Human ReproductionOxford AQA IGCSE Biology: Revision notes
Section 1
What is the structure and function of the male reproductive system?
The male reproductive system produces sperm and transports them to the female reproductive system during sexual intercourse.
Key structures and their functions:
| Structure | Function |
|---|---|
| Testes | Paired organs that produce sperm through meiosis; also secrete testosterone |
| Sperm duct (vas deferens) | Transports sperm from the testes to the urethra; stores sperm |
| Prostate gland | Secretes alkaline fluid that activates sperm and provides nutrients |
| Urethra | Tube that carries sperm and urine through the penis to the outside |
| Penis | External organ that delivers sperm into the female reproductive system during sexual intercourse |
Sperm structure: Sperm are specialised gametes with a head (containing the nucleus), midpiece (packed with mitochondria for energy), and tail (flagellum for movement). This design allows sperm to swim towards the egg.
Think of the male system as a delivery pipeline: the testes manufacture the package (sperm), the sperm duct stores and transports it, the prostate gland adds protective packaging (alkaline fluid), and the penis delivers it to its destination.
Examiners expect you to link structure to function – always explain why sperm have a tail (for swimming) and mitochondria (for energy). Simply naming structures without function will lose marks.
Section 2
What is the structure and function of the female reproductive system?
The female reproductive system produces eggs, provides a site for fertilisation, and supports foetal development.
Key structures and their functions:
| Structure | Function |
|---|---|
| Ovaries | Paired organs that produce eggs (ova) through meiosis; secrete oestrogen and progesterone |
| Fallopian tubes (oviducts) | Transport eggs from ovaries to uterus; site of fertilisation; have cilia to move egg along |
| Uterus (womb) | Muscular organ where the fertilised egg implants and the foetus develops |
| Cervix | Muscular ring at the base of the uterus; acts as a barrier; dilates during labour |
| Vagina | Elastic tube that receives the penis during intercourse and forms the birth canal |
Egg structure: Eggs are large gametes containing abundant cytoplasm (nutrients for early embryonic development) and one nucleus with genetic material.
The female system is like a specialised hotel: the ovaries produce guests (eggs), the fallopian tubes are the reception area where guests are greeted (fertilisation), and the uterus is the luxury suite where guests stay and develop (foetus).
Students often confuse the role of the fallopian tubes – they are NOT just a passive tube. The cilia lining them actively beat to move the egg towards the uterus, and fertilisation happens inside the tube, not in the uterus.
Section 3
How does the menstrual cycle prepare the body for pregnancy?
The menstrual cycle is a monthly cycle in females of reproductive age that involves ovulation (release of an egg) and preparation of the uterus lining for potential implantation. The cycle is controlled by hormones and typically lasts 28 days (range: 21–35 days).
The four phases of the menstrual cycle:
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Menstruation (Days 1–5): The uterus lining breaks down and is shed through the vagina as a bloody discharge (lasting 3–7 days). Oestrogen and progesterone levels are low.
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Follicular phase (Days 1–13): The pituitary gland releases follicle-stimulating hormone (FSH), which stimulates the ovaries to produce follicles containing developing eggs. Oestrogen levels rise, causing the uterus lining to thicken and rebuild.
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Ovulation (Day 14): A surge in luteinising hormone (LH) from the pituitary triggers the release of a mature egg from the ovary. The egg travels down the fallopian tube.
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Luteal phase (Days 15–28): The empty follicle becomes the corpus luteum, which secretes progesterone. This hormone maintains the thick uterus lining in preparation for implantation. If fertilisation does not occur, progesterone and oestrogen levels drop, triggering menstruation.
Hormonal control:
| Hormone | Source | Role |
|---|---|---|
| FSH | Pituitary gland | Stimulates follicle development and oestrogen production |
| LH | Pituitary gland | Triggers ovulation |
| Oestrogen | Ovarian follicles | Rebuilds and thickens uterus lining; triggers LH surge |
| Progesterone | Corpus luteum | Maintains uterus lining; inhibits FSH and LH (negative feedback) |
If fertilisation occurs: Progesterone levels remain high, the uterus lining is maintained, and menstruation does not occur. If no fertilisation occurs, the cycle repeats.
Examiners test your ability to explain why hormones change at each stage and how these changes prepare the uterus. Describe the rising and falling of hormones with reference to their effects on the uterus lining thickness and egg development.
Between days 1–13, FSH rises → ovarian follicles develop → oestrogen rises → uterus lining thickens. On day 14, oestrogen peaks → LH surge → ovulation occurs. This sequence shows how hormones orchestrate the cycle's progression.
Section 4
What happens during fertilisation and implantation?
Fertilisation is the fusion of male and female gametes to form a zygote. Implantation is the embedding of the developing embryo into the uterus lining. Together, these processes establish pregnancy.
The process of fertilisation:
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Sperm travel: Millions of sperm are deposited in the vagina during intercourse. They swim through the cervix and uterus, powered by their flagella and aided by muscular contractions of the uterus.
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Sperm meet egg: Sperm travel up the fallopian tube where they meet the egg, typically within 12–24 hours of ovulation.
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Acrosome reaction: The head of the sperm contains enzymes that digest the egg's protective layers (corona radiata and zona pellucida), allowing the sperm nucleus to enter.
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Fusion: The sperm nucleus fuses with the egg nucleus, combining genetic material from both parents. This creates a zygote with a full set of chromosomes (46 in humans).
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Cortical reaction: The egg membrane hardens to prevent other sperm from entering (polyspermy prevention).
Early development and implantation:
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The zygote undergoes mitotic divisions as it travels down the fallopian tube, forming a morula (ball of cells) and then a blastocyst (hollow sphere with an inner cell mass).
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After approximately 6–10 days, the blastocyst reaches the uterus and implants into the thick, nutrient-rich uterus lining (endometrium). The inner cell mass develops into the embryo, whilst outer cells form the trophoblast (which becomes part of the placenta).
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Once implanted, the blastocyst secretes human chorionic gonadotrophin (hCG), which maintains progesterone production by the corpus luteum, preventing menstruation and maintaining pregnancy.
Key distinction: Fertilisation creates a zygote (genetic combination); implantation establishes the physical connection to the mother.
Questions often ask why sperm are produced in large numbers (millions)—emphasise that most are lost during the journey, so high numbers increase the probability that at least one reaches the egg. Examiners expect you to explain this adaptive advantage.
Students often say fertilisation happens in the uterus, but it occurs in the fallopian tube. The zygote then travels to the uterus where implantation occurs about 6–10 days later.
Section 5
How does the foetus develop, and what is the role of the placenta?
Foetal development spans approximately 40 weeks (9 months) from conception. During this time, the embryo (weeks 1–8) develops into a foetus (weeks 9–40), undergoing rapid cell division, differentiation, and growth.
Key stages of foetal development:
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Weeks 1–8 (Embryonic stage): Rapid mitotic divisions form three germ layers (ectoderm, mesoderm, endoderm). Major organs begin to develop (organogenesis). The heart begins beating by week 4.
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Weeks 9–40 (Foetal stage): Organs mature and specialise. Bones ossify (harden). The foetus grows significantly. By week 12, the foetus is recognisably human. By week 24, many organs are sufficiently developed for survival outside the uterus (viability threshold).
The placenta: structure and function
The placenta is a disc-shaped organ formed from both foetal tissue (from the trophoblast) and maternal tissue. It attaches to the uterus lining (endometrium) and is connected to the foetus via the umbilical cord.
Functions of the placenta:
| Function | Mechanism |
|---|---|
| Gas exchange | Maternal blood in placental pools comes into close contact with foetal capillaries; O₂ diffuses from mother to foetus; CO₂ diffuses from foetus to mother |
| Nutrient absorption | Glucose, amino acids, and other nutrients diffuse from maternal blood into foetal blood by active transport and diffusion |
| Waste removal | Urea and other foetal metabolic wastes diffuse into maternal blood for maternal kidneys to excrete |
| Hormone production | Produces progesterone to maintain the uterus lining (replacing corpus luteum after week 12) |
| Antibody transfer | Maternal antibodies (mainly IgG) pass into foetal blood, providing passive immunity |
| Barrier function | Prevents direct mixing of maternal and foetal blood; blocks most pathogens (though some viruses, e.g., rubella, can cross) |
The umbilical cord
The umbilical cord contains:
- Two umbilical arteries: Carry deoxygenated blood and waste from foetus to placenta
- One umbilical vein: Carries oxygenated, nutrient-rich blood from placenta to foetus
The cord is not simply a pipe; it contains Wharton's jelly, a gelatinous substance that protects the blood vessels and prevents the cord from being compressed.
Gas and nutrient exchange in the placenta
Although maternal and foetal blood do not mix directly, they come into close proximity within the placenta. The large surface area of the placenta (formed by chorionic villi) and the thin barrier between maternal and foetal capillaries allow efficient exchange of gases, nutrients, and waste by diffusion and active transport.
The placenta is like a sophisticated air-conditioning and filtration system: it brings maternal and foetal circulations into close contact but keeps them separate (preventing mixing), efficiently exchanges gases and nutrients, and filters out most harmful substances—though not perfectly.
Examiners expect you to explain why the placenta is effective: emphasise the large surface area (from villi), the thin barrier between capillaries, and the close contact between maternal and foetal blood. Simply stating 'the placenta exchanges gases' will not earn full marks.
Section 6
How do maternal lifestyle choices affect foetal development?
Maternal choices during pregnancy significantly impact foetal development. The placenta, whilst offering some protection, does not block all harmful substances, making the foetus vulnerable to teratogens (substances causing birth defects).
Effects of smoking during pregnancy:
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Reduced oxygen supply: Carbon monoxide in cigarette smoke binds to haemoglobin more readily than oxygen, reducing oxygen delivery to the foetus.
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Restricted foetal growth: Nicotine constricts blood vessels, reducing placental blood flow and nutrient transfer.
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Low birth weight: Babies born to smoking mothers typically weigh 200–300 g less than those of non-smoking mothers.
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Increased miscarriage and stillbirth risk: Smoking increases the risk of pregnancy loss.
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Long-term effects: Increased risk of respiratory problems, sudden infant death syndrome (SIDS), and developmental delays.
Effects of alcohol during pregnancy:
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Teratogenic effects: Alcohol crosses the placenta freely and damages developing foetal cells, particularly the brain and nervous system.
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Foetal alcohol spectrum disorder (FASD): Excessive alcohol consumption causes intellectual disability, facial abnormalities, heart defects, and behavioural problems. No safe level of alcohol has been established during pregnancy.
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Foetal growth restriction: Alcohol reduces nutrient transfer and interferes with foetal metabolism.
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Increased miscarriage risk: Heavy alcohol use increases the likelihood of pregnancy loss.
Effects of drug use during pregnancy:
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Heroin and opioids: Cross the placenta and cause foetal addiction. Babies are born dependent and experience withdrawal symptoms (tremors, irritability, respiratory distress).
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Cocaine: Constricts blood vessels, reducing placental blood flow and oxygen delivery. Causes foetal growth restriction and increases miscarriage risk.
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Cannabis: Limited evidence, but some studies suggest increased risk of low birth weight and developmental delays.
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Prescription drugs: Some medications (e.g., thalidomide) are highly teratogenic and cause severe birth defects. Pregnant women should only take medications prescribed as safe in pregnancy.
Why these substances harm the foetus:
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Direct toxic effects: Chemicals damage developing cells and organs, particularly during critical periods of organogenesis (weeks 3–8).
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Reduced nutrient/oxygen transfer: Many substances constrict blood vessels or interfere with placental exchange, depriving the foetus of essential supplies.
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Placental barrier limitations: Whilst the placenta filters some molecules, many harmful substances (smoking, alcohol, drugs) cross freely due to their small size or lipid solubility.
Concept table:
| Substance | Primary Effect | Outcome |
|---|---|---|
| Smoking | Reduced oxygen and nutrients | Low birth weight, SIDS risk, respiratory problems |
| Alcohol | Direct cellular damage; reduced nutrients | FASD, intellectual disability, facial defects |
| Heroin | Foetal addiction | Withdrawal symptoms in newborn; long-term developmental issues |
| Cocaine | Constricted blood vessels; reduced oxygen | Foetal growth restriction; miscarriage |
Key principle: The first 12 weeks (embryonic period) are most critical, as major organs are forming. However, harmful exposure at any stage of pregnancy can cause problems.
Students often think the placenta completely protects the foetus from all harmful substances. In reality, many teratogens (smoking, alcohol, drugs) cross the placenta freely because they are small or fat-soluble, so the placenta offers incomplete protection.
A pregnant woman who smokes: nicotine constricts placental blood vessels → reduced blood flow to foetus → less oxygen and nutrients transferred → foetal cells receive insufficient oxygen (hypoxia) and nutrients → foetal growth restriction and low birth weight. This chain of cause and effect is what examiners expect.
Must Know
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Male reproductive system: Testes produce sperm; sperm duct and urethra transport sperm; prostate gland provides alkaline fluid; penis delivers sperm. Sperm have a head (nucleus), midpiece (mitochondria), and tail (flagellum) for swimming.
-
Female reproductive system: Ovaries produce eggs; fallopian tubes transport eggs and are the site of fertilisation; uterus is where the foetus develops; cervix is the muscular barrier; vagina receives sperm and forms the birth canal.
-
Menstrual cycle: FSH stimulates follicle development and oestrogen production (days 1–13); oestrogen surge triggers LH release and ovulation (day 14); progesterone from corpus luteum maintains the thickened uterus lining (days 15–28). If no fertilisation occurs, progesterone drops, triggering menstruation.
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Fertilisation and implantation: Sperm acrosome reaction penetrates egg protective layers; sperm and egg nuclei fuse to form a zygote (6–10 days before implantation). Blastocyst implants into endometrium; trophoblast secretes hCG to maintain pregnancy.
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Foetal development and placenta: Foetus develops over 40 weeks; placenta (formed from foetal and maternal tissue) facilitates gas/nutrient exchange and waste removal via a large surface area and thin barrier. Umbilical cord contains two arteries (deoxygenated blood to placenta) and one vein (oxygenated blood to foetus).
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Maternal lifestyle effects: Smoking reduces oxygen delivery and causes low birth weight; alcohol causes FASD and intellectual disability; drugs (heroin, cocaine) cause addiction, withdrawal, and growth restriction. The placenta does not completely block these teratogens, making the foetus vulnerable during the critical periods of organogenesis (weeks 3–8) and beyond.
That's the notes covered.
Carry on to the next subtopic.