Final answer:
The placenta handles nutrient and waste requirements for the developing fetus during the first few weeks, eventually taking over the role from the endometrial cells. It facilitates the transfer of oxygen and essential nutrients to the fetus, while also removing carbon dioxide and waste via the umbilical arteries and vein.
Step-by-step explanation:
Nutrient and waste requirements for the developing fetus during the first few weeks are handled primarily by the placenta. Initially, the cells of the endometrium nourish the embryo, but as the embryo grows, its needs surpass what the endometrial cells can provide. The placenta, a unique circulatory organ that develops from the embryo and the uterine wall, then takes over this crucial role. It is through the umbilical vein that oxygen-rich blood is carried from the mother to the fetus, and through the umbilical arteries, the oxygen-depleted blood and wastes are transported back to the placenta.
The placenta is fully developed by weeks 14 to 16 of gestation, providing nutrition and excretion, as well as respiration and endocrine functions, essential for the growth and development of the fetus. Nutrients and oxygen are transferred to the fetal bloodstream through capillaries in the chorionic villi, which are surrounded by maternal blood. Substances like oxygen and carbon dioxide move across the placenta by simple diffusion, while others, such as glucose, utilize facilitated diffusion, and certain nutrients like amino acids and iron rely on active transport.