The Placenta & the Exchange of Materials
- Mammals can be split into different categories on the basis of their mechanism for foetal nourishment
- Placental mammals, such as humans
- Monotremes, or egg laying mammals, such as a platypus
- Marsupials, such as kangaroos, whose offspring develop in a pouch
- Placental mammals rely on the complex system of blood vessels specially designed to maximise exchange of substances between mother and foetus without any direct connection between them
- The placenta is also responsible for production of key pregnancy hormones oestrogen and progesterone
- The foetus is connected to the placenta via the umbilical cord and is contained within the amniotic sac filled with amniotic fluid which protects the foetus
Structure of the placenta
- The placenta is an organ primarily made up of a complex arrangement of blood vessels arranged into placental, or chorionic, villi with maternal blood flow distributed around the villi
- Throughout the course of the pregnancy, the number of villi increases to meet the demands of the growing foetus
- Maternal blood and foetal blood never mix directly, but flow either side of a layer of cells that make up the placental barrier; there is a very short distance between the maternal and foetal blood to allow exchange of nutrients and gases
- The mother's blood flows out of the mother's blood vessels and forms pools in the spaces surrounding the placental villi; these spaces are known as the inter-villous spaces
- The placental membrane, or barrier, provides a selectively permeable barrier which restricts the exchange of substances between mother and baby
- Substances that move across the barrier from mother to foetus include
- Oxygen
- Antibodies
- Antibodies cross the placenta using a mechanism called endocytosis
- Water
- Glucose
- Unwanted or harmful substances may also cross the placental barrier, including alcohol, drugs or small pathogens such as viruses
- Bacterial pathogens are too large to cross the barrier
- Substances that move across the barrier from foetus to mother include
- Carbon dioxide
- Water
- Urea
- The placenta is connected to the growing foetus by the umbilical cord
The placenta diagram
The placenta brings the maternal and foetal blood systems close but without directly mixing
Exam Tip
You are not required to know details of placental structure apart from the large surface area of the placental villi. You should understand which exchange processes occur in the placenta and that it allows the foetus to be retained in the uterus to a later stage of development than in mammals that do not develop a placenta.