Human reproduction and growth

Every human starts life as a single cell, made when a sperm cell from the father joins an egg cell from the mother. That cell grows into a baby, the baby grows into a child, and during puberty the body changes so that one day it could have children of its own.

Key facts

• The human life cycle: baby → child → adolescent (teenager) → adult → older adult. Before birth, the growing baby is called a foetus.
• Puberty usually happens between about 10 and 16. In both boys and girls, body hair grows, there is a growth spurt and sweat glands become more active.
• In boys the voice deepens, the shoulders widen and the testes start to make sperm. In girls the breasts develop, the hips widen and periods start.
• Male reproductive system: the testes make sperm and the hormone testosterone; sperm ducts carry the sperm; glands add fluid to make semen; it leaves through the urethra in the penis.
• Female reproductive system: the ovaries hold the eggs and release one about every 28 days; an oviduct carries it to the uterus (womb), where a baby can grow; the cervix is a ring of muscle at the entrance to the uterus.
• Fertilisation happens in an oviduct when the nucleus of a sperm joins the nucleus of an egg. The fertilised egg divides into a ball of cells (an embryo), which settles into the lining of the uterus.
• Pregnancy lasts about 40 weeks (9 months). The placenta passes oxygen and food from the mother’s blood to the foetus and takes away carbon dioxide and waste; the umbilical cord joins them, and fluid in the amniotic sac cushions the foetus.

Worked examples