The placenta is a temporary organ that develops during pregnancy, serving as the interface between the mother and the developing fetus, facilitating all necessary exchanges for fetal growth and health. Within the placenta is a specialized area called the intervillous space, where the mother’s blood interacts with the fetal tissue. This space enables the close proximity required for exchange without allowing the direct mixing of maternal and fetal blood supplies.
Defining the Intervillous Space
The intervillous space is not a vessel but rather a large, open pool of maternal blood situated within the placenta. This area is bordered on the maternal side by the basal plate and on the fetal side by the chorionic plate. The space contains numerous tree-like structures known as chorionic villi, which are projections of fetal tissue that float within this blood pool.
Maternal blood enters this space through specialized openings called spiral arteries, which are remodeled vessels from the mother’s uterine wall. The space at term can contain approximately 150 milliliters of blood. This blood then washes over the surfaces of the chorionic villi, allowing for the exchange of substances before the deoxygenated blood drains back into the mother’s circulation through uterine venules. The blood within the intervillous space is exchanged constantly, being replenished about three to four times every minute.
The Critical Role in Nutrient and Waste Exchange
The intervillous space is the site for all maternal-fetal exchange, which occurs across the thin barrier of the chorionic villi. These villi contain an extensive network of fetal capillaries, bringing fetal blood very close to the maternal blood surrounding them. The maternal blood in the intervillous space and the fetal blood inside the villi never mix directly, as they are separated by the placental barrier.
This barrier, formed by layers of cells including the syncytiotrophoblast, facilitates the transfer of necessary materials via processes like diffusion and active transport. Oxygen from the mother’s blood moves across the villi and into the fetal capillaries, while carbon dioxide moves in the opposite direction for maternal disposal. Nutrients such as glucose and amino acids are transferred from the maternal blood to the fetal circulation to support growth.
The syncytiotrophoblast, the outermost layer of the villi, functions as a highly selective membrane. This barrier also allows the transfer of maternal antibodies, providing the fetus with passive immunity. The efficiency of this exchange is enhanced by the volume of blood flow, which reaches 600 to 700 milliliters per minute at term, and the large surface area of the villi, which is roughly 12 square meters.
Clinical Conditions Related to Intervillous Space Dysfunction
Disorders of the intervillous space are often rooted in a failure of the spiral arteries to remodel. Normally, specialized fetal cells invade the uterine wall and convert these arteries into wide, low-resistance vessels. When this conversion is incomplete, the arteries retain their muscular walls, leading to reduced and often intermittent blood flow to the intervillous space.
This restricted flow can result in uteroplacental insufficiency, where the placenta cannot deliver sufficient oxygen and nutrients to the fetus, leading to fetal growth restriction (FGR). Preeclampsia is also linked to this failed arterial remodeling, causing chronic oxidative stress due to cycles of poor perfusion. The resulting placental dysfunction can lead to the release of factors into the maternal bloodstream that cause high blood pressure and other systemic symptoms.
Other conditions, such as placental abruption, involve issues where the placenta prematurely separates from the uterine wall, which can be linked to damage within the basal plate boundary. Monitoring blood flow patterns within the maternal circulation of the placenta assesses the health of the intervillous space and identifies high-risk situations. Early detection and management are focused on minimizing the impact of these perfusion issues on fetal well-being.