166 ANIMAL BIOLOGY 



C. Respiratory Interchange 



The respiratory mechanism has attained its objective when air 

 and blood are brought into such close relationship in the lungs 

 that gaseous interchange can occur, and heat be transferred. 

 Inhaled air varies very widely in temperature but in our homes is 

 perhaps most frequently about 20° C. (70° F.). Exhaled air is 

 about 36° C, or very nearly the same as that of the human body. 

 Thus under usual circumstances the exhaled air is warmer than 

 when it entered the lungs — the blood has lost heat. Again, the 

 amount of water in the inhaled air is variable, being low on a dry 

 day and high on a wet day; but the exhaled air is practically 

 saturated with water vapor — the blood has lost water. Further- 

 more, the inhaled air contains merely a trace of carbon dioxide, 

 while when it leaves the lungs it bears about 4 per cent — the 

 blood has given up carbon dioxide. And finally, air entering the 

 lungs comprises approximately 20 per cent oxygen, while when 

 exhaled there is only about 16 per cent — the blood has received 

 oxygen. In short, the blood by its traffic with the air in the lungs 

 gives up heat, moisture, and carbon dioxide, and takes up oxygen. 



One naturally is interested to know how the blood while passing 

 through the lungs acquires the oxygen, since this is the element 

 demanded by every cell in its life processes. At least two phe^ 

 nomena are involved. In the first place, the air contains a con- 

 siderable amount of oxygen under relatively high pressure and 

 therefore some oxygen passes into the liquid plasma of the blood 

 where the oxygen conditions are just the opposite. But the amount 

 of oxygen gained in this way by the blood is by no means equal to 

 the demands of the tissues, and so the emergency is met by special 

 blood cells, known as red rlood corpuscles, of which there are 

 many trillions in the human body. These carry a complex chemical 

 substance, hemoglorin, which has a high chemical affinity for 

 free oxygen : it is oxidized to form an unstable chemical compound, 

 oxyhemoglorin. Accordingly the millions of red blood corpuscles 

 leave the lungs with the oxygen affinity of the hemoglobin satisfied 

 and return to the heart to be distributed throughout the body. 



The oxygen is actually delivered to the tissue cells through the 

 capillaries in the tissues where the oxygen content is low — just 

 the opposite of the condition in the lungs — because the various 

 cells use the oxygen nearly as rapidly as it is received. So the 



