ABSTRACT
During the embryonic period (4 to 8 weeks), theembryonic circulation is transformed into the fetal circulation. BLOOD Hemopoiesis takes place in the bone marrow after the formation of primary centers of ossification (8th week), in the membrane bones, e.g., in the cranium, and in car tilage bones, e.g., vertebrae and limb bones. With the growth of the bony skeleton, blood formation is increas ingly confined to bone marrow, and extramedullary hemopoiesis regresses. After the 4th month, circulating erythrocytes are packed with fetal hemoglobin (Hb.F) whose 0 2 dissociation curve shows a shift to the left of the postnatal Hb curve (see Chapter 34, Figure 3). This signifies that fetal red cells take up more oxygen in the placenta, at a given Pq , than normal Hb in the postna tal lungs.
