126 



THE MECHANISM OF THE CIRCULATION. 



the natural expansion of the lungs be imitated by the aspiration of this 

 chamber, it is then found that a How of blood through the pulmonary 

 vessels is quickened by expansion and lessened by collapse. Experi- 

 ments 3 and 4 thus yield exactly contrary results. Now, on examining 

 the diagram (Fig. 79), it becomes obvious that the elastic tube which 

 passes through the thorax and along the base of the lung would be 



compressed by a rise of pressure in the 

 lung. On the other hand, if the thorax 

 were aspirated, this elastic tube would 

 expand. 1 



It must be noted, however, that the 

 conditions in the thorax are not exactly 

 imitated in this model, for the head 

 of pressure, or pump, should also lie 

 within the chamber. 



Funke and Latschenberger have 

 more exactly imitated the natural con- 

 ditions in the following way (Fig. 80). 2 

 The lungs and heart are placed in an 

 artificial thorax ; the pulmonary artery 

 is fed with defibrinated blood (tube 2) 

 from a pressure-bottle, which is so 

 arranged as to come under the influ- 

 ence of the intrathoracic pressure 

 (tube 6). The outflow from the left 

 auricle into the thoracic chamber is 

 measured (tubes 3 and 4). Under 

 these conditions an expansion of the 

 lungs brought about by thoracic aspira- 

 tion (tube 5) favours the flow, while 

 an expansion produced by blowing 

 down the trachea (tube 1) lessens the 

 pulmonary circulation. 



There are four chief factors which 

 influence the pulmonary circulation : 



1. The positive pressure in the force-pump, the right ventricle. 



2. The negative pressure in the suck-pump, the left auricle. 



3. The pressure of the alveolar air exerted on the pulmonary 

 capillaries. 



4. The calibre of the pulmonary capillaries dependent upon the 

 expansion of the lung. 



On artificial expansion of the lungs, the thorax being opened, the 

 third and fourth factors are increased, but the former is greater in 

 effect than the latter, and the circulation is lessened. By natural 

 inspiration the third factor is decreased, the second and fourth increased ; 

 the circulation is therefore augmented. 



In artificial respiration, when the lungs are blown out, the pulmonary 

 vessels are compressed by the rise of intra-pulmonary pressure. At 

 first the blood within them is expelled, and subsequently the flow of 

 blood through the lungs is diminished. When the lungs collapse the 



1 Quincke and Pfeiffer, Arch. f. PhysioL, Leipzig, 1871, S. 90; de Jager, Arch. f. d. 

 yea. PhysioL, Bonn, 1879, Bd. xx. S. 426. 



2 Arch./, d. ges. PhysioL, Bonn, 1878, Bd. xvii. S. 547. 



FIG. 80. Model illustrating the effect 

 of intrathoracic pressure on the 

 circulation. Funke and Latschen- 

 berger. 



