Reproductive Physiology

Reproduction is the biological process by which organisms produce new individuals. In humans, reproduction is sexual — requiring genetic material from two parents, combined to create a new individual with a unique mix of both parents' genes. The reproductive system has two main goals: 1. Produce gametes (sex cells — sperm in males, eggs/ova in females) that carry half the normal number of chromosomes (23 instead of 46) 2. In females, provide the environment for fertilisation, development, and birth of a baby The hormones that control reproduction are produced in a cascade starting at the brain: - The hypothalamus releases GnRH (gonadotropin-releasing hormone) - GnRH tells the pituitary gland to release FSH (follicle-stimulating hormone) and LH (luteinising hormone) - FSH and LH act on the gonads (testes in males, ovaries in females) to produce sex hormones and gametes This same hypothalamus-pituitary-gonad axis controls both male and female reproduction — but in very different rhythms. In males, the system runs continuously and fairly steadily from puberty onwards. In females, it runs in a monthly cycle — the menstrual cycle.

The main jobs of the male reproductive system are to produce sperm continuously and deliver them during sexual intercourse. Sperm production (spermatogenesis): Sperm are produced in the testes — specifically in coiled tubes called seminiferous tubules. The process takes about 74 days from start to finish. The testes hang outside the body in the scrotum for an important reason: sperm production requires a temperature about 2°C below normal body temperature (35°C rather than 37°C). If the testes are too warm (as in undescended testes — cryptorchidism), sperm production is impaired. Each day, a healthy male produces approximately 300 million sperm — an extraordinarily large number designed to maximise the chance of one reaching an egg. Most sperm never get close. The hormone system in males: - FSH stimulates the seminiferous tubules to produce sperm - LH stimulates cells in the testes (called Leydig cells) to produce testosterone - Testosterone is responsible for the development of male sexual characteristics at puberty (deeper voice, facial hair, muscle mass, bone density) and maintaining sperm production - Testosterone feeds back to the hypothalamus and pituitary to regulate its own production (negative feedback) The path sperm travel: Sperm produced in the testes mature and are stored in the epididymis. During ejaculation, they travel through the vas deferens (a muscular tube) and mix with secretions from the seminal vesicles and prostate gland — forming semen. Seminal vesicle secretions provide fructose (energy for sperm). Prostate secretions make the semen slightly alkaline, protecting sperm from the acidic vaginal environment. Clinical connection: Prostate cancer is the most common cancer in men. The prostate sits around the urethra — so enlargement (benign or cancerous) causes urinary symptoms: difficulty starting urination, weak stream, frequent urination at night.

The menstrual cycle is a monthly cycle of hormonal changes that prepares the uterus for pregnancy. An average cycle is 28 days, though 21–35 days is normal. It has two phases separated by ovulation. Days 1–14: The Follicular Phase - Day 1 is the first day of menstruation (the period). Menstruation is the shedding of the uterine lining (endometrium) built up in the previous cycle. - FSH from the pituitary stimulates several follicles in the ovary to develop. Each follicle is a fluid-filled sac containing an egg. - The developing follicles produce oestrogen. Rising oestrogen causes the endometrium to thicken and rebuild. - By around day 14, one follicle becomes dominant and oestrogen levels peak. Ovulation — around day 14: The peak oestrogen level triggers a massive surge of LH from the pituitary (the LH surge). About 36 hours later, the dominant follicle bursts and releases the egg — ovulation. The egg is swept into the fallopian tube and lives for about 24 hours. Sperm can survive in the female reproductive tract for up to 5 days, so the fertile window is approximately days 10–15. Days 14–28: The Luteal Phase - The burst follicle transforms into the corpus luteum (Latin for "yellow body"). - The corpus luteum produces progesterone (and some oestrogen). - Progesterone maintains and thickens the endometrium, making it ready to receive a fertilised egg. - If fertilisation does not occur: the corpus luteum degenerates after about 14 days → progesterone and oestrogen fall → the endometrium breaks down and menstruation begins (back to day 1). - If fertilisation occurs: the embryo produces hCG (human chorionic gonadotropin) which keeps the corpus luteum alive and progesterone high, preventing menstruation. This is the hormone detected by pregnancy tests.

Fertilisation: If sperm are present in the fallopian tube when an egg arrives, one sperm may penetrate the egg's outer coat and fuse with it — combining 23 chromosomes from the sperm with 23 from the egg to make a complete 46-chromosome cell called a zygote. The moment of fertilisation triggers changes in the egg's surface that prevent any other sperm from entering. The zygote immediately begins dividing. As it travels down the fallopian tube over 5–7 days, it divides repeatedly, becoming a solid ball of cells (morula), then a hollow ball (blastocyst). The blastocyst reaches the uterus and burrows into the thickened endometrium — this is implantation. Early pregnancy: The implanted blastocyst produces hCG, which maintains the corpus luteum. The outer cells of the blastocyst form the placenta — the organ that connects mother and baby, allowing oxygen, nutrients, and waste to be exchanged without the mother's and baby's blood mixing directly. The inner cells form the embryo. The placenta becomes the main source of progesterone and oestrogen from around week 10, taking over from the corpus luteum. Key hormones of pregnancy: - hCG — produced from implantation, peaks at ~10 weeks, detected by pregnancy tests (present in urine) - Progesterone — maintains the pregnancy, prevents the uterus from contracting prematurely - Oestrogen — stimulates growth of the uterus and breast tissue, prepares for labour - Oxytocin — triggers uterine contractions during labour (positive feedback — contractions stimulate more oxytocin → stronger contractions → delivery) Labour is triggered by a complex interplay of signals. Oxytocin (and synthetic versions like Syntocinon) are used medically to induce or speed up labour.

Puberty is the period of development when a child's body matures into an adult body capable of reproduction. It is triggered when the hypothalamus begins releasing GnRH in pulses, starting the hormonal cascade. In girls (average onset 10–11 years): - Breast development (usually first sign) - Pubic and underarm hair - Growth spurt - Widening of hips - Onset of menstruation (menarche) — average age 12–13 years - Driven by oestrogen from the developing ovaries In boys (average onset 11–12 years): - Testicular enlargement (first sign) - Pubic, facial, body hair - Growth of the penis - Voice deepening (larynx grows → longer vocal cords) - Growth spurt (slightly later than girls) - Muscle mass increase - Driven by testosterone from the testes Contraception — how different methods work: - Combined oral contraceptive pill — contains synthetic oestrogen + progesterone. High levels suppress GnRH and LH → no LH surge → no ovulation. Also thickens cervical mucus and thins the endometrium. Over 99% effective if taken correctly. - Progesterone-only pill (mini-pill) — thickens cervical mucus and alters the endometrium. May or may not suppress ovulation. - Intrauterine device (IUD) — copper versions are toxic to sperm; hormonal versions release progesterone locally to thicken mucus and suppress the endometrium. - Barrier methods (condoms, diaphragm) — physical barrier preventing sperm reaching the egg. Condoms are the only method that also prevents sexually transmitted infections (STIs). - Emergency contraception — high-dose progesterone (Plan B/morning-after pill) delays ovulation if taken within 72 hours. Does not end an established pregnancy.