Circulation within the intervillous space is described in Chapter 5 (p. 94). Intervillous and uteroplacental blood flow increases throughout the irst trimester of normal pregnancies (Merce, 2009). At term, the residual volume of the intervillous space approximates 140 mL. Moreover, utero placental blood flow near term is estimated to be 700 to 900 mLl min, and most of this blood apparently goes to the intervillous space (Pates, 2010).
Active labor contractions reduce blood low into the intervillous space to a degree that depends on contraction intensity. Blood pressure within the intervillous space is significantly less than uterine arterial pressure, but somewhat greater than venous pressure. he latter, in turn, varies depending on several factors, including maternal position (Nelson, 2015). When supine, for example, pressure in the lower part of the inferior vena cava is elevated, and consequently, pressure in the uterine and ovarian veins, and in turn in the intervillous space, is increased.
Substances that pass from maternal to fetal blood must irst traverse the syncytiotrophoblast, the attenuated cytotrophoblast layer, the villous stroma, and inally, the fetal capillary wall. Although this histological barrier separates maternal and fetal circulations, it is not a simple physical barrier. First, throughout pregnancy, syncytiotrophoblast actively or passively permits, facilitates, and adjusts the amount and rate of substance transfer to the fetus. The maternal-facing syncytiotrophoblast surface is characterized by a complex microvillous structure. he fetal-facing basal cell membrane is the site of transfer to the intravillous space. Finally, the villous capillaries are an additional site for transport from the intravillous space into fetal blood, or vice versa. In determining the efectiveness of the human placenta as an organ of transfer, several variables are important and shown in Table
Zhao and coworkers (2014) have provided a review of the pharmacology of these interactions.
Mechanisms of Transfer
Most substances with a molecular mass <500 Da pass readily through placental tissue by simple difusion. These include
TABLE 7-1. Variables of Maternal-Fetal Substance binding of the substance
Maternal blood flow rate through the intervillous space Trophoblast surface area size available for exchange Physical trophoblast properties to permit simple difusion Trophoblast biochemical machinery for active transport Substance metabolism by the placenta during transfer Fetal intervillous capillary surface area size for exchange Fetal blood concentration of the substance
Specific binding or carrier proteins in the fetal or maternal ci rcu lation oxygen, CO2, water, most electrolytes, and anesthetic gases (Carter, 2009). Some low-molecular-weight compounds undergo transfer facilitated by syncytiotrophoblast. These are usually those that have low concentrations in maternal plasma but are essential for normal fetal development.
Insulin, steroid hormones, and thyroid hormones cross the placenta, but very slowly. he hormones synthesized in situ in the syncytiotrophoblast enter both the maternal and fetal circulations, but not equally (Chap. 5, p. 98). Examples are hCG and hPL concentrations, which are much lower in fetal plasma than in maternal plasma. High-molecularweight substances usually do not traverse the placenta, but there are important exceptions. One is immunoglobulin G-molecular weight 160,000 Da-which is transferred by way of a speciic trophoblast receptor-mediated mechanism (Stach, 2014).
Transfer of Oxygen and Carbon Dioxide