132 



RESPIRATION 



The respiratory movements have a marked effect upon the blood 

 pressure. In the carotid artery with every inspiration it rises, 

 and with every expiration it falls (Fig. 7). The two events 

 are, however, not exactly synchronous, the pressure changes 

 lagging a little behind the respiratory changes. The effects 

 are readily understood when it is remembered that the thoracic 

 cavity is an air-tight chamber, and that the elastic fibers of 



FIG. 7 



Comparison of blood-pressure curve with curve of intrathoracic pressure 

 (Foster) (dog) : a is the blood-pressure curve taken by means of a mercury 

 manometer; it shows the respiratory undulation, the slower beats on the 

 descent being very marked; b is the curve of intrathoracic pressure obtained 

 by connecting one limb of a manometer with the pleural cavity. Inspira- 

 tion begins at i, expiration at e. With the beginning of inspiration (i) 

 the expansion of the chest causes a marked fall of the mercury in the intra- 

 thoracic manometer; but the effect soon diminishes, since the lessening of 

 intrathoracic pressure does not bear on the manometer alone, but on the 

 lungs also; and as the lungs* expand more and more the fall in the mercury 

 becomes less and less until toward the end of inspiration the curve becomes 

 very nearly a straight line. Conversely, the return of the chest at the begin- 

 ning of expiration (e) produces at first a marked rise of the mercury in the 

 manometer; but this soon ceases as the air leaves the chest and the lungs 

 shrink, whereupon the mercury falls slowly. 



the lung which fill it are constantly pulling on the heart and 

 bloodvessels. This effect is increased during inspiration when 

 the thorax is enlarged. The pressure of the blood is conse- 

 quently lowered in the intrathoracic vessels, and the blood 

 rushes in from extrathoracic regions, where it exists under 

 atmospheric pressure. The effect of the pull of the lungs is 

 greater upon the flaccid walls of the veins than upon the more 

 rigid walls of the arteries, so that the inflow through the veins 



