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Definition: placenta from The Penguin Dictionary of Psychology

In mammals, an organ consisting of embryonic and maternal tissue to which the foetus is attached via the umbilical cord. The placenta is the ‘life-support system‘ for the foetus: oxygen, food substances and antibodies enter through it, and metabolic waste products leave through it. In general, there is no admixture of maternal and foetal blood. The placenta also serves as an endocrine organ producing various hormones so that the uterus remains properly adapted during pregnancy.


Summary Article: placenta
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Organ that attaches the developing embryo or fetus to the uterus (womb) in placental mammals (mammals other than marsupials, platypuses, and echidnas). Composed of maternal and embryonic tissue, it links the blood supply of the embryo to the blood supply of the mother, allowing the exchange of oxygen, nutrients, and waste products. The two blood systems are not in direct contact, but are separated by thin membranes, with materials diffusing across from one system to the other. The placenta also produces hormones that maintain and regulate pregnancy. It is shed as part of the afterbirth.

The placenta is an organ essential to the development of the fetus. In mammals, the fetus develops inside the body of the mother. The placenta is attached to the fetus by a cord containing blood vessels – the umbilical cord. The placenta is also firmly attached to the uterus of the mother. The role of the placenta is to bring the blood of the fetus close to the blood of the mother. This then allows oxygen and mineral salts to pass from the mother to the fetus, while carbon dioxide and other waste materials pass in the opposite direction. A variety of materials, including drugs and viruses, can pass across the placenta and enter the fetus. HIV, the virus that causes AIDS, can be transmitted in this way. However, the mother provides some protection against infection. There are chemicals that the mother makes to defend against disease that can cross into the fetus, such as some antibodies.

The tissue in plants that joins the ovary to the ovules is also called a placenta.

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External and Internal Fertilization

Development of fetus and function of placenta

Placenta: a Life-Support System

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