i6 4 



RESPIRATION 



back upon and distends the heart, so that this organ is finally 

 paralyzed and ceases to beat. This is the cause of death 

 from asphyxia. 



Effect of Respiration on Blood-Pressure. The lowest 

 blood-pressure is just after the beginning of inspiration, 

 from which time it increases during inspiration to reach its 

 maximum just after the beginning of expiration; it gradu- 



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



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



ally decreases from this time to the minimum just after the 

 beginning of inspiration. The general effect, then, of in- 

 spiration is to increase 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 arterial system at each ventricular systole. The explana- 

 tion is somewhat complex, but if the mechanics of respira- 

 tion be understood it may be made satisfactory. 



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

 cavity, the chest, and that they expand with the chest be- 

 cause of negative 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 communicating with smaller extrathoracic ves- 

 sels. The heart and these great thoracic vessels are elastic 

 and distensible. Consequently the expansion of the thorax 



