316 THE CHEMISTRY OF ANIMAL PHYSIOLOGY 



(e) Gaseous Exchange in the Blood. — Reference has 

 been made occasionally to the fact that the blood in 

 one way or another carries oxygen and carbon dioxide. 



The oxygen is necessary to oxidize 

 dextrose and supply energy to the 

 various tissues. Carbon dioxide is a 

 result of the oxidation of dextrose and 

 must be eliminated. The lungs are the 

 seat of exchange between the oxygen 

 and the carbon dioxide. They are a 

 mass of tissue containing very many 

 minute cells or alveoli surrounded by 

 capillaries. The venous blood coming 

 into the left side of the heart is charged 

 with carbon dioxide. It is forced from 

 there to the lungs where carbon dioxide 

 is given off and oxygen taken in. This 

 renewed blood passes then to the right 

 side of the heart where it is discharged 

 into the arterial system. Fig. 80 gives 

 in diagrammatic form the circulation 

 of the blood. 



The lungs contract and expand by 



involuntary muscular effort as they 



exhale and inhale air. From Table 



XVIII, which gives the composition 



of 100 volumes of inspired and expired 



air, it is to be noted that the inspired air is richer in oxygen 



than the expired air, and contains much less carbon dioxide 



and water. 



Fig. 80. — Diagram of 

 blood circulation: L, 

 lung capillaries; c, other 

 capillaries; r, rV, right 

 compartments of heart; 

 I, IV, left compartments 

 of heart. 



Table XVIII. — Composition of Air 



100 volumes 

 Inspired air. Expired air. 



Oxygen 20.80 Oxygen 



Carbon dioxide. . . . trace Carbon dioxide 



Nitrogen 79.20 Nitrogen 



Water variable Water . 



Organic matter 



. 16.02 

 4.38 



. 79.60 



saturated 



trace 



From Table XIX, which gives the composition of 100 

 volumes of arterial and venous blood at 0° and 760 mm. 



