The placenta is what connects the baby to their mother's circulatory system. It is essentially the lining of the uterus while the baby is in the mother's body. The umbilical cord is attached to the baby and via a network of blood vessels the baby is nourished by the mother's supply of oxygent and nutrients. During the gestation period the baby's every need is take care of by the mother.
The mother shares her food, her air, her blood and her chemistry with her unborn child - or children if there happen to be multiple births. Nature had to plan a way of feeding this little inward papoose. The placenta that envelopes the baby is the answer. It allows the baby to derive nourishment needed for growth indirectly from the mother. In this way it is a transitory auxiliary mechanism by which a fetus is nourished and kept alive by its mother until fully developed and it is born.
Essentially, nature prepares the way for this event by menstruation. The lining of the uterus becomes engorged with blood mixed in with a thin membrane of a mucus-like substance. If no pregnancy takes place during ovulation - peak time for pregnancy - after the readying of the incubator, the lining is sloughed off. In other words, menstruation happens. When the ovum and sperm meet and travel together up the fallopian tube and attach to the prepared lining in the uterus, new life begins.
The placenta itself is the means by which nourishment, oxygen, and waste removal of the growing fetus is carried out. Fetal circulation is different from the mother's circulation in that the lungs are bypassed by the blood circulating throughout the fetus. Only a small amount of blood - the needed amount - gets to this structure but mainly it is shunted away from the lungs. The blood of the newly developing infant and that of the mother never intermingle.
Picture the placenta as an inside covering for a cathedral dome, as an example. The difference being, however, is that the fetus' temporary home is soft and cushioning and not hard and lifeless. This membrane encloses the embryo and expands as the fetus grows and matures. It is composed of small blood vessels and nerves and is filled with fluid. This fluid cushions the fetus from outside shocks. For instance if the mother falls, the baby is probably not harmed. Only extreme force, not minor pushes and shoves harm the fetus.
An umbilical cord attaches itself to the center of the placenta and gathers in nutrients and oxygen form the mother's circulatory system and is the means by which waste is carried off and deposited in the mother's blood stream for dispersal. (Picture this as an internal chain by which means the mother is caring for her unborn infant.) Its structure is not unlike the equally unseen DNA strands that connect all living cells. It too is a blueprint for the developing fetus with information and instructions on how to build a new human being from two separate fore-ordained sources.
The actual meeting of the mother and developing child is done by osmosis. Clumps of villi, or little hair like projections from the fetal part of the membrane fit into indentation of the maternal part of the membrane. The larger blood vessels of the uterus are the conduits by which all this interaction between mother and child take place. Temporary minute capillaries or spiral arteries and veins allow the passage of blood from the mother and back to the mother after circulating through the fetus. The placenta is a multifaceted temporary organ that also acts as an endocrine system with regulatory hormones directing the pregnancy from beginning to end. Its hormone is what is what is detected in the mother's urine when pregnancy tests are positive.
The circulation itself, moving from right to left is unique. The placental veins empty into a duct that connects to the blood and oxygenates and from the inferior vena cava blood flows into the right atrium, bypassing the lungs. Some small amount of blood enters the left atrium by way of an opening that closes after birth. Another duct connects the main pulmonary artery to the aorta.
Three shunts are in place in the fetal development. One an arterial shunt protects delicate lungs from the force of the circulation, allows the ventricles to strengthen. A venous shunt connects to the umbilical cord that regulates the flow of blood by way of sphincters. The third shunt is the opening between the right atrium and left atrium. These close after birth. Occasionally there will be failures in closing due to some mishap and babies are born with heart defects.
The placenta is the commonly known afterbirth. After it stops pulsating the doctor or midwife will cut the cord. The remaining stub will gradually dry and fall off within a week or so of delivery. Only an indentation known as the naval will remain. Placenta's are gathered and are used for various purposes by medical scientists. They carry important messages via their DNA.