324 RESPIRATION 



far as the heart and great vessels are concerned, never quite equals the intra- 

 pulmonary pressure, and at the conclusion of inspiration is considerably 

 less than the atmospheric pressure. It has been ascertained that the amount 

 of the pressure used up in the way above described varies from 5 to 7 mm. of 

 mercury in ordinary inspiration, to 30 mm. of mercury at the end of a deep 

 inspiration. So it will be understood that the pressure to which the heart 

 and great vessels are subjected diminishes as inspiration progresses, and at 

 its summit is less by from 7 to 30 mm. than the normal atmospheric pres- 

 sure, 760 mm. of mercury. It will be understood from the accompanying 

 diagram how an increase in the volume of the thorax will have the effect of 

 pumping blood into the heart from the veins. During inspiration the pres- 

 sure outside the heart and great vessels is diminished, and they, by virtue of 

 their elasticity, have therefore a tendency to expand and to diminish the intra- 

 vascular pressure. The diminution of pressure within the veins passing 

 to the right auricle and within the right auricle itself, will draw the blood 

 into the thorax, and so assist the circulation. This suction action of the 

 thorax is the cause of the slight negative pressure of the ventricles previously 

 described. The effect of more blood in the right auricle will, cateris paribus, 

 increase the amount passing through the right ventricle, and through the 

 lungs into the left auricle and ventricle, and thus into the aorta. This all 

 tends to increase the blood-pressure. The effect of the diminished pressure 

 upon the pulmonary vessels will also help toward the same end, an increased 

 flow through the lungs, so that, as far as the mechanical effects on the heart 

 and its veins are concerned, inspiration increases the blood-pressure in the 

 arteries. The effect of inspiration upon the aorta and its branches within 

 the thorax would be, however, contrary; for as the external pressure is dimin- 

 ished, the vessels would tend to expand, and thus to diminish the tension of 

 the blood within them, but, inasmuch as the relative variation in pressure 

 on the large arteries is slight, the diminution of arterial tension caused by 

 this means will be insufficient to counteract the increase of blood-pressure 

 produced by the effect of inspiration upon the volume of discharge of the 

 veins of the chest, and the balance of the whole action would be in favor of 

 an increase of blood-pressure during the inspiratory period. When a blood- 

 pressure tracing is taken at the same time that the respiratory movements 

 are being recorded, it will be found that, although, speaking generally, the 

 arterial tension is increased during inspiration, the maximum of arterial 

 tension does not correspond with the acme of inspiration, figure 243. In 

 fact, at the beginning of inspiration the pressure continues to fall for a brief 

 moment, then gradually rises until the end of inspiration, and continues to 

 do so for a moment after expiration has commenced. For explanation of 

 the influence of heart rate in this variation of blood-pressure, associated 

 with the respiratory movement, see page 212. 



In ordinary expiration all this would be reversed, but if the abdominal 



