Final answer:
Normal arterial pO2 in healthy newborns delivered vaginally is approximately 80 mm Hg, facilitated by the greater affinity of fetal hemoglobin for oxygen and the pressure gradients established in systemic capillaries.
Step-by-step explanation:
The normal partial pressure of oxygen (pO2) in arterial blood for a healthy newborn delivered vaginally is approximately 80 mm Hg. This value can be derived from the information given in Figure 20.10, where the maternal arterial oxygen pressure is about 80-100 mm Hg.
However, when blood reaches the systemic capillaries, there arises a pressure gradient as the blood pO2 is 100 mm Hg and tissue cells pO2 is 40 mm Hg, facilitating the diffusion of oxygen out of the capillaries into the tissue cells. Since the fetal hemoglobin has a greater affinity for oxygen than maternal hemoglobin, the actual pO2 in fetal blood at the placenta is about 20 mm Hg. The newborn's pO2 levels will increase significantly after taking the first breath when the circulatory pattern changes to accommodate the independent respiratory function.