Respiratory Functions uf Body Fluids 



297 



four atoms ot iron and a molecular weight of about 67,000. Muscle hemoglobin 

 contains two heme units. In all of the vertebrates, with the exception of the 

 cyclostomes (Myxine), the blood hemoglobin contains four atoms of iron; in 

 Myxine there are two. The in\'ertebrate hemoglobins are composed of two or 

 many, but never four, units. 



The function of hemoglobin as an oxygen carrier depends on the loose 

 combination of the ferrous iron with oxygen (oxygenation). The ferrous iron 

 can be oxidized by strong oxidants, resulting in methemoglobin. The resulting 

 ferric iron is incapable of combining further with oxygen. Oxygen combines 

 in the proportion of one molecule per atom of iron. 



The rate of dissociation or unloading of oxygen has been measured photo- 

 electrically.-*^' ^^■"'' ^•^'•' The time for 50 per cent dissociation (deoxygenation) 



CH 



is similar in man, sheep, frog, and Glycera, but is longer in Linnbricus (Table 

 5 1 ) and very long in Ascaris.-^ 



The protein portion of the hemoglobin molecule differs from species to 

 species and even from embryo to adult in an individual. The protein differ- 

 ences are shown by slight differences in spectral absorption, by differences in 

 oxygen affinity and in the effect of carbon dioxide on oxygen affinity, and by 

 differences in amino acid composition. For example, the amount of methionine 

 is higher in the hemoglobin of man, monkey, and cow than in that of dog and 

 fox, and cystine is higher in the hemoglobin of man, monkey, dog, and fox 

 than in that of the cow and horse, whereas the hemoglobin in all these species 

 contains similar amounts of arginine, lysine, histidine, and tryptophane.^-^ 



Absorption Bands. All hemin compounds show characteristic absorption 



