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, the pressure in the pul- 

 monary arteries does not amount to more than about 20 mm. Hg, or 

 about one-sixth of that of the systemic arteries, so that the peripHeTat 

 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 iTit.rai.Tim^^jft_jrrfissLUJ!fi with eagli 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 although nerve fibers have been seen run- 

 ning to the w T alls of the pulmonic blood vessels no changes in their caliber 

 can be demonstrated, in mammals, by stimulation of the vagus or sympa- 

 thetic. In the frog, however, constriction of the pulmonary artery occurs 

 when the vagus is stimulated (Carlson and Luckhardt). When the pul- 

 monary vessels are perfused and the outflow measured, a diminution 

 in the latter is said to occur when epinephrine is added to the injection 

 fluid. In short, the conclusion which we must draw is much the same as 

 that for the blood vessels of the brain namely, that although we must 

 admit that a vasomotor supply may be present, yet it is one which is so 

 feeble that it is " readily overpowered by changes in the * * * dis- 

 charge of the right heart or by back pressure effects from the systemic 

 side of the circulation" (Wiggers 57 ). 



When the venous inflow to the right heart is increased, the discharge 



into the pulmonary artery correspondingly increases, in obedience to the 



law of the heart (page 216) and a certain degree of congestion of the 



fpulmonary vessels results. But when this becomes sufficient to cause an 



[effect on the blood flow in the pulmonary veins, the left ventricle im- 



I mediately also responds by augmented output so that the engorgement 



I in the pulmonary circuit does not become excessive. It has indeed been 



I thought by some that the left ventricle is so much more sensitive to 



changes in venous inflow than is the right that it succeeds in preventing 



