SECTION VII 

 THE PULMONARY CIRCULATION 



IN the lungs there is an extensive system of wide capillaries pre- 

 senting 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 determination of the normal average pressure 

 in the pulmonary artery presents considerable 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 sys- 

 temic blood pressure and are largely responsible for the respiratory 

 variations observed in the systemic blood pressure. On the other 

 hand, the distensibility of the lung capillaries may play an impor- 

 tant 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 doubtful. Brodie and Dixon obtained no vaso -constriction in the 

 lungs on injection of adrenalin, a drug which causes excitation of the 

 sympathetic vaso -constrictor fibres in all other parts of the body. 



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 con- 

 tinues for a short time after the beginning of expiration before it 



1053 



