AN INTRODUCTION TO ANIMAL BIOLOGY 55 



supplying food and oxygen to these animals ( 45, 48), 

 there is no need of blood as a carrier of excretions. 



51. Respiration. This is a term which has long been used 

 in physiology as almost a synonym for the popular word 

 " breathing." It includes two of the processes which have 

 been described ; namely, taking in oxygen and giving out or 

 excreting carbon dioxide. In most animals the two processes 

 go on at the same time and in the same organs. Air taken 

 into the lungs supplies oxygen to the blood and at the same 

 time absorbs carbon dioxide from blood circulating in the 

 lungs ; and hence the lungs are often called respiratory 

 organs. Keep in mind for use in future lessons that respira- 

 tion includes (1) supplying oxygen, and (2) excreting carbon 

 dioxide. 



52. Summary of Functions of the Blood. We have 

 found that blood and a mechanism for its circulation are 

 necessary in the frog for communication between the living 

 cells and certain organs which communicate with the ex- 

 ternal world. As we have seen, all cells must have a supply 

 of food and oxygen and must get rid of the substances 

 (excretions) resulting from the oxidation of cell-substances. 

 Some small animals have all their cells near the places where 

 food and oxygen must be absorbed and excretions eliminated ; 

 and for this reason such small animals need no blood-system. 

 But in the frog, and in all except the simplest animals, there 

 are cells at some distance from the organs which supply oxygen 

 and food, and also far from those which eliminate excretions ; 

 and in all such animals the blood acts as a transporting 

 medium which (1) carries food and oxygen to the cells from 

 the organs (lungs and digestive organs) which obtain these 

 substances directly from the exterior, and (2) carries ex- 

 cretions from the cells to the organs (lungs, skin, or kidneys) 

 which pass them out of the body. In animals higher than 

 frogs and reptiles the blood is also important in distributing 

 heat. 



