RESPIRATION 307 



effects which we have seen are produced on the movement of air in and 

 out of the lungs by the changes in capacity of the thorax with each res- 

 piration, we naturally assume that the increase in blood pressure may 

 be due to the fact that on inspiration more blood is sucked out of the 

 systemic veins into those of the thorax, that this excess when it is pro- 

 pelled by the heart into the arteries raises the blood pressure, and that 

 on expiration the opposite condition obtains. That the movements of 

 the thorax on inspiration do accelerate the speed with which the venous 

 blood is traveling towards the heart can easily be shown by measure- 

 ments of bloodflow. 



This explanation, however, does not suffice to account for all the 

 changes of blood pressure which occur in respiration, for if we take 

 very accurate tracings of blood pressure and of the respiratory move- 

 ments side by side, we shall find that, although, in general, the blood 

 pressure rises with inspiration, yet the beginning of the rise is consid- 

 erably delayed ; that is, immediately following the beginning of the 

 inspiratory act the arterial blood pressure continues for some time to 

 fall, and at the beginning of expiration it continues for some time to 

 rise (Fig. 22). Moreover, it will be found, if tracings taken from dif- 

 ferent animals are compared, 'that frequently the general. effect of ex- 

 piration is to cause more rise than fall, and of inspiration more fall 

 than rise. It will be found that these differences are dependent largely 

 on the type of respiration, whether thoracic or abdominal (Lewis). 11 



Let us consider first of all exactly what will happen in an animal 

 breathing entirely by the thorax (e.g., the rabbit). The first 'effect of 

 the inspiration is to cause the veins leading to the auricles, the auricles 

 themselves and the blood vessels of the lungs to become suddenly ex- 

 panded. More blood therefore will flow into them. For a moment or 

 two this blood will, however, tend to stagnate in the more capacious 

 vessels, and it will be some time until it finds its way to the left side 

 of the heart; therefore the initial effect of inspiration is a distinct fall 

 in arterial blood pressure. When the extra space created in the blood 

 vessels has been filled with blood, that is, when inspiration has prac- 

 tically ceased, the blood will flow on in increased volume to the left 

 side of the heart, and, therefore, raise the arterial blood pressure. On 

 expiration the first effect is that the diminishing negative pressure will 

 cause the thin-walled vessels mentioned above to constrict and thus 

 squeeze the blood inside them into the left side of the heart and raise 

 the pressure ; but the ultimate effect in the later stages of expiration 

 will be that the vessels, being constricted, will allow less blood through 

 them and the arterial blood pressure will fall. 



Take now the case of abdominal respiration. 'In inspiration the dia- 



