METABOLISM IN VERTEBRATES 



they lose their -NH2 or amine groups. The liver is the chief site of degrada- 

 tion of excess or worn-out amino acids. In mammals, urea, the chief nitrog- 

 enous waste product, is formed during this process. Birds and reptiles build 

 the amine groups from the breakdown of amino acids into uric acid. In 

 aquatic vertebrates, the amine groups become ammonia (NH3). Studies with 

 labeled amino acids reveal a constant state of flux or shifting of individual 

 molecules between the so-called structureless pool and the formed proteins 

 throughout the body. 



Respiration 



The term respiration has been widely used to cover the so-called gaseous 

 metabolism of the organism. In the discussion of cellular metabolism 

 (p. 35), it was shown that oxygen was required for the final steps in the 

 complete extraction of energy from the organic foods. The oxidative reac- 

 tions release carbon dioxide, which is a gaseous waste product and must be 

 removed if cellular metabolism is to continue normally. In unicellular 

 organisms, oxygen enters the cell as a result of diflfusion gradients between 

 the contents of the cell and its surroundings. In multicellular organisms and 

 especially in the complex vertebrate, a well-defined respiratory system func- 

 tions, together with the circulatory system, to satisfy the gaseous require- 

 ments for cellular metabolism. 



The discussion of respiration in terrestrial vertebrates will be broken down 

 into three distinct, though continuous, phases. The first of these is pulmonary 

 respiration, or external respiration as it is sometimes called. This is subdivided 

 into ventilation, the movement of the air mass in and out of the lungs, and 

 gas exchange, the simultaneous diffusion of oxygen and carbon dioxide be- 

 tween the alveoli of the lungs and the blood stream. The second aspect of 

 respiration to be discussed is the circulatory phase. This includes gas transport 

 between the lungs and cells in all parts of the body and gas exchange 

 between the blood and these cells. The third phase of respiration is cellular 

 respiration or biological oxidation, which has been discussed previously (p. 37). 



Air usually contains about 20 per cent oxygen, 0.04 per cent carbon dioxide, 

 and 79 per cent nitrogen. Terrestrial vertebrates live at the bottom of a 

 sea of air, and just as in the ocean the pressure is greatest at the bottom. At 

 sea level, the air we breathe has a pressure of 14.7 pounds per square inch. 

 This is usually indicated by stating that the air pressure will support a column 

 of mercury 760 mm. high. Animals must breathe air under pressure in order 

 to live. At high altitudes, the atmospheric pressure decreases although the 

 actual gas content varies little. At altitudes greater than 18,000 feet, where 

 the atmospheric pressure is only 380 mm. of mercury, oxygenation will be 

 inadequate unless corrective measures are taken. At 50,000 feet, man loses 

 consciousness in about 14 seconds and will die unless pure oxygen is im- 

 mediately supplied under pressure. There is so little pressure at 63,000 feet 



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