Pregnancy, the placenta, STIs and HIV

Last time

Before we start: last time, in The human reproductive systems and the menstrual cycle, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Nine months

For nine months, a growing baby gets everything it needs from its mother, and her blood never mixes with its own. The placenta makes that possible. Today: pregnancy, the placenta and the amniotic fluid, and sexually transmitted infections, especially H I V.

After fertilisation

The zygote divides as it moves down the oviduct, forming a ball of cells, the embryo. The embryo sinks into the thick lining of the uterus. This is implantation. Once the organs have formed, at about eight weeks, it is called a fetus.

Support for the fetus

The placenta is attached to the uterus wall. It is where substances are exchanged. The umbilical cord joins the fetus to the placenta, carrying its blood vessels. The amniotic sac holds amniotic fluid, which cushions and protects the fetus.

Across the placenta

From mother to fetus: oxygen, glucose, amino acids, water, mineral ions and antibodies. From fetus to mother: carbon dioxide and urea, the fetus's waste products. They cross by diffusion and active transport. The two bloods are kept separate.

Harmful substances

Some harmful substances cross the placenta: nicotine, alcohol and some drugs. Some pathogens, such as the rubella virus, can cross and harm the fetus. This is why pregnant women are advised not to smoke or drink alcohol.

STIs

A sexually transmitted infection is an infection that is passed from one person to another by sexual contact. H I V is a virus that is an S T I. It can lead to A I D S.

HIV

H I V infects and destroys lymphocytes, a type of white blood cell. Fewer lymphocytes means fewer antibodies, so the immune system is weakened. The person then catches other infections easily. This stage is A I D S.

How HIV spreads

Learn how H I V is and is not passed on. It spreads through sexual contact, sharing needles, and from mother to baby. It does NOT spread by hugging, shaking hands, or sharing plates and cups.

Controlling STIs

Use condoms, which act as a barrier. Never share needles, and screen donated blood. Get tested, and use treatment to lower the amount of virus in the body.