226 Tin: INTERNAL r.NVlRONMHN T OF THI- BODY Part III 



lungs, and under the skin (Fig. 13.2). The condition is well known to divers as 

 the bends or the caisson disease. It can be prevented by bringing them to the 

 surface in a series of decompression chambers, so that the adjustment of the 

 nitrogen content of their body fluids to ground level pressure is gradual and 

 harmless. 



Water. Wherever water comes in contact with air it absorbs gases that be- 

 come dissolved in it. Thus air and water are continually being mixed at the 

 surfaces of all bodies of water. In lakes and seas the aerated water is rolled 

 under by the winds and distributed by currents to considerable depths. Green 

 plants, included in the microscopic plankton, contribute to the dissolved oxy- 



FiG. 13.2. Bubbles of nitrogen in the veins of animals subjected to very low 

 atmospheric pressure. This is aeroembolism, produced by rapid decrease of pres- 

 sure such as occurs in aircraft flights to high altitudes and is marked by the 

 formation of nitrogen bubbles in the fluids and tissues of the body, especially in 

 fat. (Courtesy, Armstrong: Principles of Aviation Medicine, ed. 2. Baltimore, 

 Williams and Wilkins Co., 1943.) 



gen. The respiratory gases are present in water as in air. Though there is much 

 less of it, dissolved oxygen takes the same important part in aquatic respira- 

 tions; so do small amounts of carbon dioxide. Nitrogen in water is an inactive 

 passenger as it is in air. Although water is a combination of hydrogen and 

 oxygen (H-O), this oxygen is chemically locked and living organisms cannot 

 utilize it for respiration. 



Respiration Liberates Energy. From mankind to the simplest animals and 

 plants all direct or aerobic (with air) respiration depends upon free oxygen. 

 The more complex the animal, whether race horse or hummingbird, and the 

 greater its activity, the more constant is Us dependence upon respiration. 



Respiration is above all the process by which plants and animals, with oxy- 

 gen as the key, release the energy locked up in food. The oxidation of food 

 is a biochemical process in which oxygen unites with carbon and hydrogen, 

 forms carbon dioxide and water, and sets free the energy that once came from 



