THE INFLUENCE OF RESPIRATION. 



125 



In smaller animals, such as the dog and rabbit, the negative pres- 

 sure within the pericardium is equal to - 3 to - 5 mm. Hg, rising in 

 dyspnoea to - 9 mm. Hg. 1 



Within the lungs the tension of the air varies but slightly during 

 the phases of quiet respiration. In inspiration the pressure of the 

 intratracheal air equals -O'l mm. Hg; in expiration +0*13 mm. Hg. 

 In certain conditions, however, the intra-pulmonary pressure may vary 

 enormously. Thus in Valsalva's experiment, when, after a deep in- 

 spiration, the mouth and nose are shut and a forcible expiratory move- 

 ment is made, the intra-pulmonary pressure may rise to 100 mm. Hg. 

 On the other hand, in Miiller's experiment, when, after a deep expira- 

 tion, a forced inspiratory movement is executed with the mouth and 

 nose shut, the negative pressure within the thorax and lungs may sink 

 as much below zero. Thus it seems clear that, while in quiet in- 

 spiration the extrathoracic vessels lie under the full pressure of the 

 atmosphere, the intrathoracic vessels are exposed to a pressure of 



- 9 mm. Hg. Likewise the pressure on the pulmonary capillaries = 



- 0*1, while the pressure on the pulmonary arteries and veins within 

 the thoracic cavity = 9 mm. Hg. In Valsalva's experiment both 

 the intrathoracic and intra-pulmonary pressure will increase, but the 

 latter will always remain higher than the former by the amount of 

 elastic tension exerted by the lungs. In Miiller's experiment both 

 pressures will diminish, but the intra-pulmonary pressure will again 

 remain higher than the intrathoracic pressure by the amount of elastic 

 tension exerted by the lungs. 



The changes in intrathoracic pressure naturally affect the calibre of 

 the thin-walled inelastic veins and auricles far more than the elastic and 

 thick-walled ventricles and arteries. The veins and auricles are flaccid 

 bags, which can be easily distended up to their full capacity, while to 

 expand the arteries and the ventricles a considerable amount of force is 

 required. A negative pressure of 9 mm. Hg has but an insignificant 

 effect on the capacity of the aorta, wherein the tension is equal to 

 120 mm. Hg, or more. The total effect of the negative pressure within 

 the thorax is to increase, during inspiration, the diastolic filling of the 

 heart, while the systolic output is not materially hindered. 



Experimental facts. 1. Poiseuille 2 discovered that the circulation 

 of a certain volume of blood through 

 the pulmonary vascular system occu- 

 pied a longer time when the lungs 

 were blown out than when they were 

 collapsed. 



2. On microscopical examination 

 of the pulmonary circulation in the 

 frog, it can be seen that the blood- 

 flow is less rapid the more the lungs 

 are distended. 



3. If the thorax be opened in a 

 mammal, and artificial respiration 



supplied, the aortic pressure mounts when the lungs collapse, while it 

 falls when the lungs are expanded. 



4. If the lungs be excised and placed within an artificial thorax, and 



1 Adamkiewicz and Jacobson, CentralbLf. d. med. Wissensch., Wien, 1873, S. 483. 



2 Compt. rend. Acad. d. sc., Paris, 1852, tome xli. p. 1072. 



FIG. 79. 



