BIOLOGY · Stage 9 · Chapter 11: Human Genetics and Development
Biology 11.5 The development of a fetus — Placenta, Amniotic Sac and Umbilical Cord
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Lesson summary
11.5 The development of a fetus — Placenta, Amniotic Sac and Umbilical Cord — Alhussan International Academy — Biology (Stages 7–9) notes: Describe healthy fetal development, how the fetus is protected and supplied with materials, and how substances are exchanged at the placenta.…
- Early development
- Dependence
- Last three months
- Functions of the fluid
- Blood flow
- Umbilical vein
Section 1 of 6Stages of development
- Gestation = the time a baby takes to develop from a fertilised egg, about 40 weeks (9 months) in humans.
- Early development
- after implantation the cells grow, divide and specialise to form tissues and organs.
- A fertilised egg that only kept dividing would make a blob of identical cells, so specialisation is essential.
- Fetus
- after 8 weeks the embryo is called a fetus.
- By 12 weeks most organ systems are formed, but they need many more weeks to develop before the fetus can survive alone.
- Growth
- the fetus grows from a few cells to a baby of about 3 to 4 kg.
- Dependence
- it cannot breathe, eat or excrete for itself, so it relies on its mother through the placenta.
- Last three months
- the fetus grows rapidly and stores fat under its skin to survive after birth.
Section 2 of 6The amniotic sac and fluid
- Amniotic sac
- the fetus develops inside this bag, filled with amniotic fluid.
- Functions of the fluid
- it supports the baby as it grows and moves.
- It acts as a shock absorber because it cannot be compressed and spreads out any force.
- It keeps the temperature steady.
- It allows free movement, which helps bones and muscles to develop.
- Birth
- when labour starts the sac breaks, which is called the waters breaking.
Section 3 of 6The placenta
- Structure
- the placenta is a disc-shaped organ that grows into the wall of the uterus.
- It has a very large surface area because of finger-like villi filled with fetal blood capillaries.
- Blood flow
- the mother's blood flows in spaces around the villi, and the two bloods flow very close together but do not mix.
- Exchange
- substances cross the thin barrier by diffusion and active transport.
- Oxygen and dissolved food molecules move from the mother's blood to the fetus.
- Carbon dioxide and urea pass from the fetus to the mother to be removed.
- Barrier
- the placenta stops most infections and harmful substances reaching the baby.
Section 4 of 6The umbilical cord
- Role
- it joins the fetus to the placenta.
- Umbilical vein
- it carries oxygenated blood full of dissolved food from the placenta to the fetus.
- Umbilical arteries
- two vessels carry deoxygenated blood loaded with waste from the fetus to the placenta.
| From mother to fetus | From fetus to mother |
|---|---|
| Oxygen | Carbon dioxide |
| Glucose, amino acids, vitamins and minerals | Urea and other waste |
| Antibodies | Water |
| Water |
Section 5 of 6Why the blood does not mix
- Blood groups
- the mother and fetus may have different blood groups, and mixing could make the mother's immune system attack the baby's cells.
- Blood pressure
- the mother's blood pressure is much higher and could damage the fetus's delicate capillaries.
- Harmful substances that can cross
- alcohol, nicotine, some drugs and viruses such as rubella.
- Pregnant women are therefore advised to avoid alcohol and smoking and to check medication with a doctor.
Section 6 of 6Birth
- After around 40 weeks the baby can no longer get enough food and oxygen through the placenta, so it must be born.
- The cervix relaxes and the uterus muscles contract in labour, pushing the baby through the cervix and vagina.
- The baby breathes for the first time, and the cord is clamped and cut.
- The placenta is expelled as the afterbirth.
- The mother produces milk, which supplies nutrients and antibodies to the baby.
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