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Human reproduction and pregnancy

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The male and female reproductive systems are adapted to make gametes and bring them together. Hormones control the cycle and puberty.

The reproductive systems

Male: testes make sperm and testosterone; the scrotum holds them outside the body where it is cooler; sperm ducts carry sperm; glands add fluid to make semen; the urethra in the penis carries semen out.
Female: ovaries release eggs and make oestrogen and progesterone; the oviduct (fallopian tube) carries the egg and is where fertilisation happens; the embryo develops in the uterus; the cervix is the entrance to the uterus; the vagina receives sperm.

Oestrogen and progesterone in the cycle

The menstrual cycle lasts about 28 days.
Oestrogen causes the uterus lining to build up again after menstruation.
Progesterone maintains the lining.
If no embryo implants, progesterone levels fall, the lining breaks down and menstruation happens.

Pregnancy and puberty

The placenta lets substances pass between the mother's blood and the embryo's blood without the bloods mixing. Oxygen, glucose and amino acids diffuse to the embryo; carbon dioxide and urea diffuse away.
Amniotic fluid surrounds the embryo and protects it from bumps.
At puberty, oestrogen causes breasts to develop, hips to widen and periods to start. Testosterone causes the voice to deepen, facial and body hair to grow, muscles to develop and sperm production to start.
Worked example

Explain why the embryo's blood needs to be close to the mother's blood in the placenta.

  1. Substances move between them by diffusion.
  2. A short distance means fast diffusion of oxygen and nutrients to the embryo, and of wastes away.

Answer: So oxygen and nutrients can diffuse quickly to the embryo, and carbon dioxide and urea away from it.

Key idea

Testes make sperm and testosterone; ovaries release eggs and make oestrogen and progesterone. Oestrogen builds up the uterus lining; progesterone maintains it. The placenta exchanges oxygen, nutrients and wastes; amniotic fluid protects the embryo. Oestrogen and testosterone cause secondary sexual characteristics.

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