CHAPTER XXV 

 THE CONTEOL OF THE CIRCULATION 



The available blood in the body is parceled out to the various organs 

 and tissues according to their relative activities, and, since these vary 

 from time to time, the question arises as to the nature of the mechanism 

 or mechanisms involved in bringing about this adjustment. Two possible 

 methods of increasing the supply are: an increase in the mass movement 

 of all the blood in circulation, and a reciprocal adjustment of the resistance 

 to the flow in different vascular areas brought about by vasodilatation 

 in one and vasoconstriction in others. Both of these methods might 

 operate together. 



Two agencies can be thought of as responsible for bringing about 

 the above changes: (1) chemical substances or hormones, present in 

 the blood, and (2) the nervous system. 



The influence of chemical substances, or hormones, (page 729) in the 

 control of the circulation is undoubtedly an important one, and of those 

 known at the present time two groups may be mentioned: (1) sub- 

 stances which alter the hydrogen-ion concentration of the blood, and 

 (2) so-called pressor and depressor substances, produced either by duct- 

 less glands, such as the adrenal, or by the activity of tissues. An in- 

 crease in hydrogen-ion concentration of the blood not only affects the 

 heartbeat (see page 168), but causes a marked dilatation of the Mood 

 vessels, so that both the central and the peripheral changes will be such 

 as to encourage an increased flow of blood through the active organs 

 or viscus. Thus, during muscular "activity of the leg muscles there will 

 be a tendency to an increase in the hydrogen-ion concentration of the 

 blood as a whole, resulting in a greater cardiac activity and a greater 

 outrush of blood through the aorta, and at the same time the vessels of 

 the acting muscle will have become especially dilated because of the 

 production by the active muscles either of lactic acid or of carbonic acid. 

 The active muscle also produces such substances as imidazole, which 

 have a powerful vasodilating action. Such substances are sometimes 

 called depressor. 



Though the hormone control of the circulation is undoubtedly of great 

 importance, it is probably much less so than that exercised through the 

 nervous system, and here again the control is centered partly in the 



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