Respiration 1 7 1 







258. The Gases of the Blood. The blood contains three 

 gases, partly dissolved in it and partly in chemical union 

 with certain of its constituents. These are oxygen, carbon 

 dioxide, and nitrogen. The nitrogen need not be taken into 

 account. 



The oxygen is the material which the tissues require to 

 carry on their work. 



The carbon dioxide is a waste substance which the tissues 

 produce by their activity, and which the blood carries away 

 from them. 



^ 259. Effects of Respiration upon the Blood. As we have 

 learned in a previous chapter, the arterial blood loses part of 

 its oxygen in the tissues, and carbon dioxide takes its place. 

 Now if the blood is to maintain its efficiency in this respect, 

 it must always be receiving new supplies of oxygen, and 

 also have some mode of throwing off its excess of carbon 

 dioxide. This, then, is the double functign of respiration. 



Again, the blood sent out from the left side of the heart 

 is of a bright scarlet color. After the blood has passed 

 through the tissues and returns to the right side of the 

 heart, it is of a more purple or claret color. This change 

 in color takes place in the systemic capillaries and is due 

 to the fact that there the blood gives up most of its oxygen 

 to the tissues and receives from them a great deal of carbon 

 dioxide. 



In brief, while passing through the capillaries of the 

 lungs the blood is changed from the venous to the arterial 

 condition. That is to say, the blood in its progress through 

 the lungs rids itself of its excess of carbon dioxide and obtains 

 a fresh supply of oxygen. 1 



1 The student must guard himself against the idea that arterial blood 

 contains no carbonic acid, and venous blood no oxygen. In passing through 

 the lungs venous blood loses only a part of its carbonic acid; and arterial 



