PROTEINS 117 



red color. If a mild reducing agent is added to this oxyhemoglo- 

 bin the color becomes much darker. The oxygen has been taken 

 away from the oxyhemoglobin, which has now become hemo- 

 globin. This substance gives venous blood its dark red color. 

 This process may be repeated indefinitely. A convenient mild 

 reducing agent is Stokes' fluid which contains 2% ferrous sul- 

 phate, 3% tartaric acid and ammonia sufficient to redissolve the 

 precipitate which first f orms on adding this reagent. The amount 

 of oxygen which can be carried by a given amount of blood varies 

 somewhat. On the average from 100 c.c. of fully oxygenated 

 blood 19-20 c.c. of oxygen can be obtained (measured at C. and 

 760 mm of mercury pressure). A portion of this is simply dis- 

 solved in the blood plasma, but most of it is combined with 

 hemoglobin. The tissues do not remove all of the oxygen from 

 the blood, for venous blood still contains considerable oxyhemo- 

 globin. From 100 c.c. of venous blood an average of 15 c.c. of 

 oxygen may be obtained, although this value varies considerably. 

 The molecular weight of hemoglobin is very large, probably 

 about 16,000, and one molecule of hemoglobin is believed to com- 

 bine with one molecule of oxygen (0 2 ). The air normally con- 

 tains about 20% oxygen. This percentage may be much lowered, 

 however, before the amount of oxyhemoglobin in a hemoglobin- 

 oxyhcmoglobin mixture will be greatly diminished. If the per 

 cent of oxygen is reduced to 13% of an atmosphere the mixture 

 will still contain 93% oxyhemoglobin and only 7% hemoglobin. 



The dissociation of oxyhemoglobin is favored by the presence 

 of salts, carbon dioxide, and lactic acid. In the presence of any 

 of these substances, or if their amounts are increased, oxyhemo- 

 globin tends to give up some of its oxygen. The biological im- 

 portance of this fact is evident. In the tissues, the production 

 of carbon dioxide or lactic acid will aid in causing oxyhemo- 

 globin to yield up its oxygen, which is then used in the various 

 oxidation processes going on in the cell. 



Hemoglobin may be split into two parts, globin, a protein, 

 perhaps a histone, and hemochromogen. From oxyhemoglobin 

 hematin is obtained in place of hemochromogen. Globin makes 



