The Reproductive System
The reproductive system is the body system that allows humans to have babies. Unlike most other body systems, it is one of the few that comes in two clearly different versions: one for biological females and one for biological males. Both versions produce special sex cells (eggs in females, sperm in males), and when an egg and a sperm join, the result is a new human life that develops over 9 months in the mother's body. The reproductive system also plays a major role in puberty, the transformation from a child's body into an adult one.
- Sex cells (gametes)Sperm and eggCarry half a person's DNA each
- Eggs at birthapprox. 1 to 2 millionIn an infant ovary, all already there
- Sperm produced per dayapprox. 100 millionIn an adult male
- Pregnancy lengthapprox. 40 weeksAround 9 months
- Chromosomes in a sex cell23Half the normal number
- Chromosomes in a fertilised egg4623 pairs, normal full set
Why reproduction is special
Reproduction is one of the most important things any species can do. Unlike all the other systems of the body, which keep one person alive, the reproductive system creates a brand new person. The system produces special cells called gametes: eggs in females (very large, slow, non-mobile, full of nutrients), and sperm in males (very small, fast-moving, designed only to reach an egg).
Each gamete contains just 23 chromosomes: half the normal number of 46. When an egg and a sperm join, the two sets combine to make a complete set of 46 chromosomes again. The fertilised egg starts dividing and develops into a new baby, with genes from both parents.
The female reproductive system
The female reproductive system has these main parts.
- Ovaries: two small almond-shaped organs in the lower abdomen, storing the female's eggs. A baby girl is born with around 1 to 2 million eggs already present; she will never make more during her lifetime.
- Fallopian tubes: two tubes that catch the eggs released by the ovaries and carry them to the uterus.
- Uterus (womb): a muscular pear-shaped organ where a baby grows during pregnancy.
- Cervix: the narrow lower part of the uterus that connects to the vagina.
- Vagina: the muscular passage to the outside.
Roughly once a month after puberty, an ovary releases one mature egg (a process called ovulation), which then travels down the fallopian tube. This monthly cycle is called the menstrual cycle.
The male reproductive system
The male reproductive system has these main parts.
- Testes (testicles): two small organs in the scrotum (a pouch outside the main body) that produce sperm. The slightly cooler temperature outside the body is essential for sperm production.
- Epididymis: long coiled tubes attached to each testis, where new sperm mature.
- Vas deferens: tubes that carry sperm from the epididymis to the urethra.
- Seminal vesicles, prostate gland, bulbourethral glands: produce the fluid that carries the sperm (together making up semen).
- Urethra: the tube through which urine and semen exit (at different times).
- Penis: the external organ.
Unlike females, males produce new sperm continuously throughout adult life: around 100 million new sperm per day.
Puberty: the transformation
Puberty is the period of physical change when a child's body matures into an adult body. It is triggered by the hormones of the endocrine system: signals from the brain tell the gonads (testes or ovaries) to start producing large amounts of sex hormones. The main changes:
- In girls (usually 9 to 14): breasts develop, hips broaden, body hair grows, periods start.
- In boys (usually 10 to 15): voice deepens, body and facial hair grows, shoulders broaden, muscles develop, the body begins producing sperm.
Puberty also includes major emotional and mental changes as the brain itself continues to develop. The whole process takes 2 to 4 years.
How a new baby starts
A new human life begins when a male sperm cell joins with a female egg cell, in a process called fertilisation. Of the hundreds of millions of sperm in a single ejaculation, only a few hundred typically reach the egg in the fallopian tube. Just one usually breaks through the egg's outer layer; once it does, the egg shuts out all the others.
The fertilised egg, now called a zygote, has the full 46 chromosomes: 23 from each parent. The zygote starts dividing within hours and travels down the fallopian tube to the uterus. About 6 days after fertilisation, it implants in the uterus wall and begins growing into an embryo.
Pregnancy and birth
A typical human pregnancy lasts about 40 weeks (just over 9 months). The growing baby is called an embryo for the first 8 weeks, then a fetus from then until birth.
- First 3 months (first trimester): the embryo's major organs all start to form. By the end of the first trimester it is recognisably human and around 7 cm long.
- Middle 3 months (second trimester): the fetus grows rapidly. The mother starts to feel movements. The baby can hear sounds from outside.
- Last 3 months (third trimester): the baby puts on weight, develops fat layers, and prepares for birth.
At birth, hormones trigger the uterus muscles to contract powerfully, eventually pushing the baby out through the vagina. Many births happen this way (vaginal birth); others are done by surgical operation (C-section, named after Julius Caesar who, legend has it, may have been born this way).
Twins and multiple births
Most pregnancies produce a single baby. Sometimes two (or more) babies are born together.
- Identical twins: one fertilised egg splits in two early in development, producing two babies with identical DNA.
- Fraternal twins: the mother releases two eggs at once, both fertilised by different sperm. The twins share only as much DNA as ordinary siblings.
- Triplets and beyond are rarer (about 1 in 8,000 natural births) but more common in pregnancies from fertility treatment.
Deeper dive: how the embryo develops
The development of a single fertilised egg into a complete baby is one of the most remarkable processes in biology. Within just 9 months, a single cell divides and specialises into approximately 26 billion cells, organised into around 200 different cell types, forming every organ and tissue of a complete human being.
The process happens in stages. In the first week, the fertilised egg divides into a ball of identical cells called a blastocyst. By two weeks, the cells have organised into three different layers (ectoderm, mesoderm, endoderm), each of which will give rise to different organ systems.
By 3 weeks, the heart starts beating, even though it is just a simple tube. By 4 weeks, the eyes start to form, and there are small bumps that will become arms and legs. By 8 weeks, all the major organs are present in basic form, the embryo is recognisably human and is now called a fetus.
The remaining 7 months of pregnancy are mostly about growth and refinement. The fetus puts on weight, develops layers of fat, grows hair and nails, and practises breathing, swallowing and other movements. By the time of birth, the baby weighs around 3 to 3.5 kg and is fully ready to start life outside the womb.
The genetic instructions for this entire process are encoded in the original fertilised egg's DNA, in just 3 billion DNA letters. The way those instructions unfold (which genes switch on and off at what times) is one of the most active areas of modern biology, called developmental biology. The fact that DNA can pack all the instructions for a human being into a single cell is one of the most extraordinary facts in nature.
For the hormones that drive puberty and reproduction, see the endocrine system. For the cells and DNA that make heredity work, see inheritance.