries, the peripheral 

 resistance in the blood vessels of the lungs is much less than that of 

 the body in general. This lower resistance is owing partly to the large 

 diameter of the arterioles and the small amount of muscular fibers in 

 their walls, and partly to the fact that the capillaries are held con- 

 stantly in a somewhat dilated condition on account of the subatmos- 

 pheric pressure in the thorax (see page 323). 



Another peculiarity of the pulmonary circulation is that the caliber 

 of the vessels is to a very large extent dependent upon the changes 

 that occur in the intrathoracic pressure with each inspiration and ex- 

 piration. They become dilated on inspiration and contracted on ex- 

 piration. The extent to which these respiratory changes affect the 

 amount of blood contained in the lungs, is very considerable. At the 

 height of inspiration it is computed that a little more than eight per 

 cent of the whole blood in the body is contained in the lungs, whereas 

 on expiration it diminishes to between five and seven per cent. 



A third peculiarity is that the pulmonic blood vessels are not sup- 

 plied with vasomotor nerve fibers at least with such as can readily be 

 demonstrated. It is said that, when the pulmonary vessels are per- 

 fused and the outflow measured, a diminution in the latter is found to 

 occur when epinephrine is added to the injection fluid a result which 

 is, however, denied by certain investigators. Changes in the bloodflow 

 have not been observed to occur when the vagus or sympathetic nerve 

 fibers running to the lungs are stimulated. In short, the conclusion 

 which we must draw is much the same as that for the blood vessels 

 of the brain namely, that although, as a result of the epinephrine ex- 

 periment, we must admit that a vasomotor supply may possibly be 

 present, yet it is one which can be of no significance under normal 

 conditions. 



When there is obstruction to the outflow of blood from the left ven- 

 tricle, as, for example, in cases of high aortic pressure, the blood is not 

 entirely discharged with each beat of the left ventricle, and therefore 

 dams back through the left auricle into the lungs. On account of the 

 marked distensibility of the pulmonary capillaries, a large amount of 

 this blood may collect there and thus make the lungs serve as a kind of 

 reservoir of the heart. When the capacity of this reservoir has, how- 

 ever ? been overstepped, an increased peripheral resistance will come to 



