EESPIRATORY UNDULATIONS. 401 



arterial constriction, begins to fall, since less and less blood is pumped into 

 the arterial system ; the boldness of the pulse-curves at this stage is chiefly 

 due to the infrequency of the strokes. As the quantity which passes from 

 the heart into the arteries becomes less second by second, the pressure gets 

 lower and lower, the descent being assisted by the exhaustion of the vaso- 

 motor centre, until almost before the last beats it has sunk to zero. Thus at 

 the close of asphyxia, while the heart and venous system are distended with 

 blood, the arterial system is less than normally full. 



332. While changes occurring primarily in the respiratory system thus 

 affect the vascular system, conversely changes occurring primarily in the 

 vascular system affect the respiratory system. Two kinds of change in the 

 vascular system bearing on two parts of the respiratory system deserve 

 special attention. 



In the first place the respiratory mechanism may be affected by changes 

 in the blood-supply to the respiratory centre in the medulla. We have 

 already seen ( 314) that the sudden cutting off of the supply of blood to 

 the medulla gives rise to dyspnoeic respiratory movements and may lead to 

 expiratory convulsions. That is an extreme case ; but, short of that, the 

 activity of the respiratory centre, the extent and character of the respiratory 

 explosions which take place in it, may be varied according as the constricted 

 or dilated condition of the small arteries branching off from the basilar 

 artery or of the basilar artery itself allows a scanty or a full flow of blood 

 through the medulla. And it is possible that some forms of dyspnoea may 

 be brought about in this way. 



Much more common and important, however, is the second kind of 

 change, that affecting the circulation through the lungs. In the normal 

 organism an adequate supply of arterial blood to the tissues is secured by an 

 adequate renewal of the air in the pulmonary alveoli, and an adequately 

 rapid flow of blood through the pulmonary capillaries. When, as by ob- 

 struction in the pulmonary arteries, or by failure of the cardiac valves, or, 

 and perhaps especially, by an insufficient cardiac stroke, the stream of blood 

 from the lungs into the left ventricle is lessened either in amount or in 

 rapidity, less oxygen is carried to the tissues, including the nervous tissue of 

 the medulla, and dyspnoea or " want of breath " follows. When the circu- 

 lation through the lungs is in full healthy swing, the haemoglobin of the red 

 corpuscles is as we have seen saturated or nearly saturated with oxygen. If 

 owing to a slower stream the red corpuscles tarry longer in their passage 

 along the walls of the pulmonary alveoli they cannot thereby take up a 

 compensating addition of oxygen, indeed, it is doubtful if they can take up 

 any additional oxygen at all. The blood falling under these circumstances 

 into the left ventricle and sent thence over the body is not more arterial 

 than usual ; at the same time the amount of blood sent out at each heart- 

 stroke is less, often much less, than the normal ; and the medulla as well as 

 other tissues suffer in consequence from a deficiency of oxygen. The de- 

 ficient supply to the medulla manifests itself in dyspnoeic or at least in 

 labored breathing, which sometimes, through the mechanical influences 

 discussed above, has the happy result of improving the pulmonary circula- 

 tion and so produces compensating effects. When the pulmonary artery is 

 suddenly plugged with a clot the primary and urgent symptom is " want 

 of breath," though air enters freely into the chest ; and " cardiac dyspnoea " 

 is a common symptom of cardiac disease. 



333. Other systems of the body are also related to the respiratory sys- 

 tem, though by ties less striking than those which bind to it the vascular 

 system. We have seen that deficient arterialization of the blood stirs up the 

 muscles of the alimentary canal to increased activity, and we shall presently 



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