While the trophoblastic cords from the blastocyst are attaching to the uterus,blood capillaries grow into the cords from the vascular system of the newly forming embryo.About 21 days after fertilization,blood also begins to be pumped by the heart of the human embryo.

Blood sinuses supplied with blood from the mother develop around the outside of the trophoblastic cords.The trophoblastic cells send put more and more projections,which become placental villi into which fetal capillarries grow.Thus the villi carrying fetal blood,are surrounded by sinuses that contain maternal blood. The fetus blood flows through two umbilical arteries,then into the capillaries of the villi,and finally back through a single umbilical vein into the fetus.At the same time,mother's blood flows from her uterine arteries into large maternal sinuses that surround the villi and then back into the uterine veins of the mother.

The total surface area of the villi of the mature placenta is only a few square meters- many times less than the area of the pulmonary membranein the lungs. Placental Permeability and Membrane diffusion conductance: The major function of the placenta is to provide for diffusion of food stuffs and oxygen from the mother's blood into the fetus's blood and diffusion of excretory products from the fetus back into the mother.In the early pregnancy its permeability is low because of thickness. In later pregnancy,the permeability increases because of thining of of the membrane.

Diffusion of Oxygen Through the Placenta Membrane:The dissolved oxygen in the blood of the large maternal sinuses passed into the fetal blood by simple diffusion,driven by an oxygen pressure gradient from the mother's blood to the fetus's blood.Oxygen to the fetal tissue as in transported by the mother's blood to her tissues.First,the hemoglobin of the fetus is mainly fetal hemoglobin,a type of hemoglobin synthesized in the fetus before birth.Second the hemoglobin concentration of the fetal blood is about 50 percent greater than that of mother

.Third ,the Bohr effect,which is explained in relation to the exchange of carbon dioxide and oxygen.By these three means,the fetus is capable of receiving more then adequate oxygen through the placental membrane. Diffusion of Carbon Dioxide Through The Placental Membrane: Carbon di oxide is continually formed in the tissue of the fetus in the same way that is formed in maternal tissue and the only means for excreating the carbon dioxide from the fetus is through the placenta into the mother's blood.

This small pressure gradient for carbon dioxide across the membrane is more than sufficient to allow adequate diffusion of carbon dioxide because the extreme solubility of carbon dioxide in the placental membrane allows carbon dioxide to diffuse about 20 times as rapidly as oxygen. Diffusion of Foodstuffs of Placental Membrane: Other metabolic substrates needed by the fetus diffuse into the fetal blood in the same manner as oxygen does.In later stage of pregnancy,the fetus often uses as much glucose as the entire body of the mother's uses.Glucose is transported by carrier molecules in the trophoblastic cells of the membrane.Also fatty acid diffuse from the maternal blood into the fetal blood.

Excretion of Waste Products Through the Placental Membrane: Fetal wast products ate excreated by excreatory products of the mother.These includes especially the nonprotein nitrogens such as urea,uric acid,and creatinine.Therefore,excretion from the fetus depends mainly,if not entirely,on the diffusion gradients across the placental membrane and its permeability because there are higher concentration of the excretory products in the fetal blood than in the maternal blood,there is continual diffusion of these substances from the fetal blood to the maternal blood.