The Human Reproductive System: GCSE Biology Guide

Zoom from organism to organ to cell to molecule: reproduction begins at the level of specialised gamete cells — sperm and egg — but the system that produces, delivers, and nurtures them spans multiple organs across the whole body. Understanding that architecture is the foundation of every GCSE question on this topic.

What are the main organs of the male reproductive system?

The male reproductive system produces, stores, and delivers sperm cells.

Organ | Function
Testes | Produce sperm (spermatogenesis) and the hormone testosterone
Epididymis | Stores and matures sperm after production
Sperm duct (vas deferens) | Carries sperm from epididymis toward the urethra
Seminal vesicles and prostate gland | Produce seminal fluid (nutrients and alkaline medium for sperm)
Urethra | Carries semen (and urine at separate times) out of the body
Penis | Delivers semen into the female reproductive tract during intercourse

Sperm are microscopic cells with a streamlined head (containing the nucleus and acrosome), a mid-piece packed with mitochondria for energy, and a tail (flagellum) for propulsion. Each sperm carries 23 chromosomes — half the number needed for a human cell.

What are the main organs of the female reproductive system?

The female reproductive system produces eggs, receives sperm, and provides the environment for fertilisation and development.

Organ | Function
Ovaries | Produce and release eggs (oocytes); produce oestrogen and progesterone
Fallopian tubes (oviducts) | Carry the egg from the ovary toward the uterus; site of fertilisation
Uterus (womb) | Muscular organ where the embryo implants and develops
Endometrium | Inner lining of the uterus; thickens to receive a fertilised egg
Cervix | Lower, narrow section of the uterus
Vagina | Birth canal; receives the penis during intercourse

Each ovary contains thousands of immature eggs (follicles). At puberty, one egg is typically released each menstrual cycle in the process called ovulation.

What is the menstrual cycle?

The menstrual cycle prepares the uterus to receive a fertilised egg each month. The average cycle is 28 days but varies naturally between individuals.

Phase | Approximate days | What happens
Menstruation | Days 1–5 | Uterus lining (endometrium) sheds; oestrogen and progesterone levels are low
Follicular phase | Days 1–13 | FSH from pituitary stimulates follicle growth; oestrogen rises, rebuilding the endometrium
Ovulation | Around day 14 | LH surge from pituitary triggers release of a mature egg from the ovary
Luteal phase | Days 15–28 | Ruptured follicle becomes corpus luteum; secretes progesterone to maintain endometrium; if no fertilisation, corpus luteum breaks down and levels fall, triggering menstruation

What is fertilisation and where does it occur?

Fertilisation is the fusion of a sperm nucleus with an egg nucleus to form a single diploid cell called a zygote. It takes place in the fallopian tube — typically in the outer third closest to the ovary.

After ovulation, the egg has roughly 12–24 hours in which it can be fertilised. Sperm can survive in the female reproductive tract for up to five days, so intercourse in the days before ovulation can also result in fertilisation.

The acrosome at the head of the sperm releases enzymes that digest through the zona pellucida (protective coating) of the egg. Once a single sperm penetrates the egg, a chemical change in the zona pellucida prevents further sperm from entering — this is the cortical reaction, which blocks polyspermy.

The zygote contains 46 chromosomes: 23 from the sperm and 23 from the egg.

What happens after fertilisation?

The zygote immediately begins dividing by mitosis as it travels down the fallopian tube toward the uterus:

Cleavage — rapid mitotic divisions produce a ball of cells called a morula.

Blastocyst — a hollow ball of cells forms, with an inner cell mass (the future embryo) and an outer layer that will become the placenta.

Implantation — the blastocyst embeds itself into the thickened endometrium approximately 6–10 days after fertilisation.

Embryo — the inner cell mass differentiates; major organ systems form during weeks 3–8.

Foetus — from about week 9, the developing human is termed a foetus; all major organs are present and continue to grow.

The placenta develops to exchange oxygen, nutrients, and waste products between mother and developing foetus across a thin membrane, without the two blood supplies mixing.

What is the role of hormones in reproduction?

Four key hormones control the reproductive cycle:

Hormone | Made by | Effect
FSH (follicle-stimulating hormone) | Pituitary gland | Stimulates a follicle to mature in the ovary
LH (luteinising hormone) | Pituitary gland | Triggers ovulation; forms corpus luteum
Oestrogen | Ovary (follicle) | Rebuilds endometrium; triggers LH surge
Progesterone | Corpus luteum | Maintains thickened endometrium; inhibits FSH and LH

If fertilisation occurs, the developing embryo produces human chorionic gonadotrophin (hCG), which maintains the corpus luteum and its progesterone output — preventing menstruation and maintaining the pregnancy. This is what pregnancy tests detect.

Frequently asked questions

What is the difference between fertilisation and implantation?

Fertilisation is the joining of a sperm and an egg to form a zygote — it happens in the fallopian tube. Implantation is when the resulting blastocyst (a ball of cells formed by mitosis of the zygote) embeds into the wall of the uterus, about 6–10 days later. Both events are necessary for a pregnancy to continue.

Where exactly does fertilisation happen?

Fertilisation occurs in the fallopian tube (oviduct), typically in the outer third closest to the ovary. The egg is swept along the tube by cilia (tiny hair-like projections) and muscular contractions, and if a sperm reaches it here, fertilisation can occur before the zygote continues its journey toward the uterus.

How do hormones prevent pregnancy in oral contraceptives?

The combined pill contains synthetic forms of oestrogen and progesterone. These maintain consistently high levels of these hormones, which suppress FSH production and prevent follicle development and ovulation. Without ovulation, fertilisation cannot occur. The progestogen-only pill mainly thickens cervical mucus so sperm cannot reach an egg.

What is meant by a diploid cell?

A diploid cell contains two full sets of chromosomes — in humans, 46 chromosomes arranged in 23 pairs. Body cells (somatic cells) are diploid. Gametes (sperm and eggs) are haploid (23 chromosomes — one from each pair). When a haploid sperm fuses with a haploid egg at fertilisation, the resulting zygote is diploid, restoring the full complement of 46 chromosomes.

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