What Is a Placenta and What Does the Placenta Do?
The developing embryo or fetus is of course unable to eat or digest food. Nutrients from the pregnant woman are therefore broken down and passed in her blood into the placenta to be used by the embryo or fetus.
Providing oxygen and carrying away carbon dioxide (CO2).
The human body needs oxygen to live. We breathe in air and use oxygen, exhaling the waste product as carbon dioxide. The pregnant woman supplies the embryo or fetus with oxygen through her blood supply. The oxygen crosses the placenta and the waste carbon dioxide is carried back in the same way.
The amniotic sac is a bag full of fluid in which the fetus develops. It is made of two thin, colourless membrane sheets, the outer one being the chorion and the inner layer the amnion. After a few weeks this sac fills the uterine cavity (space inside the uterus). The amniotic fluid (sometimes known as ‘the waters’) surrounds the fetus and acts as a protective environment. There are 150 ml (5 fl. oz) of liquid in the sac by the time the fetus is ten weeks. By the end of pregnancy there are 1.5-2 litres (about 21/2-31/2 pints). The amniotic fluid is made up of water, salts, fats and small amounts of foetal urine. It carries out the following functions:
- providing a liquid environment at a constant temperature suitable for growth
- protecting against knocks and bumps
- allowing the fetus to begin its swallowing and breathing movements in preparation for breathing after birth
- helping to guard the fetus against infection.
As the fetus develops it sheds live cells from its skin into the amniotic fluid. At 14 to 16 weeks some of the fluid can be drawn off and these cells tested to check for abnormalities. This is called amniocentesis.
Other waste such as urea is passed across the placenta to be got rid of in the woman’s urine.
The placenta can filter out many of the germs, infections and drugs which the pregnant woman may have in her body. Unfortunately it cannot filter all of them, and some harmful viruses, such as the rubella virus, and drugs do get through. The harmful effects of smoking can also get past the placenta and damage the fetus, especially during the early weeks when the embryo is still forming various parts of the body. However, some of the antibodies cross the placenta and help to protect the embryo or fetus and the baby from diseases.
Large amounts of hormones such as oestrogen and progesterone are produced, and the pregnancy hormones that maintain the placenta and regulate the pregnancy.
A layer of cells forming a fine membrane divides the foetal blood from the mother’s blood. In this way the blood of the fetus and of the mother are completely separate.