SECTION VII 

 THE PULMONARY CIRCULATION 



IN the lungs there is an extensive system of wide capillaries presenting 

 very little resistance to the flow of blood. The arterioles are wide and have 

 only a slight amount of muscular fibre in their walls, so that a slight pressure 

 suffices to drive the blood from the right to the left heart. The determina- 

 tion of the normal average pressure in the pulmonary artery presents con- 

 siderable difficulties, but it probably does not exceed 15 to 20 mm. Hg., 

 i. e. about one-sixth of the mean aortic pressure. 



The capillaries of the lungs may vary passively in size according to the 

 condition under which they may be placed. Thus, whereas at the height of 

 inspiration the blood contained in the lungs is about one-twelfth of the 

 whole blood in the body, this amount is diminished during expiration to 

 between one-fifteenth and one-eighteenth, and by forcible artificial inflation 

 of the lungs may be lessened to one-sixtieth. These changes exercise a 

 considerable effect on the systemic blood pressure and are largely responsible 

 for the respiratory variations observed therein. On the other hand, the 

 distensibility of the lung capillaries may play an important part in enabling 

 the lungs to act, so to speak, as a reservoir for the left side of the heart. 

 If, in consequence of raised arterial pressure or other factor, there is a 

 temporary excess of output on the right side that cannot be dealt with, at 

 once by the left heart, the excess is taken up for a time in the lung 

 capillaries. 



Vaso-motor fibres to the lung vessels have been described as running in 

 the anterior roots of the third, fourth, and fifth dorsal nerves. Their action 

 is however of little importance, and their very existence is questioned 

 by some observers. The fact, that injection of adrenaline causes some vaso- 

 constriction in the lungs, points to the presence of a vaso-motor sympathetic 

 supply to those organs. 



If we examine a tracing of the arterial blood pressure, we notice that it 

 presents certain periodic oscillations which accompany the movements of 

 respiration. With each inspiration the blood pressure rises ; with each 

 expiration it falls. The synchronism of the rise and fall with the respiratory ' 

 movements is not exact, since the rise continues for a short time after the 

 beginning of expiration before it begins to fall, and the fall continues right 

 into the beginning of the next inspiration, so that the highest point of 

 the curve occurs at the beginning of expiration and the lowest point at the 

 beginning of inspiration. During the fall which accompanies expiration the 

 heart beats may become less frequent. This change of rate is marked in 

 the dog, but is by no means constant in man. On dividing both vagi, 



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