CHEMICAL CONTROL 91 



nerves are cut, but this difference can be explained as due 

 to the direct stimulating effect of the carbonic acid upon 

 the vagus centre. 



THE CHEMICAL CONTROL OF THE BLOOD-VESSELS 



(a) Metabolites. — It has long been known that blood 

 percolates more freely through an organ as its content of 

 CO2 rises. Other acids — as, for example, lactic acid — 

 have a like effect. This is due to a direct effect upon the 

 arterioles and, as we shall see later on, the capillaries. 

 When a tissue such as a muscle or gland becomes active, 

 the acids produced dilate the neighbouring blood-vessels. 

 But the same substances passing into the blood-stream 

 stimulate the vaso-motor centre. These acids therefore 

 produce two contrary effects- — a dilator effect, which is 

 local, and a constrictor effect, which is general. Locally, 

 the dilator effect is greater than the constrictor. The result 

 will therefore be an increased flow of blood through the 

 active organ, and a decreased flow through the inactive 

 tissues — in particular the viscera. More blood is diverted 

 to the tissues which require it. 



(6) Pressor Substances. — ^Adrenalin, the product of the 

 suprarenal glands, has the same effect upon any organ as 

 stimulation of the sympathetic nerve. It is itself dis- 

 charged into the blood-stream when the sympathetic fibres 

 to the suprarenal are stimulated. It follows, therefore, that 

 when the sympathetic system enters upon a state of 

 increased activity, as in emotional states or asphyxia, 

 the physiological effect may be caused directly by nervous 

 impulses passing to the various organs, or indirectly to 

 the secretion of adrenalin. There is evidence that in the 

 resulting rise of blood-pressure both factors contribute. 

 It is sometimes found that in asphyxia rise of blood- 

 pressure occurs in two stages : the first due to stimulation 

 of the vaso-constrictor fibres, the second due to the action 

 of adrenahn poured into the circulation. 



