THE NERVOUS CONTROL OF THE BLOOD VESSELS 1045 



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 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. 



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 given below 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. The general result therefore will be to cause dilatation of the 

 blood vessels of the part where the carbon dioxide is produced and where it 

 is present in greatest concentration, and vascular constriction elsewhere 

 under the influence of the sensitive nervous centres. 



FLOW IN CUBIC CENTIMETRES PER MINUTE PER 100 GRM. TISSUE 



