Placenta

The **placenta** is a temporary organ in the uterus (womb) of pregnant female mammals. Its main role is to transfer oxygen and nutrients from the mother's heart to the baby, and to transfer waste back from the baby to the mother through blood circulation. The placenta has blood supply from the umbilical cord, which takes blood through one vein. Waste is taken out via two arteries. The placenta also protects the baby from harmful substances in the mother's blood, and produces hormones that secrete and support the pregnancy.  

The placenta is attached to the wall of the uterus and connected to the baby by big blood vessels in the umbilical cord. After the baby is born, the placenta and cord are also pushed out of the uterus. At this time the placenta and cord are often called the **afterbirth**.

## Functions and roles 
### Feeding baby  
The baby in the womb gets everything it needs through the placenta. Oxygen travels in the mother's blood from the lungs to the placenta through two arteries. It reaches the placenta through the umbilical cord to baby. 

### Removing waste 
Carbon dioxide waste travels out through one vein, in reverse. The blood carrying away waste product goes from baby's blood to the mothers blood and out through her lungs. Nutrient molecules (sugars, fats, proteins and vitamins) travel through her blood from her digestive tract to the placenta, and then in the baby's blood to the baby, and waste products travel back to her kidneys for disposal.

### Protection 
One of the placenta's jobs is to protect the fetus and mother. It also needs to make sure blood from the mother and fetus never mixes. The placenta acts as an exchange surface between the mother and the fetus. Nutrients and oxygen are passed from one bloodstream to the other by diffusion only. If the mother's and fetus's blood mixed, it could be deadly for both of them. If the mother and the fetus had different blood types, they might both die if their blood mixed.

### Hormone production 
Throughout pregnancy the placenta produces hormones that travel through blood from the mother and help her body adapt to the pregnancy. One of these hormones is detected by pregnancy tests and also is responsible for the nausea many women experience early in pregnancy ('morning sickness'). Once the baby is ready to be born, the placenta produces hormones that stimulate labor by increasing bleeding and muscle relaxation.

## Placental barrier  
The placenta has two parts, one of which is genetically and biologically part of the fetus, the other part of the mother. It is implanted in the wall of the uterus, where it receives nutrients and oxygen from the mother's blood and passes out waste. This forms a barrier called the placental barrier, which filters out some substances which could harm the fetus. 

The placental barrier is not able to protect the fetus from everything. For example, alcohol goes through the placental barrier into the fetus. This is why drinking alcohol during pregnancy can cause disorder and conditions such as Fetal Alcohol Syndrome. The placenta is not able to filter out many other things. Chemicals can cross the placental barrier and hurt the fetus. Things which do this are called teratogens. Some viruses can also cross this barrier and infect the fetus with a disease. 

On the good side, many of the mother's antibodies can pass through the placenta, and these can help the baby resist diseases in its first year.

## Placenta-like organs in other animals 
The egg-laying mammals (echidna and platypus) and marsupial mammals produce a type of placenta that provides nutrients mostly from the egg sac, instead of from the mother's blood. It is positioned in the female's body similar to eutherian mammals. Non-mammals who give birth to live young rather than laying eggs (some snakes and lizards, and even some fish), have also evolved systems of internal development with a placenta-like tissue: these are examples of convergent evolution.

## Society and culture 
Many countries consider the placenta (afterbirth) to be edible. However most say that it is not healthy or beneficial, and is also unsafe. The afterbirth might contain bacteria which might be harmful to the person eating it, causing them to get sick. and done mostly to bury it as well as for consumption (eating). Other people, such as new mothers, may take it home to freeze it. After birth the placenta will break down quickly if it is not kept cold. Because it needs to be frozen as soon as possible, fast transport to where it needs to be keeps it in-tact. Burial of the placenta in some countries including the UK is illegal because it is bodily tissue and it is illegal to bury remains (except in a cemetery). This is likely due to prevent murder. The placenta must be burned (cremated) at high temperatures or put into the bin. The placenta is mostly  of blood, which is a connective tissue.

## References