Human Reproduction, Growth and Development
FoundationThe menstrual cycle lasts about twenty-eight days: oestrogen and progesterone take turns thickening and shedding the uterine lining, and a surge of luteinizing hormone triggers ovulation; sperm and egg meet in the oviduct, the embryo implants in the lining, the placenta handles exchange between mother and child, and the foetus grows along a growth curve until birth.
- The gametes divide the work: sperm are tiny, tailed, numerous swimmers; the egg is large, nutrient-stocked and rare — structure matching function in both.
- The menstrual cycle runs about twenty-eight days: oestrogen thickens the lining, the luteinizing hormone peak triggers ovulation around day fourteen, progesterone maintains the lining, and without fertilisation it is shed as menstruation.
- Fertilisation happens in the oviduct and the embryo implants in the uterine lining; identical twins share genes, non-identical twins are no closer than ordinary siblings.
文體小知識：科學知識說明文——以臨床教育角度講解人類生殖系統的結構與功能、月經週期的激素讀圖、受精與著床過程、胎盤交換及人類生長曲線。
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1Humans reproduce sexually: the testes of the male produce sperm and the ovaries of the female produce eggs. Sperm are tiny with long tails for swimming, and are made in huge numbers; the egg is the largest cell in the human body, large and stocked with nutrients, and usually only one is released at a time. Every part of the male and female reproductive systems is built to match its job of making and delivering gametes and nurturing the next generation.
2After puberty a woman has a menstrual cycle averaging about twenty-eight days. Hormones from the pituitary gland stimulate follicles in the ovary to develop, and the follicles secrete oestrogen, which thickens the uterine lining and increases its blood supply to prepare for a possible embryo. In mid-cycle, luteinizing hormone surges to a peak that triggers ovulation, releasing the mature egg from the ovary.
3After ovulation the follicle turns into the corpus luteum, which secretes progesterone to maintain the uterine lining. If the egg is not fertilised, the corpus luteum degenerates, progesterone and oestrogen fall, and the thickened lining breaks down and passes out through the vagina with some blood — menstruation. The rises and falls of the hormones are the key to reading cycle graphs: oestrogen peaks first, the luteinizing hormone peak coincides with ovulation, and progesterone stays high afterwards.
4During sexual intercourse sperm enter the female reproductive tract and swim through the uterus to the oviduct; if one meets an egg there, its nucleus joins the egg's nucleus and fertilisation is complete. The fertilised egg moves toward the uterus, dividing as it goes, and a few days later embeds itself in the thickened uterine lining — implantation. The embryo then develops in the uterus into a foetus, exchanging materials with the mother through the placenta; if the early embryo splits in two, identical twins result.
5Inside the placenta the foetal blood vessels never mix directly with the mother's blood; the two stay separated by a thin wall: oxygen and nutrients diffuse from the maternal blood into the foetal blood, while carbon dioxide and metabolic wastes diffuse the other way to be carried away by the mother. The foetus floats in amniotic fluid that cushions it, linked to the placenta by the umbilical cord; after a pregnancy of about forty weeks the baby is delivered by contractions of the uterus. The newborn depends on parental care, and breast milk supplies both nutrients and the mother's antibodies to help the infant fight infection.
1Summary 重點摘要
The menstrual cycle lasts about twenty-eight days: oestrogen and progesterone take turns thickening and shedding the uterine lining, and a surge of luteinizing hormone triggers ovulation; sperm and egg meet in the oviduct, the embryo implants in the lining, the placenta handles exchange between mother and child, and the foetus grows along a growth curve until birth.
2Vocabulary cards Vocabulary
A reproductive cell for sexual reproduction — sperm in the male, egg in the female — each carrying half the chromosomes.
The union of a sperm and an egg in the oviduct, their nuclei fusing to form a fertilised egg.
The release of a mature egg from the ovary, triggered by the peak of luteinizing hormone around day fourteen of the cycle.
The roughly twenty-eight-day cycle of changes during a woman's fertile years — thickening of the uterine lining, ovulation and shedding.
A hormone secreted by follicles that builds and repairs the uterine lining, preparing it for implantation.
A hormone from the corpus luteum that maintains the uterine lining; if implantation occurs its level stays high and menstruation does not come.
The embedding of the dividing fertilised egg into the thickened uterine lining after several days.
The organ of exchange between mother and foetus: their bloods never mix, and oxygen, nutrients and wastes move across by diffusion.
The fluid surrounding the foetus, cushioning shocks, protecting it and keeping a stable temperature.
A curve of a growth parameter such as mass or height against time, used to identify the stages of growth.
3Step by step Steps
- Step 1: The two systems and their gametesHuman sexual reproduction is the work of two gametes. The testes of the male make sperm and secrete male hormones; sperm are tiny, long-tailed swimmers produced in hundreds of millions, each head carrying a nucleus. The ovaries of the female make eggs and secrete oestrogen and progesterone; the egg is large and stocked with nutrients to feed the early embryo, few in number and precious. Each gamete carries half the chromosomes, restored to the full set at fertilisation. For example, picture the sperm as a lightweight swimmer and the egg as a fully provisioned camp — only when they meet can a new life begin its journey.
- Step 2: The four phases of the menstrual cycleThe menstrual cycle averages about twenty-eight days and falls into four phases. In the follicular phase, pituitary hormones stimulate follicles to develop; the follicles secrete oestrogen, and the uterine lining thickens with new blood vessels. At ovulation, a surge of luteinizing hormone, around day fourteen, releases the mature egg. In the luteal phase, the corpus luteum secretes progesterone to hold the lining ready for an embryo. In menstruation, if the egg is unfertilised, the corpus luteum degenerates, progesterone falls, and the lining is shed with blood. For example, when reading a cycle graph, locate the luteinizing hormone peak first to fix ovulation day, then track oestrogen and progesterone in that order.
- Step 3: How to read the hormone curvesCycle-graph questions hinge on three landmarks: the oestrogen peak comes first, marking a mature follicle and a thickening lining; the luteinizing hormone peak follows closely and coincides with ovulation day — the key marker; and the progesterone peak comes after ovulation, showing a busy corpus luteum holding the lining in place. If uterine lining thickness is plotted too, you will see it rise with oestrogen and be shed when progesterone falls. For example, exam papers often jumble the four curves with mixed labels; fasten onto 'the luteinizing hormone peak equals ovulation day' and every other curve falls into place.
- Step 4: Fertilisation and implantationDuring intercourse semen enters the female reproductive tract and sperm swim through the uterus into the oviduct. There a sperm meets the egg, its nucleus fuses with the egg's nucleus, and fertilisation is complete. The fertilised egg travels toward the uterus, dividing as it goes, and a few days later embeds in the thickened lining — implantation. If the early embryo splits into two separate embryos before implanting, identical twins develop, with identical genes; non-identical twins come from two eggs fertilised at once, no more alike than ordinary siblings. For example, identical twins are always the same sex, while non-identical twins can be a boy and a girl.
- Step 5: Structure and function of the placentaThe placenta is the exchange organ between mother and foetus: villi on the foetal side reach deep into the maternal blood spaces, multiplying the exchange surface. The foetal and maternal bloods stay separated by a thin wall and never mix: oxygen and nutrients diffuse from the maternal blood into the foetal blood, while carbon dioxide and wastes diffuse the other way for the mother to dispose of. The foetus is joined to the placenta by the umbilical cord and floats in amniotic fluid for cushioning; the placenta also secretes hormones that maintain the pregnancy. For example, when a pregnant woman smokes, drinks or takes certain medicines, harmful substances can diffuse across the placenta into the foetal blood and hinder development, which is why these must be avoided in pregnancy.
- Step 6: Birth and parental careAfter about forty weeks of pregnancy the foetus is ready: the uterus contracts in strong, rhythmic waves and delivers the baby through the birth canal; the umbilical cord is tied and cut, and the infant begins breathing with its own lungs. A human newborn is utterly dependent on parental care — feeding, warmth and protection are all essential. Breast milk carries nutrients in just the right proportions plus the mother's antibodies, granting the infant passive immunity against infection while deepening the bond. For example, the colostrum of the first few days after birth is especially rich in antibodies — the infant's first line of borrowed defence against pathogens.
- Step 7: Growth versus developmentGrowth is the increase in the body's size and mass — a measurable change of quantity; development is the differentiation and maturing of structure and function — a change of quality. Common growth parameters include mass, height and surface area, each with strengths: mass is easy to measure but swayed by recent meals, height is stable but changes slowly. Plotting a parameter against time yields a growth curve that marks out the stages: humans have two spurts — infancy and adolescence — a steady childhood, and a plateau in adulthood. For example, parents who measure a child's height regularly and record it are drawing an individual growth curve, with a clearly steeper stretch through adolescence.
4Diagrams Diagrams
5Worked example Example
A woman's menstrual cycle lasts about twenty-eight days. Her record shows: oestrogen rises slowly from day 1, peaking around day 12 before easing; luteinizing hormone surges on day 13 and peaks on day 14; progesterone climbs steadily after ovulation, peaking around day 21 and then falling, with menstruation arriving on day 29. Determine the day of ovulation with your reasoning, explain the role of progesterone in the cycle, and state how the progesterone level would differ if the egg were fertilised shortly after day 14 and implantation succeeded.
- Ovulation occurs around day 14: the peak of luteinizing hormone directly triggers ovulation, and the two coincide in time — the key evidence when reading the graph.
- The oestrogen peak on day 12 comes first, reflecting a mature follicle and the thickening of the uterine lining in readiness; it is not the direct marker of ovulation.
- After ovulation the corpus luteum forms and secretes progesterone, holding the uterine lining in its thickened state; the progesterone peak around day 21 marks the corpus luteum at its most active.
- If the egg is fertilised and implants, the corpus luteum does not degenerate at once: progesterone stays high, the lining is not shed, and menstruation does not arrive on day 29 — the hormonal signature of early pregnancy.
- Answer: Ovulation falls around day 14, evidenced by the luteinizing hormone peak coinciding with it; oestrogen peaks first to prepare the lining, and progesterone then maintains it. If fertilisation and implantation succeed, progesterone does not fall after its day-21 peak as it would otherwise, but stays high — the lining holds, and menstruation does not come.
6Common mistakes Mistakes
7Quick check Quick check
Q1How long is the average menstrual cycle, and on roughly which day does ovulation occur, triggered by the peak of which hormone?
Q2Where does fertilisation occur, and how does the fertilised egg reach the uterus?
Q3How does the placenta supply the foetus with oxygen and nutrients?
Q4What are the functions of amniotic fluid?
Q5What benefits does breast-feeding give an infant?
8Recap Recap
- The gametes divide the work: sperm are tiny, tailed, numerous swimmers; the egg is large, nutrient-stocked and rare — structure matching function in both.
- The menstrual cycle runs about twenty-eight days: oestrogen thickens the lining, the luteinizing hormone peak triggers ovulation around day fourteen, progesterone maintains the lining, and without fertilisation it is shed as menstruation.
- Fertilisation happens in the oviduct and the embryo implants in the uterine lining; identical twins share genes, non-identical twins are no closer than ordinary siblings.
- The placenta is the organ of exchange: the two bloods never mix, and oxygen, nutrients and wastes move by diffusion; amniotic fluid cushions and protects the foetus.
- The human growth curve shows two spurts — infancy and adolescence; parental care and the antibodies in breast milk greatly raise an infant's chances of survival.