placenta

An organ of metabolic exchange between fetus & mother. Structure: - it has both embryonic (fetal) & maternal components - the embryonic component is formed from the outermost embryonic membrane (chorion forndosum) - the maternal component is formed from the uterine mucosa (decidua basalis) Physiology: - within the placenta, chorionic villi contain capillaries carrying blood of embryonic circulation - maternal blood fills intervillous spaces creating an interface of contact - no direct mixing of blood occurs; but the placental membrane separating the maternal & fetal circulation is thin & permeable to small molecules including nutrients, metabolites, many pharmaceutical agents, & IgG - the placenta secretes the hormone chorionic gonadotropin, found in the maternal circulation & measured clinically as an indicator of pregnancy. - at term, the human placenta is disk shaped, about 4 cm in thickness & 18 cm in diameter. - its weight is about 15% of the weight of the fetus - the fetal surface is smooth with the umbilical cord normally attached near its center - the maternal surface of a detached placenta is rough because of the torn decidual tissue adhering to the chorion - lobular elevations in the maternal surface are called cotyledons or lobes

References

Stedman's Medical Dictionary 27th ed, Williams & Wilkins, Baltimore, 1999