Reproductive System
The anatomy and signaling that produce gametes, run the fertility cycle, and drive sexual response.
The reproductive system does two jobs. It makes gametes — sperm in men, eggs in women — and it runs the chemistry and anatomy of sex, pregnancy, and menopause. Everything else called "sexual" in everyday language is built on top of those two jobs.
At a glance
What it does
Produces gametes, then moves them through anatomy that either delivers them or receives them. Beyond reproduction, the sex hormones released by the testes and ovaries are potent systemic signals: they shape bone density, muscle mass, fat distribution, cognition, mood, skin, hair, cardiovascular risk, and libido. You cannot "just" modulate sex hormones in isolation — they are upstream of too much.
Male anatomy
Testes sit outside the body in the scrotum because sperm production requires temperatures 2-4 degrees C below core. Inside the testes, Leydig cells make testosterone in response to LH from the pituitary, and Sertoli cells produce sperm in response to FSH and local testosterone. Sperm mature in the epididymis, travel up the vas deferens at ejaculation, mix with fluid from the seminal vesicles and prostate, and exit via the urethra. Erection is vascular, controlled by nitric oxide and parasympathetic tone; ejaculation is sympathetic.
Female anatomy
Ovaries hold a finite pool of eggs, set at birth and never added to. Each cycle a cohort of follicles begins maturing; one usually dominates and releases its egg at ovulation. The egg travels down a fallopian tube, where fertilization happens if sperm are present, then implants in the uterine lining if fertilized. If not, the lining sheds — that is menstruation.
The fallopian tubes, uterus, and cervix form the upper tract; the vagina and vulva form the lower tract. The clitoris is the anatomical and functional equivalent of the penis, with the same embryonic origin and most of its tissue internal rather than external.
The fertility cycle
Roughly 28 days on average, though 21-35 is normal. Runs on a GnRH -> FSH/LH -> ovary -> estradiol/progesterone feedback loop. The follicular phase (days 1-14) is estrogen-dominant and builds the uterine lining. Ovulation is triggered by an LH surge mid-cycle. The luteal phase (days 14-28) is progesterone-dominant and prepares for implantation; if no pregnancy, progesterone falls and the lining sheds.
Hormonal contraception works by flattening this cycle. Combined pills suppress ovulation with a constant dose of synthetic estrogen and progestin; progestin-only methods thicken cervical mucus and thin the endometrium.
Puberty and sexual response
Puberty is a hypothalamic clock releasing GnRH that was suppressed since infancy. It kicks off around 9-13 in girls and 10-14 in boys, driving secondary sex characteristics, growth spurt, and fertility. Onset is trending earlier, especially in girls; body fat (and leptin signaling) appears to be a key trigger.
Sexual response involves both reflex arcs in the spinal cord and cortical drive from the brain. Arousal is parasympathetic (vasocongestion, lubrication, erection). Orgasm is sympathetic and involves rhythmic pelvic muscle contractions plus a brief surge of oxytocin, prolactin, and dopamine. Libido itself tracks sex hormones loosely — testosterone correlates more strongly with desire than any other single variable in both sexes — but runs through dopamine and context, not steroid levels alone.
When it goes wrong
Male infertility affects roughly half of couples struggling to conceive, usually low sperm count or motility; hormonal causes are a minority. Hypogonadism rises with age and metabolic disease — by 60, roughly 20% of men are clinically below range. Erectile dysfunction is mostly vascular (endothelial dysfunction) rather than hormonal.
PCOS affects 8-13% of women of reproductive age, presents as irregular cycles, elevated androgens, and insulin resistance. Endometriosis affects around 10% and is chronically under-diagnosed. Menopause is the ovary running out of responsive follicles, usually between 45 and 55; the resulting estrogen collapse drives hot flashes, bone loss, genitourinary symptoms, and elevated cardiovascular risk.
Interactions
Cortisol suppresses GnRH and knocks down sex hormones under chronic stress. Leptin signals nutritional sufficiency and permits reproduction; severe calorie restriction or extreme athletic training shuts down the cycle. Thyroid hormone affects both cycle regularity and sperm production. Alcohol is an aromatase-activating, testicular-toxic, estrogen-raising drug, and the pattern is linear.
Honest take
The most consequential thing most adults can do for their reproductive health is unglamorous — maintain body composition, sleep properly, avoid chronic alcohol, lift weights — before worrying about supplements or exotic protocols. For women past 45, hormone replacement therapy was set back two decades by a misread of the WHI study; current evidence favors starting within ten years of menopause for most women without contraindications, and the quality-of-life effect is large. For men, TRT works when hypogonadism is real and is overprescribed when it is not.
Sources
- Speroff's Clinical Gynecologic Endocrinology and Infertility — the reference on cycle physiology and disorders.
- Manson et al. (2017), JAMA — pooled WHI long-term follow-up reshaping HRT risk-benefit analysis.
- Basaria et al., Endocrine Reviews — clinical consensus on male hypogonadism and TRT.