Human Reproduction
Understand the structure and function of the male and female reproductive systems, the process of fertilisation, and how a human develops from a single cell to a newborn.
The Reproductive Systems
Human reproduction is sexual — it involves the combination of genetic material from two parents to produce offspring. The male and female reproductive systems each produce specialised sex cells called gametes.
Male Reproductive System
| Structure | Function |
|---|---|
| Testes | Produce sperm (spermatogenesis) and testosterone |
| Epididymis | Stores and matures sperm |
| Vas deferens | Tube that carries sperm from epididymis to urethra |
| Seminal vesicles | Produce fluid that nourishes sperm (part of semen) |
| Urethra | Carries semen (and urine separately) out of the body |
Sperm cell: Smallest human cell; has a head (nucleus with 23 chromosomes), midpiece (mitochondria for energy), and tail (flagellum for swimming).
Female Reproductive System
| Structure | Function |
|---|---|
| Ovaries | Produce eggs (oocytes) and hormones (oestrogen, progesterone) |
| Fallopian tubes | Carry eggs from ovary to uterus; site of fertilisation |
| Uterus | Muscular organ where the embryo implants and develops |
| Cervix | Lower narrow part of the uterus connecting to the vagina |
| Vagina | Birth canal; receives sperm during intercourse |
Egg cell (ovum): Largest human cell; spherical, contains 23 chromosomes. One egg is typically released per menstrual cycle (ovulation).
Fertilisation
Fertilisation is the fusion of a sperm cell and an egg cell to form a zygote. It typically occurs in the fallopian tube:
1
Ovulation
An egg is released from the ovary into the fallopian tube (~day 14 of the menstrual cycle).
2
Sperm reach the egg
Millions of sperm are deposited; only hundreds reach the fallopian tube. Sperm can survive up to 5 days.
3
Fertilisation
One sperm penetrates the egg. Their nuclei fuse, combining 23 + 23 = 46 chromosomes to form the zygote.
4
Implantation
The zygote divides (mitosis) to form an embryo, which travels to the uterus and implants in the uterine wall (~day 6–10).
Development: From Embryo to Birth
Human pregnancy lasts approximately 38–40 weeks (about 9 months) from fertilisation to birth. Development is divided into three trimesters.
First Trimester (Weeks 1–12)
- Zygote becomes an embryo through repeated cell division
- The placenta and umbilical cord form — provide nutrients and oxygen, remove waste
- All major organs begin forming by week 8
- By week 12 the embryo is now called a foetus and is about 6 cm long
Second Trimester (Weeks 13–27)
- Rapid growth; the foetus develops muscles, bones and distinct facial features
- Movement is felt by the mother (quickening) around weeks 16–20
- The foetus can hear sounds and responds to light
- By week 24, the foetus weighs about 600 g
Third Trimester (Weeks 28–40)
- Continued growth and organ maturation; lungs develop fully
- The foetus turns head-down in preparation for birth
- At birth, the average baby weighs approximately 3.5 kg and is 50 cm long
- Labour is triggered by hormonal changes; the baby is delivered through the cervix and vagina
The Placenta
The placenta is a specialised organ connecting the foetus to the uterine wall. It allows oxygen and nutrients to pass from the mother’s blood to the foetal blood, and carbon dioxide and waste to pass back — all without the two bloodstreams mixing. The umbilical cord connects the placenta to the foetus.
Key Vocabulary
| Term | Definition |
|---|---|
| Gamete | A sex cell (sperm or egg) containing half the normal number of chromosomes (23 in humans). |
| Zygote | The single cell formed when a sperm and egg fuse during fertilisation; contains 46 chromosomes. |
| Placenta | The organ that forms during pregnancy to supply nutrients and oxygen to the foetus and remove waste products. |
| Ovulation | The release of a mature egg from the ovary, usually occurring around day 14 of the menstrual cycle. |
Worked Examples
Explaining how a zygote gets 46 chromosomes.
Context: A normal human body cell has 46 chromosomes. Sex cells (gametes) are produced by meiosis.
Step 1: During meiosis, the chromosome number is halved: sperm contains 23 chromosomes; egg contains 23 chromosomes.
Step 2: At fertilisation, sperm and egg fuse. 23 + 23 = 46 chromosomes in the zygote.
Answer: The halving of chromosomes in gametes and their combination at fertilisation ensures the offspring has the correct number of chromosomes (46) for a human. This prevents the chromosome number from doubling each generation.
Describing the role of the placenta in foetal development.
Question: How does the foetus get oxygen during pregnancy if the foetal and maternal blood do not mix?
Explanation: The placenta has a very large surface area with thin walls separating maternal and foetal blood vessels. Oxygen and glucose diffuse from the mother’s blood into the foetal blood down a concentration gradient. CO2 and urea diffuse in the opposite direction into the mother’s blood for removal.
Answer: Exchange occurs by diffusion across the thin placental membrane, not by mixing of blood. The umbilical cord carries oxygenated blood to the foetus and deoxygenated blood away.
Comparing sperm and egg cells: structure linked to function.
Sperm: Small, streamlined head (to reduce drag), many mitochondria in midpiece (provide ATP for swimming), long flagellum (tail for propulsion). Adapted to swim long distances to reach the egg.
Egg (ovum): Large cell with many nutrients (yolk) to support early embryo development. Has a thick outer layer (zona pellucida) that hardens after one sperm enters, preventing polyspermy (entry of multiple sperm).
Key contrast: Sperm are numerous and mobile; eggs are large and nutrient-rich. Both carry 23 chromosomes to combine at fertilisation.
Knowledge Check
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Key Concepts Summary
- •The testes produce sperm; the ovaries produce eggs (ova). Both gametes carry 23 chromosomes.
- •Fertilisation occurs in the fallopian tube; sperm and egg fuse to form a zygote with 46 chromosomes.
- •The zygote divides by mitosis to form an embryo, which implants in the uterine wall.
- •The placenta enables nutrient, oxygen and waste exchange between mother and foetus without blood mixing.
- •Human pregnancy lasts ~40 weeks; the embryo becomes a foetus at week 9 and is born at around week 38–40.