The placenta is the site of nutrient, gas, and waste exchange between mother and fetus.
Maternal and fetal blood do not mix due to potential blood type incompatibility.
Diffusion is the primary method for transferring water, glucose, amino acids, and inorganic salts.
Diffusion requires a concentration gradient; the partial pressure of oxygen is higher in maternal blood than in fetal blood.
Fetal Hemoglobin (HbF)
Fetal blood cells contain HbF, which has a greater affinity for oxygen compared to adult hemoglobin (HbA).
This facilitates the transfer and retention of oxygen in the fetal circulatory system.
Waste materials and carbon dioxide move from fetal to maternal circulation.
Placental Barrier and Immunity
The placenta provides a barrier and immunity to the fetus.
The fetus is immunologically naive and can be accidentally exposed to pathogens in utero.
Antibodies cross the placental membrane, providing protection.
Placenta as an Endocrine Organ
The placenta produces:
Progesterone
Estrogen
Human Chorionic Gonadotropin (HCG)
These hormones are essential for maintaining pregnancy.
Umbilical Vessels
Umbilical arteries carry blood away from the fetus to the placenta (deoxygenated).
Umbilical vein carries blood towards the fetus from the placenta (oxygenated).
Oxygenation occurs at the placenta, not the fetal lungs.
Differences Between Fetal and Adult Circulation
Fetal lungs and liver have limited function before birth.
Gas exchange occurs at the placenta.
The mother's liver handles detoxification and metabolism.
Nutrient and waste exchange occurs at the placenta.
The fetal lungs and liver are underdeveloped and sensitive to high blood pressure after birth.
Fetal Shunts
Fetal shunts direct blood away from the lungs and liver during development.
Foramen Ovale
A one-way valve that connects the right atrium to the left atrium.
Allows blood from the inferior vena cava to flow from the right atrium to the left atrium, bypassing the right ventricle, and then into the aorta and systemic circulation directly.
In the fetus, pressure is higher on the right side of the heart than the left, pushing blood through the foramen ovale.
After birth, the pressure differential reverses, causing the foramen ovale to close.
Ductus Arteriosus
Shunts leftover blood from the pulmonary artery to the aorta.
Driven by the pressure differential between the right and left sides of the heart.
Ductus Venosus
Shunts blood returning from the placenta via the umbilical vein directly into the inferior vena cava, bypassing the liver.
The liver still receives some blood supply from smaller hepatic arteries through systemic circulation.