RESPIRA TION 



rest of this period. At the commencement of expiration it is still 

 mounting, but soon reaches its maximum, begins to fall, and con- 

 tinues falling through the remainder of the expiratory phase. 



A partial explanation is afforded by a consideration of the mechan- 

 ical changes produced in the thorax by the respiratory movements. 

 Of these, the influence of variations in the intrathoracic pressure 

 on the filling of the heart is of special importance. With deep 

 abdominal breathing the changes of intra-abdominal pressure also 

 affect the filling of the heart, an increase of pressure (in inspiration) 

 tending to cause more blood to be squeezed from the abdominal 

 veins towards the chest. The changes of vascular resistance in the 

 lungs, due to the alteration in the calibre of the pulmonary vessels, 

 may also contribute, but, for such variations of intrathoracic 

 pressure as normally occur, only in a minor degree. The changes 

 in the vascular capacity of the lungs that is, in the amount of 

 blood contained in the pulmonary vessels are of importance espe- 

 cially in delaying or accelerating the alterations of blood- pressure in 

 the systemic arteries due to the other factors. 



The intrathoracic pressure, which, as we have seen, is always less 

 than that of the atmosphere, unless during a forced expiration when 

 the free escape of air from the lungs is obstructed, diminishes in 

 inspiration and increases in expiration. The great veins outside the 

 chest, the jugular veins in the neck, for example, are under the 

 atmospheric pressure, which is readily transmitted through their 



thin walls, while the heart and 

 thoracic veins are under a 

 smaller pressure. The venous 

 blood both in inspiration and ex- 

 piration will, accordingly, tend 

 to be drawn into the right 

 auricle. In inspiration the ven- 

 ous flow will be increased, since 

 the pressure in the thorax, and 

 therefore in the pericardial 

 cavity, is diminished; and upon 

 the whole more venous blood 

 will pass into the right heart 

 during inspiration than during expiration. Now, the right ventricle is 

 not in general working as hard as it can work. Hence, the excess of 

 blood which reaches it during an inspiration is at once sent into the 

 lungs, although not even the first of it can have passed through to 

 the left side of the heart at the end of the inspiration, since the 

 pulmonary circulation-time (four to five seconds in a small dog, 

 two to three seconds in a rabbit) is longer than the time of a com- 

 plete inspiration at any ordinary rate. The increase in the quantity 

 of blood pumped into the pulmonary artery will, if not counteracted 

 by other circumstances, tend to raise the blood-pressure in the 



Fig. 128. The upper tracing shows the 

 respiratory movements in a rabbit with 

 rather deep and slow diaphragmatic 

 breathing; the lower tracing is the 

 blood-pressure curve; /, inspiration; 

 E, expiration, including the pause. 



