At full term, the human placenta is a discoid organ with a diameter of roughly 15 to 20 centimeters and a thickness of about 2 to 3 centimeters. It weighs approximately 500 grams, which represents roughly one-sixth of the newborn's weight.
The placental membrane separates maternal blood from fetal blood, preventing direct mixing while allowing the exchange of substances. This membrane is not a single layer but a complex barrier composed of fetal tissues.
Blood circulation through the placenta is divided into two separate circuits that do not normally mix.
Maternal blood enters the intervillous space through spiral endometrial arteries under high pressure, washing over the branching villi where exchange occurs, before draining back through endometrial veins. Meanwhile, deoxygenated fetal blood travels from the fetus through the two umbilical arteries to the placental capillaries, where it picks up oxygen and nutrients, and returns to the fetus via a single umbilical vein.
The placenta acts as the lungs and kidneys for the developing fetus. Oxygen diffuses across the placental membrane from maternal blood to fetal blood due to a concentration gradient, assisted by the high affinity of fetal hemoglobin. Carbon dioxide moves in the opposite direction, passing easily from the fetus to the maternal circulation for elimination by the mother's lungs.
For excretion, fetal metabolic waste products such as urea, uric acid, and creatinine cross the placental membrane into the maternal blood, allowing the maternal kidneys to filter and excrete them.
Because the fetus cannot eat, the placenta extracts and processes nutrients from the maternal blood. Glucose is the primary energy source for the fetus and crosses the membrane rapidly via facilitated diffusion. Amino acids are actively transported against a concentration gradient to ensure adequate protein synthesis for fetal growth.
Vitamins, electrolytes, and water cross readily through diffusion or active mechanisms. Additionally, the placenta synthesizes glycogen, cholesterol, and fatty acids to support its own high metabolic demands and fetal nutrition.
At term, the placenta functions as a major temporary endocrine gland, secreting vital hormones directly into both maternal and fetal circulations to maintain pregnancy and prepare the body for lactation.
The placenta shields the semi-allograft fetus—which carries foreign paternal antigens—from rejection by the maternal immune system. It achieves this through local immunosuppressive factors and the absence of certain HLA molecules on trophoblast cells.
Furthermore, the placenta serves as a selective barrier. While it allows maternal immunoglobulin G (IgG) antibodies to pass through via pinocytosis to provide passive immunity to the newborn, it blocks many harmful bacteria and large molecules. However, many viruses, drugs, and alcohol can cross the barrier and pose severe risks to fetal development.