1002 PHYSIOLOGY 



the stimulation is continued, whereas in the latter case the effect shows signs 

 of fatigue and disappears before the excitation is shut off. 



So far as the general blood-pressure is concerned the most important 

 impulses arriving at the centre are those from the vascular system, especially 

 from the heart itself, and those from the higher parts of the brain. Whatever 

 the condition of the heart the brain always demands a normal arterial 

 pressure, since on this depends the supply of a proper quantum of blood to the 

 master tissues of the body. A failing heart therefore evokes indirectly con- 

 striction of the blood-vessels, a fact which may lead to a vicious circle in 

 cases where the heart is unable to perform its normal functions and to 

 empty itself against the resistance of the blood-vessels. In this case the 

 heart dilates more and more, until the slightest increase in the demands upon 

 it, as by a slight muscular exertion, may suffice to stop its action altogether. 



Under normal circumstances every part of the body receives just so 

 much blood as it needs for its metabolic requirements. Hence activity must 

 be associated with an increased flow of blood through the part. Two 

 mechanisms are involved in the production of this adaptation. In the first 

 place, stimuli arising in any part of the body many affect the vascular system 

 in two directions, causing reflexly dilatation of blood-vessels in the part 

 which is the origin of the impulses and constriction of the blood-vessels in 

 the rest of the body, so that a normal or raised blood-pressure is available 

 for driving an increased supply of blood through the dilated vessels of the 

 part. Thus, if both hind limbs of an animal be placed in a plethysmograph, 

 it will be seen that stimulation of the anterior crural or peroneal nerve in 

 the left leg causes dilatation of this leg and constriction of the leg of the other 

 side. At rest the organs of the chest and abdomen contain more than half 

 of the total quantity of blood in the body, so that very little change in the 

 capacity of these organs suffices to furnish the extra supply of blood needed 

 by any part during a state of increased activity. 



THE CHEMICAL REGULATION OF THE BLOOD-VESSELS 

 Another factor which is possibly involved in the production of the 

 increased blood- flow through active organs is a chemical stimulation of the 

 vessels themselves, by means of substances (metabolites) produced as a 

 result of the chemical changes accompanying activity. The great increase 

 in the flow through the muscles which accompanies muscular exercise is 

 probably brought about largely by this means. It has been shown that the 

 passage of blood containing lactic acid or carbon dioxide (both results of 

 muscular metabolism) causes a marked dilatation of the blood-vessels of a 

 limb. The Table on the next page shows the influence of activity on the 

 blood- flow through various organs. 



We thus see that carbon dioxide, which is the universal hormone set free 

 in the circulation when the activity of the body as a whole is increased, has . 

 a double effect on the blood-vessels a central effect through the vaso- 

 motor centres, medulla and spinal cord, causing contraction of the blood- 

 vessels, and a local peripheral effect causing dilatation of the blood-vessels. 



