The placenta & the amnion (7)

Question 1

What is the placenta?

Answer 1

The placenta is a disc-shaped temporary organ richly supplied with maternal and foetal blood vessels. It connects the foetus with its mother’s blood at the end of the umbilical cord.

Question 2

What does the placenta develop from?

Answer 2

As mentioned the placenta develops from the chorion and its villi that penetrate the endometrium.

Question 3

What is the function of the villi?

Answer 3

The villi provide a large surface area of attachment of the placenta to the endometrium. Each villus contains a capillary network, which provides a large surface area for exchange of substances by diffusion.

Question 4

What are placental villi surrounded?

Answer 4

The placental villi are surrounded by the mother’s blood, so that mother’s blood supply and foetal capillaries are very close to each other.

Question 5

Do the foetal and mother’s blood mix?

Answer 5

However, the foetal and mother’s blood do not mix because the foetal capillaries have thin walls. Substances move between the maternal and foetal blood by diffusion.

Question 6

What is the function of the placenta? (5)

Answer 6

1. Oxygen and dissolved food substances such as glucose, amino acids, fatty acids, ions and vitamins pass from the mother to the foetal blood system for respiration and nutrition.
2. Foetal waste products of metabolism, e.g. urea and carbon dioxide pass into the maternal blood system for excretion.
3. Some maternal antibodies pass to the foetus providing temporary passive immunity to certain diseases. As a result the child enjoys immunity to most infectious diseases during the first six months of life.
4. The placenta acts as a barrier (micro filter) preventing many pathogenic (harmful) micro-organisms and drugs from entering the foetus from the mother. However, some pathogens can pass through the placenta, e.g. those causing German measles (rubella), syphilis and HIV.
5. The placenta also has an endocrine function.

Question 7

How does the endocrine function? (2)

Answer 7

• After three months of pregnancy the placenta takes over the function of secreting progesterone and oestrogen from the ovaries (corpus luteum). These hormones are essential for bringing about the necessary changes in the uterus and for the maintenance of pregnancy. Progesterone prevents ovulation and menstruation which would result in the loss of the foetus.
• Late in pregnancy it secretes relaxin, a hormone that relaxes joints and ligaments to assist in the delivery of the baby.

Question 8

What is the amnion?

Answer 8

The amnion is a membrane that surrounds the developing embryo and foetus. It secretes a fluid called amniotic fluid that fills the amniotic cavity.

Question 9

What is amniotic fluid?

Answer 9

Amniotic fluid surrounds the developing foetus and serves to provide a protective environment. It consists of about 99% water, foetal cells and waste products.

Question 10

What information do the cells contain?

Answer 10

The cells contain foetal genetic information. This is why amniotic fluid is used during a process called amniocentesis to determine whether the baby has any chromosomal abnormalities.

Question 11

What is the function of amniotic fluid? (5)

Answer 11

1. It supports the developing foetus, allowing it freedom to move about easily and so prevent malformations due to gravity/pressure.
2. It cushions and protects the foetus from damage by external injury or from impact from the mother.
3. It provides a medium in which to practice breathing and swallowing movements.
4. It holds urine as it is released from the foetus.
   5.It protects the foetus against:
   • changes in temperature
   • dehydration.