The plasma of the blood, Hke the water of the sea, carries in solution 

 varying amounts of the atmospheric gases. Ordinarily, these seem to make 

 no difference. When men are exposed to high atmospheric pressures, as in 

 deep tunnel work or in deep diving, the amount of nitrogen in solution 

 seems to increase. On returning to the surface, nitrogen bubbles out of the 

 blood and expands in the capillaries. That results in a very painful and 

 sometimes fatal condition known as the "bends". It is possible to prevent 

 that by having the workers come back to normal air pressure very slowly, 

 through so-called "decompression chambers". A similar difficulty arises in 

 aviation when airplanes are brought rapidly from the surface to very high 

 altitudes, where the air pressure is very low: here again the nitrogen may 

 "boil" out as bubbles. It is customary to prepare aviators who are about to 

 make high ascents by having them spend some hours in low-pressure 

 chambers, where they can breathe the needed amount of oxygen and slowly 

 eliminate some of the nitrogen dissolved in the blood. 



Another problem arising out of high flying is the impossibility of breath- 

 ing in and distributing enough oxygen at the highest levels, where the air 

 is so very "thin". Aviators are supplied with special masks, through which 

 needed amounts of oxygen are delivered from flasks or tanks. 



Many persons find that merely going to the mountains, not to mention 

 flying up into the air several miles, puts too much strain upon the heart. 

 And those who always live in high mountains have relatively larger hearts 

 than those who dwell at the seashore. 



Strange as it may seem, the real blue bloods of the animal kingdom are 

 cold-blooded arthropods, not man. In crabs, lobsters, and the like the blood 

 contains hemocyanin^ a pigment in which the metallic element is copper. 

 Hemocyanin turns blue when it combines with oxygen, and is colorless in 

 the absence of oxygen. It is not carried in special corpuscles, but dissolved 

 in the body fluid. 



In all animals that have blood, cell respiration is related to the blood. 

 That is, the cells get their oxygen from the blood, and they discharge their 

 carbon dioxide to the blood. In all such animals we therefore apply the 

 term respiration to the process by which the air is brought from the outside 

 to the blood, -and by which the carbon dioxide is thrown out. 



Air-tubes^ Insects use relatively large amounts of oxygen. Movements 

 of the body compress and release the delicate branching air-tubes, which 

 reach all parts, thus aiding in the circulation of air (see illustration, p. 16). 

 In some insects, as the common locust, rhythmic movements alternately 

 empty and fill the air-pipes, and so accelerate the diffusion of oxygen and 

 the removal of carbon dioxide. 



^See No. 7. p. 213. 

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