The Placenta: Anatomy, Physiology, and Transfer of Drugs

The placenta, which presents unceremoniously after delivery of the neonate, has been given the undignified name afterbirth. This often ignored structure is, in fact, a critical organ that should not be an afterthought in the study of obstetric anesthesia. Revered by ancient cultures as the seat of the external soul or the bundle of life, the placenta has been the subject of many cultural rituals.1 However, a true understanding of the indispensable role of the placenta in the development of the fetus did not evolve until the seventeenth century. Much of the placenta’s function remained a mystery until the development of microanatomic, biochemical, and molecular biologic techniques during the past 50 years. The concept of the placenta as a passive sieve (which does little more than serve as a conduit for oxygen, nutrients, and waste) has been dispelled with the realization that the placenta is a complex and dynamic organ. The placenta brings the maternal and fetal circulations into close apposition, without substantial interchange of maternal and fetal blood, for the physiologic transfer of gases, nutrients, and wastes.2 This important exchange is accomplished within a complex structure that is almost entirely of fetal origin. ANATOMY