EFFECT OF RESPIRATION ON CIRCULATION 401 



of CO2 in alveolar air is maintained at an almost constant level 

 of about 40 mm. Hg (see Chap. XXXI.). 



Regulation of Depth. Impulses are constantly passing from 

 the lungs, through the N^agi, to the respiratory centre and, by a 

 reflex act, inspiration is checked when a certain tension is set 

 up in the lung substance, i.e. the respiratory mechanism carries 

 out the inspiratory phase of its function till this stretch-reflex 

 inhibits it. In cases where the vagi are hyper-irritable, the stop 

 mechanism acts too soon and breathing becomes shallow and 

 rapid. 



Effect of the Respiratory Movements on Mass Movements of the Blood. 



Thoracic Respiration. During each inspiration the increase in 

 the capacity of the thorax not only tends to produce a negative 

 pressure in the trachea and pharynx, so causing air to be sucked 

 into the lungs, but negative pressure is effectively applied to the 

 thin-walled veins in the thoracic cavity, causing them to dilate 

 with blood aspirated from the extra- thoracic venous system. 

 This dilatation causes more blood to collect in the intrathoracic 

 vessels, and to some extent delays the passage of this blood to 

 the atria. During expiration, however, positive pressure is applied 

 to these veins ; the extra blood is forced into the heart. As we 

 have seen in a previous chapter, the increased venous filling leads 

 to an increased ventricular output and a rise of blood-pressure. 



Diaphragmatic Respiration. During inspiration the diaphragm 

 contracts and presses down on the abdominal viscera. They, in 

 turn, pass on the pressure to the blood in the vena cava, with the 

 result that, at first, more blood is forced into the thoracic vena 

 cava, and, consequently, to the right atrium ; and the blood pressure 

 tends to rise a little. Continued compression of the abdominal 

 vena cava cuts down the supply of blood to the thorax, and blood 

 pressure falls. During the expiratory relaxation of the diaphragm 

 the reverse process takes place. 



Most people breathe diaphragmatically with a slight rising of the 

 thoracic cage. The effect on their blood pressure will, therefore, 

 be the algebraic sum of the two opposing effects : viz. an initial 

 rise due to thoracic negative pressure and abdominal pressure 

 on the veins. This rise, which would continue in purely chest 

 breathing, is cut short by the fall of pressure due to continued 

 abdominal pressure. 



Exercise. The above factors are more marked in their influence 

 on cardiac output during ordinary nniscular exercise. There are 

 certain types of exercise, however, in which, in order to produce a 

 maximum leverage (q.v.), the chest wall is fixed and respiration is 



B. 26 



