RESPIRATION 



309 



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 

 down the flow of blood through the capillaries and causing bulging of 

 the veins and, if the effect is sustained, cyanosis. On the arterial 

 side of the vascular system, after a momentary rise caused by the 

 squeezing out into the left side of the heart of the blood in the capil- 

 laries of the lungs, there is a more permanent fall in pressure due to 

 the fact that less blood is now getting from the right side to the left 

 side of the heart. After some time the pressure begins to rise again, 

 partly on account of the back pressure through the capillary vessels 

 and partly because of vasoconstriction as a result of asphyxial 

 conditions. 



In the opposite condition, during a forced inspiratory movement with 

 the glottis closed or with the mouth attached to some tube through 

 which the attempt is made to suck air, the thoracic cavities open up 

 without the lungs being able to occupy completely the extra space. 

 The dilatation of the veins and other thin-walled structures in the tho- 

 rax thus causes an immediate fall in both the venous and the arterial 

 pressure in the venous, because the blood is sucked toward the large 

 vessels in the thorax and lungs, and in the arterial, because the blood is 

 now delayed in its passage from the right to the left side of the heart. 

 If this condition is maintained, the arterial pressure may recover some- 

 what, but that in the veins is permanently lowered. 



