In viviparous animals, how does embryo get nutrition and oxygen?

In viviparous animals, particularly placental mammals like humans, the embryo receives nutrition and oxygen through a specialized temporary organ called the placenta that forms during pregnancy. The placenta develops where the embryo implants in the uterine wall, creating an interface between maternal and fetal circulatory systems. Blood vessels from the embryo extend through the umbilical cord to the placenta, where they branch into extensive networks of tiny capillaries. These fetal blood vessels come into extremely close contact with maternal blood vessels in the placental tissue, though maternal and fetal blood never actually mix—they remain separated by thin membrane barriers.

This arrangement enables efficient exchange through diffusion: oxygen and nutrients (glucose, amino acids, vitamins, minerals) pass from maternal blood across the placental membranes into fetal blood, while carbon dioxide and metabolic waste products move in the opposite direction from fetal to maternal circulation for elimination. The placenta essentially functions as the fetus's lungs, digestive system, and kidneys combined, supporting all vital processes until birth when the baby must transition to independent function. Additionally, the placenta transfers maternal antibodies to provide temporary immune protection, filters out many harmful substances (though not all—some toxins, drugs, and pathogens can cross), and produces hormones essential for maintaining pregnancy. In some viviparous animals lacking true placentas (like certain sharks and reptiles), embryos receive nourishment through other mechanisms such as absorbing uterine secretions or consuming unfertilized eggs, but the principle remains the same: maternal resources sustain the developing offspring within a protected internal environment until birth.