152 THE HUMAN MECHANISM 



O. THE ADJUSTMENT OF THE CIRCULATION TO THE 

 NEEDS OF EVERYDAY LIFE 



The total quantity of. blood in the body (ten to fourteen 

 pints) is riot enough to furnish a working supply to all 

 organs at the same time ; and since, in general, whenever 

 an organ works it receives more blood, and when it is at rest 

 it receives less, our daily life with its changes of activity 

 among the organs makes necessary frequent adjustments of 

 the circulation to the needs of the organs at various times. 



Some of these adjustments are matters of familiar expe- 

 rience. The increased flow of blood to the skin on a warm 

 day makes the veins stand out and the face red, and we are 

 conscious of the more rapid heart beat during muscular 

 activity, even in an act so simple as running upstairs. Other 

 adjustments are not so evident, but betray themselves by 

 their results, as happens after a hearty meal when the 

 demand of the digestive organs for blood lessens the supply 

 to the brain and we feel disinclined to hard mental work. 

 We may begin our study of these adjustments by learning 

 what occurs in the circulation during some of the more 

 common activities and events of daily life. 



15. The circulation during exposure to heat and cold. When 

 the skin is exposed to cold its blood supply is greatly dimin- 

 ished ; the veins no longer stand out prominently on the hand, 

 and if a small area of skin be made pale by pressing upon it 

 (thus driving the blood out of its capillaries), the pallor passes 

 off very slowly. This simple experiment shows that blood is 

 flowing but slowly from the arterial reservoir into the skin. 

 Conversely, on a warm day the veins stand out prominently 

 and the red color instantly returns upon the removal of pres- 

 sure. These variations in the supply to the skin are due, as 

 we have already seen (p. 147), to changes in the diameter of 

 the arteries of the skin, which changes serve, like the spigot 

 of an ordinary water faucet, to regulate the flow of liquid. 



