(A) Umbilical arteries (dark blue) transport oxygen and nutrient-depleted blood to the placenta, while the umbilical vein (dark red) carries oxygen and nutrients to the fetus. During pregnancy, the umbilical arteries (cross-section, left side) have a thickened middle layer (the tunica media), whose outer layer is populated with contractile smooth muscle cells (SMCs; orange) and whose inner layer contains a high levels of proteins known as proteoglycans (light blue). (B) At birth, the umbilical arteries close rapidly (indicated by the yellow x) to prevent blood loss in the newborn. This is caused by the high local concentrations of proteoglycans, which enable the inner layer of the tunica media to swell and help the smooth muscle cells to contract more strongly (cross section, right side).
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