📚 Reproduction in Humans | 人类生殖
Reproduction is essential for the continuation of a species. In humans, sexual reproduction involves the fusion of male and female gametes to form a zygote, which develops into a new individual. This process relies on specialised reproductive organs, hormones and a carefully timed menstrual cycle.
繁殖对于物种延续至关重要。人类通过有性生殖繁衍，精子和卵细胞融合形成受精卵，进而发育为新个体。这一过程依赖特化的生殖器官、激素以及精确调控的月经周期。
1. Male Reproductive System | 男性生殖系统
The male reproductive system is designed to produce, store and deliver sperm. The testes produce both sperm and the male sex hormone testosterone. The scrotum holds the testes outside the body to keep them at a temperature slightly below core body temperature, which is essential for sperm production.
男性生殖系统负责产生、储存和输送精子。睾丸产生精子和雄性激素睾酮。阴囊将睾丸托于体外，使其温度略低于体温，这对精子的生成至关重要。
Sperm are conveyed through the sperm duct (vas deferens). Along the way, glands such as the prostate gland and seminal vesicles add fluids to nourish the sperm and enable them to swim. This mixture is called semen, which leaves the body via the urethra inside the penis during ejaculation.
精子通过输精管输送。途中的前列腺和精囊等腺体分泌液体，为精子提供营养并助其游动，形成的混合物即为精液。射精时精液从阴茎内的尿道排出。
2. Female Reproductive System | 女性生殖系统
The female reproductive system produces eggs (ova), provides a site for fertilisation and supports the development of the foetus. The ovaries produce eggs and the hormones oestrogen and progesterone. Each month, an egg is released from an ovary in a process called ovulation.
女性生殖系统产生卵子，提供受精场所并支持胎儿发育。卵巢产生卵子和雌激素、孕激素。每月会有一颗卵子从卵巢排出，这一过程称为排卵。
The oviduct (fallopian tube) carries the egg towards the uterus. Fertilisation normally occurs in the oviduct. The uterus has a muscular wall and a lining (endometrium) that thickens each month in preparation for implantation of an embryo. The cervix is the narrow neck of the uterus, and the vagina receives the penis during intercourse and serves as the birth canal.
输卵管将卵子向子宫输送，受精通常发生在输卵管内。子宫具有肌肉壁，其内膜每月增厚，为胚胎着床做准备。宫颈是子宫狭窄的颈部，阴道在性交时接纳阴茎并充当产道。
3. Gametes: Sperm and Egg Cells | 配子：精子与卵细胞
Gametes are haploid cells containing half the number of chromosomes (23 in humans). Sperm cells are highly specialised for movement and fertilisation. They have a streamlined head containing the nucleus, a midpiece packed with mitochondria for energy, and a flagellum (tail) for swimming.
配子是含有半数染色体（人类为23条）的单倍体细胞。精子高度特化，适于运动与受精。其头部呈流线型，内含细胞核；中段富含线粒体提供能量；尾部（鞭毛）用于游动。
Egg cells are among the largest cells in the body, containing a large amount of cytoplasm with nutrient reserves. The cell membrane of the egg changes after fertilisation to prevent the entry of more than one sperm. A jelly coat surrounds the egg and must be penetrated by sperm enzymes.
卵细胞是人体最大的细胞之一，胞质丰富，储存营养物质。受精后卵细胞膜发生变化，阻止多个精子进入。卵子外有胶状层，需精子释放的酶方能穿透。
| Feature | Sperm | Egg cell |
|---|---|---|
| Size | Very small, mobile | Much larger, immobile |
| Cytoplasm | Very little | Large amount with food stores |
| Specialisations | Flagellum, many mitochondria, acrosome with enzymes | Jelly coat, membrane changes after fertilisation |
精子与卵细胞特征比较表（英文如上），中文同义对照：精子极小、可游动；卵细胞大而静止；精子胞质极少，卵细胞胞质丰富且储存养料；精子特化结构包括鞭毛、大量线粒体和含酶的顶体；卵子特化包括胶状层和受精后膜变化。
4. Fertilisation and Implantation | 受精与着床
Fertilisation is the fusion of a sperm nucleus with an egg nucleus to form a diploid zygote. It normally takes place in the oviduct. Many sperm release enzymes from their acrosomes to digest a path through the jelly coat of the egg. Once one sperm penetrates, the egg membrane becomes impermeable to other sperm.
受精是精子细胞核与卵细胞核融合形成二倍体受精卵的过程，通常发生在输卵管内。众多精子从顶体释放酶类，消化卵子胶状层开辟通道。一旦一条精子进入，卵膜即阻止其他精子再进入。
After fertilisation, the zygote divides by mitosis while moving slowly down the oviduct. It becomes a hollow ball of cells called a blastocyst, which embeds into the thickened endometrium of the uterus. This process is called implantation and marks the beginning of pregnancy.
受精后，受精卵通过有丝分裂不断分裂，同时沿输卵管缓慢移动，形成一个中空细胞球，称为囊胚。囊胚植入增厚的子宫内膜，这一过程称为着床，标志着怀孕的开始。
5. Development of the Foetus | 胎儿发育
After implantation, the embryo develops within the uterus. For the first eight weeks it is called an embryo; thereafter it is referred to as a foetus. During this period, the main organs and body systems form through cell differentiation and growth. By week 12, the foetus has recognisable human features.
着床后，胚胎在子宫内发育。前8周称为胚胎，之后称为胎儿。此期间通过细胞分化和生长，主要器官和系统逐渐形成。到第12周，胎儿已具备可辨认的人形特征。
The foetus is surrounded by the amniotic sac and connected to the placenta by the umbilical cord. The placenta and umbilical cord allow exchange of substances between the mother and foetus, but their blood does not mix.
胎儿被羊膜囊包裹，并通过脐带与胎盘相连。胎盘和脐带使母体与胎儿之间实现物质交换，但两者的血液并不混合。
6. The Placenta and Umbilical Cord | 胎盘与脐带
The placenta is a disc-shaped organ embedded in the uterine wall. It contains many villi, which provide a large surface area for diffusion of substances. In the placenta, oxygen, glucose, amino acids, vitamins, mineral ions and water diffuse from the mother's blood to the foetus's blood. Carbon dioxide, urea and other waste substances diffuse from the foetus to the mother for excretion.
胎盘是嵌入子宫壁的圆盘状器官，含大量绒毛，提供了极大的物质扩散表面积。在胎盘中，氧气、葡萄糖、氨基酸、维生素、无机盐和水从母体血液扩散至胎儿血液；二氧化碳、尿素等代谢废物则由胎儿扩散至母体排出。
Harmful substances such as alcohol, nicotine from smoking and certain drugs can also cross the placenta and damage the developing foetus. The placenta also allows antibodies from the mother to pass into the foetus, providing passive immunity after birth.
酒精、香烟中的尼古丁和某些药物等有害物质也能穿过胎盘，损害胎儿发育。此外，胎盘允许母体抗体进入胎儿体内，为新生儿提供出生后的被动免疫。
The umbilical cord contains two arteries and one vein. The vein carries oxygenated blood and nutrients to the foetus, while the arteries carry deoxygenated blood and waste back to the placenta.
脐带含有两条动脉和一条静脉。脐静脉将含氧血和营养物质输送给胎儿，脐动脉将去氧血和废物送回胎盘。
7. Amniotic Fluid | 羊水
The foetus floats in amniotic fluid within the amniotic sac. This fluid acts as a shock absorber, protecting the foetus from physical bumps. It also allows the foetus to move freely, which aids muscle and bone development. The fluid helps maintain a constant temperature around the foetus.
胎儿悬浮在羊膜囊内的羊水中。羊水起缓冲作用，保护胎儿免受物理撞击。它也使胎儿能自由活动，促进骨骼和肌肉发育，并帮助维持胎儿周围恒定的温度。
8. The Menstrual Cycle | 月经周期
The menstrual cycle is a roughly 28‑day sequence of events that prepares the female body for pregnancy. It involves the maturation and release of an egg, thickening of the uterine lining, and shedding of the lining if fertilisation does not occur. The cycle can be divided into menstruation, the follicular phase, ovulation and the luteal phase.
月经周期大约是28天的一系列事件，使女性身体为可能怀孕做好准备。它包括卵子成熟与排出、子宫内膜增厚，以及未受精时的内膜脱落。周期可分为月经期、卵泡期、排卵期和黄体期。
Menstruation is the shedding of the endometrium, resulting in bleeding from day 1 to about day 5. After menstruation, under the influence of oestrogen, the endometrium thickens again (proliferative phase). Ovulation occurs around day 14, when a mature egg is released.
月经是子宫内膜脱落，第1天至大约第5天出血。月经过后，在雌激素影响下子宫内膜再次增生（增殖期）。排卵约在第14天发生，此时一颗成熟卵子排出。
After ovulation, the empty follicle becomes a corpus luteum which secretes progesterone. Progesterone maintains the thick, spongy endometrium ready for implantation. If the egg is not fertilised, the corpus luteum breaks down, oestrogen and progesterone levels fall, and the uterine lining is shed, restarting the cycle.
排卵后，空卵泡变为黄体，分泌孕激素。孕激素维持厚实、多血供的内膜等待胚胎着床。若卵子未受精，黄体退化，雌激素和孕激素水平下降，子宫内膜脱落，周期重新开始。
9. Hormones in the Menstrual Cycle | 月经周期中的激素
Two ovarian hormones play central roles in the menstrual cycle. Oestrogen is produced by the developing follicles. It stimulates the thickening of the uterine lining and, at high levels, triggers a surge of hormones that leads to ovulation. Oestrogen also causes the cervix to produce thinner mucus, aiding sperm entry.
两种卵巢激素在月经周期中起着核心作用。雌激素由发育中的卵泡分泌，刺激子宫内膜增厚，并在高水平时触发引起排卵的激素高峰。雌激素还使宫颈分泌稀薄黏液，便于精子通过。
Progesterone is secreted mainly by the corpus luteum after ovulation. It maintains the thickened endometrium and increases its blood supply. Progesterone also thickens cervical mucus, preventing further sperm entry and protecting the potential embryo. If pregnancy occurs, the placenta takes over progesterone production to sustain the uterine lining.
孕激素主要由排卵后的黄体分泌，维持增厚的内膜并增加其血液供应。孕激素还会使宫颈黏液变稠，阻止更多精子进入，保护潜在的胚胎。如果怀孕，胎盘会接替分泌孕激素，维持子宫内膜。
| Hormone | Source | Main effects |
|---|---|---|
| Oestrogen | Ovarian follicles | Repairs and thickens uterine lining; triggers ovulation |
| Progesterone | Corpus luteum (later placenta) | Maintains uterine lining; thickens cervical mucus |
激素功能表（英文如上），中文解释：雌激素来自卵泡，修复并增厚子宫内膜、促发排卵；孕激素来自黄体（后期来自胎盘），维持子宫内膜、增稠宫颈黏液。
10. Puberty and Secondary Sexual Characteristics | 青春期与第二性征
Puberty is the stage when humans become capable of reproduction. In males, testosterone stimulates the growth of the penis and testes, the production of sperm, and secondary sexual characteristics such as facial hair, deepening of the voice and increased muscle mass.
青春期是人类具备生育能力的阶段。在男性，睾酮刺激阴茎和睾丸的生长、精子的生成，以及出现胡须、声音低沉和肌肉量增加等第二性征。
In females, oestrogen initiates breast development, widening of the hips, and the onset of the menstrual cycle. Progesterone works alongside oestrogen to regulate menstrual cycle events. Both sexes experience a growth spurt and develop pubic and underarm hair.
在女性，雌激素引发乳房发育、骨盆增宽和月经初潮。孕激素与雌激素共同调节月经周期。两性均经历身高快速增长（生长突增），并长出阴毛和腋毛。
11. Birth and Lactation | 分娩与泌乳
After about 40 weeks of pregnancy, labour begins. The muscular walls of the uterus contract rhythmically, and the cervix dilates to allow the baby to pass through the vagina. Hormones such as oxytocin stimulate contractions. The amniotic sac usually ruptures, releasing amniotic fluid ('water breaking').
大约怀孕40周后分娩开始。子宫肌壁有节奏地收缩，宫颈扩张，让婴儿通过阴道。催产素等激素刺激宫缩。羊膜囊通常破裂，流出羊水（“破水”）。
After birth, the placenta is expelled as the afterbirth. The mother's breasts produce a nutrient‑rich fluid called colostrum before milk comes in. Suckling by the infant stimulates release of prolactin (milk production) and oxytocin (milk ejection). Breastfeeding provides antibodies and ideal nutrition for the newborn.
婴儿娩出后，胎盘作为“胞衣”排出。母亲的乳房先产生富含营养的初乳，之后才会分泌成熟乳汁。婴儿吸吮刺激催乳素（产生乳汁）和催产素（排乳）释放。母乳喂养提供抗体和理想的新生儿营养。
12. Sexually Transmitted Infections | 性传播感染
Unprotected sexual intercourse can transmit sexually transmitted infections (STIs). Common examples include chlamydia, gonorrhoea and HIV. Chlamydia and gonorrhoea are caused by bacteria and can be treated with antibiotics. They may lead to pelvic inflammatory disease and infertility if left untreated.
无保护的性行为可能传播性传播感染。常见的有衣原体感染、淋病和 HIV。衣原体和淋病由细菌引起，可用抗生素治疗。如不及时治疗，可导致盆腔炎和不孕。
HIV is a virus that attacks white blood cells, weakening the immune system. It can be managed with antiviral drugs but is not curable. HIV can be transmitted through bodily fluids and from mother to baby across the placenta or during birth. Prevention includes barrier methods such as condoms.
HIV 是一种攻击白细胞的病毒，会削弱免疫系统。它可以通过抗病毒药物控制，但无法治愈。HIV 可通过体液传播，也可通过胎盘或在分娩时母婴传播。预防措施包括使用避孕套等屏障法。
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