At birth the newborn finds itself with excess oxygen carrying capacity. With the rise in arterial PO2 from 20-25 mmHg to 100 mmHg, nearly all the hemoglobin can be saturated at the lung, as opposed to only 60-80% in utero. Consequently, erythropoiesis is sharply depressed and erythrocyte breakdown takes place at an accelerated rate. This is the source of the bilirubin which can cause some newborns to turn yellow, necessitating ultraviolet light treatment.
The fall in hemoglobin shortly after birth is known as postnatal anemia (Table 3.16), and is also due to the rapidly expanding blood volume which occurs with increasing body weight, and the fact that milk contains little iron.
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