Uteroplacental Blood Flow

Abstract

Sexual reproduction in the ocean necessitates only the combination of gametes, followed by absorption of nutrients and oxygen from the surrounding watery medium. As life moved from the sea to the land, reproductive strategies required compensation for the loss of this aquatic environment. For mammals and a few other animals, the solution to this problem was the development of the placenta, the means by which the fetus extracts nutrients from its environment. As the animals that used the placenta evolved from small rodent-like creatures with short gestations to larger animals with prolonged gestations, the demands of the developing fetus grew. Whereas the placenta of the fetal pig, with a gestational period of a little less than 4 months, can extract sufficient nutrients from the mother by simple diffusion across the uterus to the placenta, the human fetus needs a far more complex uteroplacental relationship. Several evolutionary solutions to the increased demands of fetuses can be observed.1Benirschke K Kaufmann P Pathology of the Human Placenta. 3d edition. Springer-Verlag, New York1995Crossref Google Scholar One approach was a larger placenta. For example, the chinchilla has a neonatal:placental weight ratio of 30:1, whereas the human has a 6:1 ratio. Another means to greater nutritional support for the fetus was to increase the surface area of contact between fetal circulation in the placenta and maternal circulation. The pig fetus has a diffuse placenta that makes contact with the mother's uterus by a simple folded contact. The human placenta, on the other hand, has a complex villous structure, similar to the sea anemone's tentacles waving in the sea, that greatly increases the contact surface area between the mother's blood space and the fetal circulation. Despite this increased fetal-maternal contact, the system is still rather inefficient. We can quantify this by considering the amount of oxygen in the maternal blood that enters the human placenta and the amount of oxygen in the fetal blood that leaves the placenta. Maternal blood has a pO2 of around 100, whereas the pO2 of umbilical vein blood is around 35 to 40. This represents an efficiency of only 35 to 40%. Therefore, it also became necessary to greatly increase the flow of maternal blood into the intervillous space during pregnancy.2Thornburg KL Jacobson SL Giraud GD Morton MJ Hemodynamic changes in pregnancy.Semin Perinatol. 2000; 24: 11-14Abstract Full Text PDF PubMed Google Scholar, 3Duvekot JJ Peeters LL Maternal cardiovascular hemodynamic adaptation to pregnancy.Obstet Gynecol Surv. 1994; 49: S1-S14Crossref PubMed Google Scholar Without this increased maternal blood flow, preterm birth and fetal loss occur.4Naeye RL Pregnancy hypertension, placental evidences of low uteroplacental blood flow, and spontaneous premature delivery.Hum Pathol. 1989; 20: 441-444Abstract Full Text PDF PubMed Google Scholar One of two mechanisms can increase maternal flow. increased total body blood flow or increased blood flow to the placental bed through the uterine spiral arteries. For the human, evolution has selected the latter mechanism, limiting the overall systemic effects that increased total body blood flow would produce. In the nonpregnant state the uterine vessels carry <1% of the maternal cardiac output.5Silver M Barnes RJ Comline RS Burton GJ Placental blood flow: some fetal and maternal cardiovascular adjustments during gestation.J Reprod Fertil Suppl. 1982; 31: 139-160PubMed Google Scholar This is not surprising in light of the fact that a nonpregnant women needs to maintain a uterus that weighs only 50 g. At term, these same vessels must support a uterus, placenta, and fetus that can weigh up to 5000 g. How can these vessels meet such a hemodynamic challenge? Doubling the number of vessels in the uterus, for example, would have only doubled the total amount of flow into the placenta. An understanding of fluid mechanics gives us insight into how such a significant increase in total blood flow can be achieved without increasing the total number of vessels in the uterus. Poiseuille's law of fluid flow in a cylinder states that flow is proportional to the radius to the fourth power.6Poston L The control of blood flow to the placenta.Exp Physiol. 1997; 82: 377-387PubMed Google Scholar Applying this law to the situation in the uterus, doubling the radius of a uterine vessel will increase the flow through that vessel 16 times. Comparison of vessels in the nonpregnant uterus to those at term reveals that these vessels can increase their radii by as much as tenfold. According to Poiseuille's law, this results in an increase in blood flow by a factor of 10,000! Clearly, the ability of uterine vessels to vary in diameter is a great advantage. The evolutionary problem then became how to convert small-caliber vessels in the nonpregnant state to large-caliber vessels during pregnancy, and then return them to their nonpregnant state and size when the pregnancy i