The placenta has been described as a pancake-shaped organ that attaches to the inside of the uterus and is connected to the fetus by the umbilical cord.

  • Found only in mammals.
  • Only organ composed of cells derived from two organisms; mother and fetus.
  • It is known as deciduous alantochorion placenta.

Structure of placenta


  • Placenta is a disc shaped structure.
  • Arises from the chorion, allantois and uterine wall.
  • Finger like processes(chorionic villi) containing capillary loops project into maternal blood space in the uterine wall.
  • These finger like processes increase the surface area
  • In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness (greatest thickness at the center and become thinner peripherally).
  • It typically weighs approximately 500 grams (1 lb).

Importance of placenta

  • Maternal and fetal blood do not mix.
  • Across the thin barrier, exchange of material by diffusion and active transport occurs.
  • From the maternal blood, foetus obtains oxygen, water, glucose, amino acids, lipids, some proteins, minerals, vitamins and hormones.
  • Several antibodies, drugs( including alcohol),viruses(Hepatitis B, Rubella),toxins and diverse substances absorbed from mother’s body tobacco smoke can also pass to the foetus.
  • From the fetus water, urea, hormones and carbon dioxide pass to the maternal blood.
  • Placenta is also an endocrine gland which produces hormones including hCG at the initial stages and later progesterone , oestrogen and placental lactogen.
  • Prevention from coagulation of blood due to Rh factor and different blood groups and relatively high blood pressure of maternal circulation

Functions of placenta
• Exchange material between mother and fetus after 12 weeks of pregnancy.
• Endocrine [oestrogen, progesteron, human placental lactogen, chorionic
• Attachment of the fetus to mother
• Acting as a barrier for certain materials
• Prevention from coagulation of blood due to Rh factor and different blood groups, and relatively high blood pressure of maternal circulation.
• Fetal membranes
The outer cells of the blastocyst (trphoblastic villi) project into the surrounding tissue of the endometrium. Nutrients are absorbed across these villi. The cells of the inner cell mass divide. The embryo grows continuously and outer cells and tissues give rise to the amnion and yolk sac. Amnion surrounds the fluid filled cavity(amniotic cavity) in which the embryo is suspended and cushioned against mechanical damage. Yolk sac has no obvious function and later becomes buried in placenta.
The fourth embryonic membrane i.e. the allantois develops from the hind gut and grows in close contact with the chorion and participates in the formation of placenta.

human placenta
• Umbilical cord
Connecting cord from the placenta to the fetus.



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