The Reproductive System

The reproductive system has one core biological purpose: to create new life and pass genetic information to the next generation. It is unique among body systems because it is not required for the survival of the individual — every other system (heart, lungs, kidneys, brain) is essential to keep you alive. The reproductive system is essential for the survival of the species. Human reproduction is sexual — it requires the genetic contribution of two individuals (male and female). This genetic mixing is one of the main reasons sexually reproducing species evolve faster than asexually reproducing ones — every offspring has a unique combination of genes. Two key processes: 1. Gametogenesis — making the sex cells (gametes): sperm in males, eggs (ova) in females 2. Fertilisation — a sperm cell meets and fuses with an egg cell → creating a zygote (the single cell that will become a new human being) The role of sex chromosomes: Every human cell contains 46 chromosomes (23 pairs). One pair determines biological sex: - XX — female (one X from mum, one X from dad) - XY — male (one X from mum, one Y from dad) The Y chromosome carries a gene called SRY (sex-determining region Y) that triggers male development in the embryo. Without SRY, the embryo develops along a female pathway by default. Chromosomes and gametes: Each gamete (sperm or egg) contains only 23 chromosomes — exactly half. When sperm and egg fuse, the resulting zygote has the full 46 again. This halving is achieved by a special type of cell division called meiosis (as opposed to the usual mitosis that makes all other body cells).

The male reproductive system is designed to produce, store, and deliver sperm. Testes: The two testes (singular: testis) are oval organs that sit in the scrotum — a skin pouch outside the main body cavity. The reason they are outside the body is important: sperm production requires a temperature about 2–3°C cooler than core body temperature (37°C). The scrotum has a unique muscle (the cremaster) that raises or lowers the testes depending on temperature — drawing them closer to the body when cold, lowering them away when warm. The testes serve two functions: 1. Produce sperm (spermatogenesis) — in tiny coiled tubes called seminiferous tubules. This produces millions of sperm per day continuously from puberty onwards. 2. Produce testosterone — the main male sex hormone, made by Leydig cells between the seminiferous tubules. The path of sperm — from production to delivery: 1. Sperm are produced in the seminiferous tubules of the testes 2. They mature in the epididymis — a coiled tube on the back of each testis where sperm spend 2–3 weeks maturing and becoming able to swim 3. During ejaculation, sperm travel through the vas deferens — a muscular tube that runs up through the inguinal canal into the pelvis 4. Along the way, three glands add fluid to create semen: - Seminal vesicles — add fructose (energy for sperm) and prostaglandins (~60% of semen volume) - Prostate gland — adds a milky, slightly alkaline fluid that helps neutralise vaginal acidity (~30% of semen volume) - Bulbourethral (Cowper's) glands — add a clear lubricating fluid 5. Semen exits through the urethra (which also carries urine — but a valve prevents both at the same time) Common male health issues: - Testicular cancer — most common cancer in young men (15–35 years). Very treatable if caught early. Monthly self-examination recommended. - Prostate cancer — most common cancer in older men. The prostate surrounds the urethra → enlargement causes difficulty urinating.

The female reproductive system produces eggs, provides the environment for fertilisation, nurtures a developing embryo, and delivers the baby. Ovaries: Two almond-sized organs in the lower abdomen. The ovaries: 1. Produce eggs (oogenesis) — a female is born with all her eggs already formed (about 1–2 million immature eggs at birth). Only ~400–500 of these will be released during her reproductive lifetime. 2. Produce hormones — oestrogen (female development, preparing the uterus) and progesterone (maintaining pregnancy) The menstrual cycle — the monthly preparation: Each month, one egg matures inside a fluid-filled sac called a follicle in the ovary. Around day 14 of a 28-day cycle, the follicle ruptures and releases the egg — this is called ovulation. The egg is swept into the fallopian tube (uterine tube), where fertilisation can occur. It takes the egg about 4–5 days to travel down the fallopian tube to the uterus. Meanwhile, hormones prepare the uterus (womb) by building up its lining (endometrium) — ready to receive a fertilised egg. If fertilisation does not occur: - The egg is not implanted → the endometrium lining is shed → menstruation (the "period") - The cycle resets If fertilisation DOES occur → the fertilised egg implants in the endometrium → pregnancy begins → the hormone hCG (human chorionic gonadotropin) is produced by the embryo → hCG maintains the uterine lining and is the hormone detected by pregnancy tests. Key structures: - Fallopian tubes — the tubes connecting the ovaries to the uterus. Fertilisation usually happens here. - Uterus — the pear-shaped muscular organ where the baby develops. Its walls are powerful smooth muscle (myometrium) that contracts during labour to push the baby out. - Cervix — the narrow lower end of the uterus that opens into the vagina. Cervical screening (smear tests) detects pre-cancerous changes caused by HPV (human papillomavirus) before cancer develops. - Vagina — the muscular canal that receives the penis during intercourse, allows menstrual blood to flow out, and is the birth canal.

Fertilisation — a sperm cell successfully penetrating and fusing with an egg — is a remarkable sequence of events. The sperm's journey: Of the 200–500 million sperm in a typical ejaculation, only a few hundred reach the fallopian tubes — and only one normally fertilises the egg. The rest: - Cannot swim well enough - Are destroyed by the acidic vaginal environment - Take wrong turns - Are killed by the female immune system (which sees sperm as foreign) The surviving sperm swim actively using their tails (flagella). The journey takes 30 minutes to several hours. Sperm can survive in the female reproductive tract for 3–5 days — this is why intercourse several days before ovulation can still result in pregnancy. Fertilisation: When a sperm reaches the egg, it releases enzymes (from the acrosome cap on its head) that dissolve through the egg's outer coat. One sperm fuses with the egg → the egg immediately changes its surface chemistry to block all other sperm from entering. The 23 chromosomes from the sperm combine with the 23 chromosomes from the egg → a zygote with 46 chromosomes. Early development: - The zygote begins dividing immediately by mitosis: 2 cells → 4 → 8 → 16... - By about day 5, it is a hollow ball of ~100 cells called a blastocyst - The blastocyst travels down the fallopian tube to the uterus - Around day 6–10, it burrows into the endometrium lining — implantation - Once implanted, the embryo produces hCG → maintains the endometrium → pregnancy established - By week 8, the major organs are beginning to form — this is the embryo stage - From week 9 until birth — the foetus stage Twins: - Identical (monozygotic) — one egg fertilised by one sperm, which then splits into two embryos. Genetically identical. - Non-identical (dizygotic/fraternal) — two separate eggs, fertilised by two separate sperm in the same cycle. Genetically as similar as any siblings.

Puberty is the period of development during which children become sexually mature and capable of reproduction. It is triggered by the brain — specifically the hypothalamus, which starts releasing pulses of GnRH (gonadotropin-releasing hormone), which stimulates the pituitary gland to release FSH and LH. These hormones signal the ovaries (in girls) or testes (in boys) to become active. In girls (typically ages 8–13): The first sign is usually breast development, followed by pubic and underarm hair growth, rapid growth in height, and eventually the first menstrual period (menarche). The average age of menarche in most Western countries is around 12–13 years. These changes are driven by oestrogen from the ovaries. In boys (typically ages 9–14): The first sign is usually testicular enlargement, followed by pubic and facial hair growth, voice deepening (the larynx/Adam's apple grows larger), rapid height growth, and the ability to produce sperm. These changes are driven by testosterone from the testes. Why puberty starts when it does: The timing is influenced by genetics, body composition (a certain amount of body fat seems to be required — which is why female athletes with very low body fat can have delayed puberty), nutrition, and health. The brain appears to detect when the body is ready. Precocious (early) puberty — before age 8 in girls, 9 in boys. Can be caused by hormonal tumours or be idiopathic (no known cause). Treated with hormone-blocking medications to avoid premature bone growth plate closure (which would cause short adult height). The menopause: Around age 50, a woman's ovaries gradually stop producing eggs and hormone levels (oestrogen, progesterone) fall. Menstruation ceases permanently — this is the menopause. The fall in oestrogen causes: hot flushes, mood changes, sleep difficulties, vaginal dryness, and accelerated bone loss (osteoporosis risk). Hormone replacement therapy (HRT) can replace the falling oestrogen and reduce these symptoms.