The placenta is an extraordinary organ that links the foetus to the mother whilst keeping the blood supply separate and carrying out functions that the unborn child cannot on its own. It acts as an endocrine organ, synthesising hormones essential for the growth and development of the foetus; it provides an immunological function, passing antibodies for immunity from mother to child; but perhaps most importantly, it provides nutrition in the form of oxygen, glucose and other substances whilst excreting waste products such as carbon dioxide.
The placenta and foetus feature many adaptations to ensure the most efficient gas exchange and to ensure high oxygen availability to the foetus . Gas exchange across the placental blood-blood barrier is dependent on many factors such as interhaemal barrier thickness, the surface area of exchange and the concentration gradient of the gases, among other factors.
Molecules of oxygen and carbon dioxide are relatively small and are able to cross the placenta by passive diffusion, just like gas exchange in the lungs of a more developed human. However, the blood-blood barrier of the placenta is thicker (3.5µm) than the blood-gas barrier of the lungs (0.5µm) (Donnelly L and Campling G, 2011) and the permeability of the blood-blood barrier is also lower (Mess and Ferner, 2010)which means diffusion of these gases takes longer and can thus be seen as less efficient than human lungs. However, the placenta and foetus have support other adaptations to make up for this difference
Surface Area of Exchange
The placenta is composed of two structures: the maternal decidua basalis and the foetal chorion composed of the foetal membranes such as the syncytiotrophoblast. (University of Michigan Medical School, 1999). The syncytiotrophoblast is covered with many structures known as chorionic villi which transport oxygen from the maternal blood circulation to the foetal circulation and vastly increase the surface area of the chorion for gas exchange to take place. They project through the syncytiotrophoblast into the intervillous space where they are directly in contact with the maternal blood. The placenta itself is averages 22cm in diameter (Yetter, J, 1998) but the chorion has an approximate surface area of 12m2 (Wang Y and Zhao S, 2010) allowing for a much greater surface over which oxygen exchange can take place.
Counter current flow + Concentration Gradient
The concentration gradient of high amounts of oxygen in the maternal vessels and the lower amounts of oxygen in the foetal vessels is what drives the diffusion of oxygen from mother to foetus. As blood is constantly flowing, the concentration gradient is refreshed and maintained at a high level. If blood were stationary, the oxygen would diffuse into the foetal vessels and then back into the maternal vessels and continue this way until an equilibrium is reached where no more gas exchange would take place. The maintenance of this gradient is also due to a counter current...