Physiology of Placenta || Vascular Supply and Circulation

 Physiology of Placenta

        In mammals, egg is yolkless, the embryo depends upon the flow of supplies from the maternal body through the placenta fro its nutrition. A close contact is therefore, maintained between the foetal tissues and material tissues but the two never mix with each other. The foetal vascular supply and the maternal vascular supply are in communication but neither the foetal blood and material blood mix with each other nor the material blood enters the foetal circulation or vice versa. 

        The two are always kept separated due to some hindrance known as the placental barrier. and maternal parts of the placenta. In some cases, as in hemochorial placenta, this barrier may be made slender or thin but never broken down. The placental barrier, physiologically acts as a semipermeable membrane which allows substances to pass through and preventing others. So the exchange of gases, food supply and removal of nitrogenous wastes take place through this semipermeable placental barrier.

Vascular Supply and Circulation

        As already mentioned, the foetal and maternal blood circulate independently in quite separated vessels. The former type of blood is contained by the spiral terminations of the uterine arteries. The foetal blood is distributed to the chorionic villi by way of chorionic plate. The blood enters the villi through capillary net and is collected by the umbilical veins and returned to the embryo. Each cotyledon of placenta contains a main villous whose apical branches extending to the basal plate, may be embedded there. 

        A good number of nozzles of uterine arteries, containing maternal blood, open directly into the intervillous space but all of them do not pass blood at the same time. Each nozzle sends a jet of blood reaching the chorionic plate independently and irregularly. The material blood, after bathing the chorionic villi which come across in the way, disperses laterally and then mostly downward in the same cotyledons. It is moved passively (forced along by the blood of subsequent jets) and passes through the most conveniently located venous outlet. the blood flows through the inter villous space at the rate of 600 ml. per minute.
The foetal and maternal blood circulations are completely separated with the only communication through diffusive inter charge. 

        The barriers encountered here are the trophoblastic covering of a villous, the connective-tissue core and the endothelium of embeded capillary. Which the advancement is of pregnancy, the thinning of the barrier takes place and it is reduced as much as to 0.002mm. Along with thinning goes a surprise increase in the transfer rate in both direction.