16.5 - Human Reproduction & Pregnancy

The male reproductive system

• The male reproductive system makes sperm (the male gametes) and delivers them into the female.
• It is made up of several parts, each with its own job.

Making and storing sperm

• Sperm are made in the testes.
• The two testes are held in the scrotum - a sac of skin that hangs behind the penis.
• Keeping the testes outside the main body keeps them slightly cooler than body temperature, which is the condition sperm need to be made.

Delivering the sperm

• Sperm ducts (sperm tubes) - Muscular tubes that carry sperm from the testes towards the urethra.
• Prostate gland - Adds a fluid that nourishes the sperm. The mixture of sperm and this fluid is called semen.
• Urethra - The tube running through the penis. It carries semen out of the body during ejaculation, and also carries urine out from the bladder at other times.
• Penis - Stiffens when it fills with blood, which lets it place sperm inside the female.

Female reproductive system, fertilisation, implantation

• The female reproductive system produces egg cells, holds and nourishes a developing baby, and is where fertilisation takes place.
• It is made up of several parts, each with its own job.

Parts of the female reproductive system

• Ovaries - Two organs that make the egg cells (ova) and also produce the sex hormones. Each month an egg is released from an ovary.
• Oviduct - A muscular tube that carries the released egg towards the uterus.
• Uterus - The organ in which an embryo can grow and develop.
• Cervix - The neck of the uterus, at its lower narrowed end; it opens into the vagina and widens during birth to let the baby pass through.
• Vagina - Where sperm are deposited; they then swim up through the cervix and uterus into the oviducts.

Fertilisation and implantation

Fertilisation happens in the oviduct.

1. A sperm reaches an egg there, and the nucleus of the sperm fuses with the nucleus of the egg cell to form a single new cell called a zygote.
2. Within about four days the zygote divides several times to form an embryo - a small ball of cells.
3. This ball of cells travels down the oviduct to the uterus and implants into the soft, thick lining of the uterus (the endometrium), which has a good blood supply to support the implanted embryo.

This implantation marks the start of a pregnancy.

Pregnancy - The placenta and exchange across it

• After the embryo implants in the lining of the uterus, structures develop to nourish and protect it as it grows.
• At about nine weeks after fertilisation, once it starts to look human, the developing baby is called a foetus.

Structures that support the foetus

• Placenta - An organ that grows in the wall of the uterus. It is the organ of exchange between the mother's blood and the foetus's blood, with a large surface area so substances pass across quickly by diffusion.
• Umbilical cord - Joins the placenta to the foetus. Substances are carried to and from the foetus in its blood vessels.
• Amniotic sac - A membrane surrounding the foetus, filled with a watery liquid called amniotic fluid.
• Amniotic fluid - Protects the foetus from knocks and bumps, supports its weight as it grows, and lets it move around.

Exchange across the placenta

• The placenta's surface is folded into many finger-like projections called villi, which push into the wall of the uterus and give it a very large surface area.
• The villi bring the foetus's blood and the mother's blood very close together, but the two blood supplies do not mix.
• With only a thin barrier between them, substances pass quickly from one blood supply to the other by diffusion:
  • To the foetus - Oxygen and dissolved nutrients (glucose and amino acids) supply food and the oxygen it needs.
  • From the foetus - Waste substances, carbon dioxide and urea, pass back to the mother.

The placenta as a barrier

• The placenta keeps most harmful substances out, but it does not stop everything.
• Some harmful substances - such as alcohol, nicotine, and certain drugs - and some pathogens can cross from the mother to the foetus, where they can harm the developing baby.