Special fetal adaptations are required for gas exchange, particularly oxygen, because of the limitations of passive exchange across the placenta. The PO2 of maternal arterial blood is about 80 to 100 mm Hg and about 20 to 25 mm Hg in the incoming blood in the umbilical artery. This difference in oxygen tension provides a large driving force for exchange; the result is an increase in the fetal blood PO2 to 30 to 35 mm Hg in the umbilical vein. Fortunately, fetal hemoglobin carries more oxygen at a low PO2 than adult hemoglobin carries at a PO2 2 to 3 times higher. In addition, the concentration of hemoglobin in fetal blood is about 20% higher than in adult blood. The net result is that the fetus has sufficient oxygen to support its metabolism and growth but does so at low oxygen tensions, using the unique properties of fetal hemoglobin. After birth, when much more efficient oxygen exchange occurs in the lung, the newborn gradually replaces the red cells containing fetal hemoglobin with red cells containing adult hemoglobin.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.