The intervillous space is a specialized area within the placenta, serving as the primary site for interaction between the mother and the developing fetus. This region acts like a vast, fluid-filled lake where maternal blood circulates, bathing tree-like fetal structures. It is a dynamic environment supporting the demands of pregnancy. The health and proper functioning of this space are fundamental for successful fetal development and a healthy pregnancy.
Anatomy of the Intervillous Space
The intervillous space is located within the placenta, positioned between the maternal decidua basalis and the fetal chorionic plate. This area is intricately structured, filled with chorionic villi. These finger-like projections extend from the chorionic plate, resembling a branching tree or a cauliflower-like structure submerged in fluid.
Each chorionic villus contains a network of tiny fetal capillaries, extensions of the umbilical vessels. The outermost layer of these villi, exposed to maternal blood, is a specialized tissue called the syncytiotrophoblast. This continuous layer of multinucleated epithelial cells forms the direct interface, supporting fetal growth and well-being. The large surface area provided by these villi, spanning up to 11-12 square meters at term, maximizes contact between maternal and fetal blood for efficient exchange.
Maternal Blood Flow and Circulation
Maternal blood enters the intervillous space through approximately 120 spiral arteries, specialized vessels originating from the uterine wall. These arteries erode into the space, allowing oxygenated maternal blood to flow directly into this open pool, unlike a closed capillary system. This creates a high-volume, low-pressure system where blood slowly bathes the chorionic villi, facilitating extensive contact with fetal structures.
Pressure within the maternal spiral arteries is around 70 mmHg, dropping to about 10 mmHg once blood enters the intervillous space, ensuring a gentle flow. This low-pressure environment allows slow, continuous circulation around the villi, promoting efficient exchange. After circulating and exchanging substances, deoxygenated maternal blood and waste products are collected and drained through uterine veins, returning to the mother’s systemic circulation.
The Exchange of Nutrients and Gases
The primary function of the intervillous space is to facilitate the transfer of essential substances between the mother and fetus without direct mixing of bloodstreams. The extensive surface area of the chorionic villi, combined with the slow flow of maternal blood, optimizes this exchange. A selective barrier, often called the placental barrier, is formed mainly by the syncytiotrophoblast layer, underlying connective tissue, and the endothelium of fetal capillaries.
This placental barrier is not an impenetrable wall but a sophisticated filter employing various transport mechanisms, including passive diffusion, facilitated diffusion, active transport, and pinocytosis, to regulate what crosses. Oxygen and nutrients like glucose and amino acids move from maternal blood into fetal circulation. Maternal antibodies, which provide passive immunity, are also transported to the fetus. In the opposite direction, waste products such as carbon dioxide and urea diffuse from fetal blood into maternal blood for elimination.
Complications Involving the Intervillous Space
Disruptions to the normal structure or blood flow within the intervillous space can lead to various pregnancy complications, directly affecting fetal health and development. Preeclampsia, characterized by high blood pressure in pregnancy, is one example. In preeclampsia, maternal spiral arteries may not remodel properly during early pregnancy, leading to narrower, less compliant vessels that restrict blood flow into the intervillous space. This reduced blood supply can cause placental insufficiency, impairing oxygen and nutrient transfer to the fetus and leading to restricted fetal growth.
Another complication is the formation of blood clots, known as intervillous thrombosis. These clots can block areas of exchange, reducing the functional surface area for nutrient and gas transfer, contributing to placental insufficiency and affecting fetal well-being. The integrity and consistent function of the intervillous space are thus directly tied to the placenta’s ability to support the growing fetus, making its health a factor in pregnancy outcomes.