164 



RESPIRATION 



blood-pressure and of expiration to decrease it. This remark 

 applies to general arterial tension. 



Taking inspiration, the increase in arterial tension is, in its 

 last analysis, due to the larger amount of blood sent into the arte- 

 rial system at each ventricular systole. The explanation is some- 

 what complex, but if the mechanics of respiration be under- 

 stood it may be made satisfactory. 



FIG. 53. Carotid blood-pressure tracing of a dog. 

 Vagi not divided; I, inspiration; E, expiration. (Stirling.) 



It was seen that the lungs are contained in an air-tight cavity, 

 the chest, and that they expand with the chest because of nega- 

 tive pressure (" suction") exerted upon them. The heart is also 

 a hollow organ situated in this cavity; it has connected with it, 

 and lying also in the thoracic cavity, large vessels communicat- 

 ing with smaller extrathoracic vessels. The heart and these 

 great thoracic vessels are elastic and distensible. Consequently 

 the expansion of the thorax also expands them slightly and tends 

 to draw blood from the extrathoracic into the intrathoracic 

 vessels and heart; in fact inspiration is one of the main forces 

 causing a flow of venous blood toward the heart. Now all this, 

 so far as it goes, tends to keep the blood out of the extrathoracic 

 vessels, and so to contradict the statement that inspiration 

 increases arterial tension. 



But, remembering that we are dealing with arterial tension 



