1128 



PHYSIOLOGY 



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 may 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. 



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) pro- 

 duced as a result of the chemical changes accompanying activity. 

 The great increase in the flow through the muscles which accom- 

 panies 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 following 

 figures may be given as showing the influence of activity on the blood- 

 flow through various organs : 



FLOW IN CUBIC CENTIMETRES PEE MINUTE PEE 100 GEM. TISSUE 



At rest 



Active 



Levator labii superioris (of the horse) 



Kidney . 



Hind limb . 



Hind limb (after section of nerves) 



Thyroid gland . 



Babbit's brain . 



17-5 



3'4 



9'9 



590-0 



136-0 



85 

 140 



