Maternal intervillous space

The membrane separating fetal blood from maternal blood in the intervillous space, the placental barrier, resembles other membranes, in that lipid-soluble substances diffuse readily but water-... 

The placenta develops from a portion of the zygote and thus has the same genetic endowment as the developing fetus (78). The embryonic/fetal component consists of trophoblastic-derived chorionic villi/ which invade the maternal endometrium and are exposed directly to maternal blood in lake-like structures called lacunae. These villi create the large surface area necessary for maternal-fetal transfer in what becomes the intervillous space of the placenta. Here the maternal blood pressure supplies pulsatile blood flow in jetlike streams from the spiral arteries of the endometriunX/ to bathe the chorionic villi and allow for transfer of gaseS/ nutrientS/ and metabolic products. Biologically/ the human placenta is classified as a hemochorial placenta because maternal blood is in direct contact with the fetal chorionic membrane. It is this membrane that determines what is transferred to the fetus. 

The placenta keeps the maternal and fetal circulation systems separate, nourishes the fetus, eliminates fetal wastes, and produces hormones vital to pregnancy. It is composed of large collections of fetal vessels called villi, which are surrounded by intervillous spaces in which maternal blood flows. For substances to move from maternal circulation to fetal circulation, they must cross through the trophoblasts and several membranes. The transfer of any substance depends largely on the concentration gradient between the maternal and fetal circulatory systems, the presence or absence of circulating binding proteins, the hpid solubility of the substance, and the presence of facilitated transport, such as ion pumps or receptor-mediated endocytosis (Box 54-1). The placenta is an effective barrier to the movement... 

The membrane separating fetal blood from maternal blood in the intervillous space, the placental barrier, resembles other membranes, in that hpid-soluble substances diffuse readily but water-soluble substances either do not or diffuse poorly. Thus, for instance, morphine-induced respiratory depression and miosis may occur in both the mother and her newborn infant. The children of narcotic-addicted mothers will be bom with an addiction to narcotics. 

Discussion of Assumptions of the Model. The pattern of maternal to fetal placental flow affects the amount of 02 transferred. It seems unlikely that the simple concurrent system shown in Figure 3 accurately represents the placental capillaries in sheep, a species in which the exchange vessels interdigitate in a nonuniform and complex manner (30, 31). The flow patterns are also complicated in humans where maternal blood enters the intervillous spaces from the base and flows upward and outward past fetal capillary loops in placental villi. Physiological studies also fail to reveal a simple geometric flow pattern. For... 

The maternal blood space (intervillous space) between terminal villi is assumed to be of capillary size (10). 

Blood entering the placenta through the arterial openings is propelled by the maternal pressure toward the chorionic plate, the blood flowing in spurts. It circulates through the intervillous space and then is collected through the venous openings. 

The capillary bed should be regarded as continuous in all three dimensions, and in the intervillous space the maternal blood channels resemble a labyrinth of canals with diameters of capillary dimensions (27). At the level of exchange, fetal and maternal blood may be considered to flow in similar channels, but the maternal channel is free of an enclosing membrane and is bounded by the villi of the capillary bed. 

Pinheiro FS, Jorge SM, Martinez FE. 1992. Plasma zinc and copper levels in maternal, placental intervillous space and cord blood. Nutrition Research 12(3) 367-373. 

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