CHAPTER XII 

 THE BLOOD 



THE intestinal lining is a barrier between the mixture 

 of food and secretions within the canal and the blood 

 which flows through the neighboring vessels. The main 

 problem to be dealt with in treating of absorption is the 

 transfer of portions of the intestinal contents to the blood. 

 We shall find that a certain fraction of the incoming nutri- 

 ment is carried for a time in the lymph, but this is destined 

 before long to blend with the main current of the circula- 

 tion. It will be for our advantage to become somewhat 

 familiar with the make-up and service of the blood before 

 we discuss the entrance into it of the products of digestion. 



Blood is a carrier. Regarding it as such, we can con- 

 veniently subdivide its functions according to the classes 

 of material which it conveys. Most people will think of it 

 as, first of all, the bearer of food to the tissues. This is, 

 indeed, a matter of prime importance. The food is added 

 to the blood mainly as it flows through the capillaries of the 

 wall of the alimentary canal x and taken from it by the 

 various parts of the living body in proportions correspond- 

 ing with the degree of the local activity. The largest tax 

 is that levied by the skeletal muscles. An unfailing sup- 

 ply of oxygen is a need even more urgent from moment to 

 moment than the presentation of food. Oxygen is added 

 to the blood in the lungs, and is withdrawn from it as it 

 makes its round through the body, the quantity consumed 

 in different organs being an even better measure of their 

 individual metabolism than is their appropriation of food. 

 Where oxygen is taken from the blood, carbon dioxid is 

 returned to it. This indicates that the blood is a bearer of 

 wastes. The excess of carbon dioxid makes its escape dur- 



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