308 



THE RESPIRATION 



phragm descends and crowds the viscera against the vena cava, with 

 the result that at first more blood is squeezed into the thorax and the 

 blood pressure tends slightly to rise. After this initial effect, how- 

 ever, the compression of the vena cava causes less blood to reach the 

 thorax, and the arterial blood pressure falls. The conditions will be 

 exactly reversed on expiration. The initial effect of thoracic inspira- 

 tion is, therefore, to make the arterial blood pressure fall, and the in- 

 itial effect of abdominal inspiration, to make it rise. The net effect 



A . ABDOMEN. 



B. CHEST 



B.P.Lmt. 



C. ABDOMEN. 



Fig. 110. Effect of abdominal and chest breathing on the pulse and blood pressure of man. 

 Abdominal inspiration raises the pressure and diminishes the amplitude of the pulse curve. Thoracic 

 inspiration less clearly lowers the pressure. Expiration has the opposite effects. (From Lewis.) 



produced will be the algebraic sum of these two opposing influences 

 (see Fig. 110). 



Another factor that comes into play in determining the effect of the 

 respiratory movements on the cardiac output acts through the changes 

 in the pericardial pressure. When this is lowered, as early in inspira- 

 tion, it encourages diastole, thus causing better filling and therefore 

 better discharge from the heart. 



These considerations taken together make it easy to understand the 

 changes in blood pressure, particularly in the veins, which occur when 

 a forced inspiratory or expiratory movement is made with the glottis 

 closed. A forced expiration of this nature occurs during the acts of 



