364 - Multicellular Animals, Especially Man 



hemoglobin, which is a very efficient respira- 

 tory pigment. 



Only a small fraction (about 2 percent) of 

 the oxygen load of the blood remains freely 

 dissolved in the plasma and in the proto- 

 plasm of the corpuscles. Most of the oxygen 

 unites chemically with the hemoglobin in 

 the erythrocytes. As fast as it diffuses into the 

 corpuscles from the plasma, free oxygen con- 

 tinues to unite with hemoglobin until all of 

 this pigment has become oxygenated. Conse- 

 quently the plasma does not reach equilib- 

 rium with the alveolar air until the hemo- 

 globin becomes saturated. 



Hemoglobin vs. Oxyhemoglobin. About 

 90 percent of the dry weight of each red 

 corpuscle represents the complex iron-con- 

 taining protein pigment, hemoglobin. The 

 chemical properties of this compound are 

 admirably adapted to the role it plays as an 

 oxygen carrier. Hemoglobin combines with 

 oxygen on a 1:4 molecular basis, forming 

 oxyhemoglobin (p. 321). Oxyhemoglobin dis- 

 plays a bright scarlet color — in contrast to the 

 dull purplish red of reduced hemoglobin. 

 Consequently, the color of whole blood alters 

 considerably while it flows through the lungs 

 and changes from an unaerated (venous) to 

 an aerated (arterial) condition. 



As a carrier of oxygen, oxyhemoglobin 

 must be an unstable compound, capable of 

 liberating free oxygen when the blood 

 reaches the various tissues of the body. In 

 other words, the oxygenation of hemoglobin 

 is a delicately poised reversible reaction. It 

 shifts in either direction, depending upon 

 small changes that occur in the chemical 

 composition of the blood in the different 

 parts of the circulation. This reaction, ex- 

 pressed in simplified equational form, may 

 be written as follows: 



The binding and freeing of oxygen in the 

 blood is mainly controlled by the quantities 

 of oxygen and carbon dioxide present in the 

 different parts of the circulatory system — a 

 conclusion (see Fig. 19-8) based on many ex- 

 periments. In leaving the lungs, arterial 

 blood remains isolated in the pulmonary 

 veins and in the various arteries, until it 

 reaches the capillaries in some other part of 

 the body. Here the HbOo is exposed to an 

 environment in which there is relatively little 

 free oxygen— since the tissue cells consume 

 oxygen — and here also carbon dioxide is 

 abundant due to the continuous production 

 of C0 2 by the tissues. Under these conditions 

 (Fig. 19-8) Hb0 2 liberates free oxygen, which 

 then diffuses from the corpuscles, across the 

 plasma and intervening membranes, into the 

 tissue cells. Carbon dioxide, in contrast, con- 

 tinually diffuses in the opposite direction. 

 Carbon dioxide passes from the cells, where 

 its concentration is maximum, into the blood. 

 This gas exchange between the blood and the 

 tissues is specified as internal respiration, 

 and internal respiration proceeds spontane- 

 ously on a diffusional basis. 



Venous blood, on leaving the tissues, like- 

 wise remains isolated in the veins and in the 

 pulmonary arteries until it reaches the lung 

 capillaries. Here the reduced Hb is exposed 

 to the relatively abundant oxygen of the 

 alveolar air, and here also carbon dioxide 

 begins to leave the blood by diffusing into 

 the alveolar air spaces. Both these factors, 

 namely high 2 and low C0 2 , favor the 

 formation of HbO.,; and in the few seconds 

 required for the corpuscles to file through the 

 capillaries bordering the alveoli, practically 

 all the hemoglobin is transformed to oxy- 

 hemoglobin. 



Hb + (4)0 2 - 



high Os; low CO2 

 at the lungs 



reduced 

 hemoglobin 



at the tissue* 

 low Oa; high COu 



Hb(0 2 ) 



oxyhemoglobin 



CARBON DIOXIDE-CARRYING CAPACITY 

 OF BLOOD 



Whole blood will absorb and carry more 

 than 50 cc of COo, which is some 10 times 

 greater than the amount that can be dis- 

 solved in an equivalent amount of plain 



