Blood Circulation in the Mature Placenta
- Fetal circulation
- Maternal circulation
Because the placenta functionally represents a rather intimate association of the fetal capillary bed to maternal blood, its gross anatomy primarily concerns vascular relations. The fetal surface of the placenta is covered by the transparent amnion beneath which the fetal chorionic vessels course. A section through the placenta in situ includes amnion, chorion, chorionic villi and intervillous spaces, decidual plate, and myometrium. The maternal surface of the placenta is divided into irregular lobes by furrows produced by septa, which consist of fibrous tissue with sparse vessels confined mainly to their bases. The broad-based septa ordinarily do not reach the chorionic plate, thus providing only incomplete partitions.
[...] The vis a tergo forces blood in discrete streams toward the chorionic plate until the head of pressure is reduced. Lateral spread then occurs. The continuing influx of arterial blood exerts pressure on the contents of the intervillous space, pushing the blood toward exits in the basal plate, from which it is drained through uterine veins. During uterine contractions, both inflow and outflow are curtailed, although the volume of blood in the intervillous space is maintained, thus providing for continual, albeit reduced, exchange. [...]
[...] The magistral pattern is characterized by arteries that traverse to the edge of the placenta without an appreciable decrease in the diameter of the vessel. The arteries are end arteries, supplying one cotyledon, as the branch turns downward to pierce the chorionic plate. The truncal arteries are the perforating branches of the surface arteries that pass through the chorionic plate. Each truncal artery supplies one cotyledon. There is a decrease in smooth muscle of the vessel wall and an increase in the caliber of the vessel as it penetrates through the basal plate; the loss in smooth muscle continues as the truncal arteries branch into the rami; the same is true of the vein walls. [...]