FUNCTIONAL ADAPTATION MAMMALIAN RESPIRATION 329 



liter, the COo/HCOs" ratio being kept constant, caused the oxygen 

 saturation to drop from 80 to 16%. A subsequent paper from the 

 same school (4i) showed that bicarbonate not only affected the equi- 

 librium constant in Pauling's equation, but also the interaction con- 

 stant (Chapter VI). Roughton {2362) considers that carbamate 

 formation takes place on one of the heme-linked groups (c/. Chapter 

 VI, Section 3.2.2.4.). 



The aggregation of Hiifner units within the mammalian erythrocyte 

 makes possible, therefore, a much finer adjustment of the hemoglobin 

 to cyclic changes in its microenvironment. Changes in the partial 

 pressure of carbon dioxide and in the pH might still affect the oxygen 

 affinity of a single Hiifner unit, and conversely the buffer capacity of 

 the latter would still be available to assist in carbon dioxide transport. 

 The reciprocal influence of hydrogen ions, carbon dioxide, and oxygen 

 on myohemoglobin is, however, almost negligible compared with that 

 exerted on a system in which heme-heme interaction occurs. 



9.3. Fetal Respiration 



In Section 6.1. the differences between fetal and adult hemoglobin 

 have been discussed. In vivo, however, differences in the composition 

 of the proteins are modified in some species by the microenvironment 

 of the cell. The reader is referred to other works (Needham, 2061; 

 Barcroft, i|7) for discussion of the nature of the placental circulation. 

 For maximum oxygen carriage, the loading tension at which fetal 

 blood approaches full saturation must be in the region of the unloading 

 tension of the maternal blood in the placenta. This is achieved by 

 shifting the dissociation curve of the fetal blood so that it lies above 

 that of the mother. This has been shown by Haselhorst and Strom- 

 berger (1151,1152) and Liebson and co-workers {17Ji.2) to take place 

 in the human species, by Barcroft and co-workers (H?) in the goat, 

 arid by Roos and Romijn {2332) in the cow. 



In the investigation of stroma-free dialyzed hemoglobins from the 

 maternal and fetal blood of the goat, McCarthy (ISOO) showed by 

 gasometric analysis that the difference in the dissociation curves was 

 a property of the hemoglobin. Samples of fetal and maternal hemo- 

 globin taken at the fifteenth week of pregnancy were saturated 58 

 and 33%, respectively, at 30 mm. oxygen, 37° C. and ;;H 6.8. The 

 differences that Barcroft and co-workers {1^7) found on the analysis 

 of the whole blood at 50 mm. carbon dioxide pressure and 38° C. 

 were 35% and 10%, respectively for the goat at a similar stage of 



