RESPIRATION 



325 



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 

 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- 



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.) 



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 

 defecation and parturition, as well as in the first stages of coughing; it 

 is also produced by blowing into a tube, or against some resistance. 

 On account of the positive pressure that is brought to bear on the veins 

 as they enter the thorax, the venous pressure suddenly rises, slowing 



