There are several reasons for the high cardiac output required during the first few postnatal weeks. At birth there is a high concentration of fetal hemoglobin (Hbf) in the newborns' blood (Figure 4.13). Because it has a high oxygen affinity, the A-V oxygen difference is small, about 2 ml/100 ml at the tissue level (Figure 4.14). This is due to the fact that oxygen affinity is so high immediately after birth that oxygen binding and release occur on the flat "shoulder" region of the ODC. This contrasts with an average A-V oxygen difference in the adult of ca. 6 ml/100 ml. Because cardiac output is equal to total oxygen consumption divided by A-V oxygen difference, the smaller the A-V oxygen difference the larger the cardiac output that is necessary to support a given level of oxygen consumption.
The necessity to maintain body temperature and the high body surface to volume ratio also contribute to the necessity for a high cardiac output in the newborn. With postnatal development Hbf is replaced by adult hemoglobin, and rising 2,3-diphosphoglycerate concentration acts on it to lower oxygen affinity. Also, growth decreases the body surface to volume ratio, decreasing the mass specific metabolic rate.
To review the Oxygen Dissociation Curve
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