Reproduction is how living things produce young ones. Humans reproduce sexually. A male cell and a female cell join to start a new life. Exams ask a fixed set of questions here. They ask about the organs, the hormones, the menstrual cycle, where fertilisation happens and how sex is decided. This post explains the human reproductive system in simple words. It ends with 15 previous year questions.
Key Points (Quick Revision)
- The male sex cell is the sperm. It is made in the testes.
- The female sex cell is the ovum (egg). It is made in the ovaries.
- Fertilisation normally happens in the fallopian tube (oviduct).
- The baby grows in the uterus. The placenta links mother and baby.
- The sex of the child depends on the father's sperm - X or Y.
1. Basic ideas
Humans show sexual reproduction. Two special cells take part. They are called gametes. The male gamete is the sperm. The female gamete is the ovum.
Each gamete carries 23 chromosomes. That is half the normal number. When a sperm and an ovum join, the new cell has 46 chromosomes. This first cell is called the zygote.
Gametes are made by a special cell division called meiosis. It halves the chromosome number. Normal body cells divide by mitosis, which keeps the number the same.
The organs that make gametes are called gonads. In males, the gonads are the testes. In females, they are the ovaries. The gonads also make sex hormones.
Humans become able to reproduce at puberty. In most girls it comes around 10 to 14 years. In most boys it comes a little later. Body changes at puberty are controlled by hormones.
2. The male reproductive system
The main male organs are listed below.
| Organ | Job |
|---|---|
| Testes | Make sperm and the hormone testosterone |
| Scrotum | A pouch outside the body that holds the testes |
| Epididymis | Stores sperm and helps them mature |
| Vas deferens | Carries sperm from the testes towards the urethra |
| Seminal vesicles and prostate gland | Add fluids that feed and carry the sperm |
| Urethra | Common passage for urine and semen |
The testes sit in the scrotum, outside the body. Why? Sperm need a temperature about 2 to 3 degrees Celsius lower than normal body temperature.
Inside the testes are thin tubes called seminiferous tubules. Sperm are made here. Cells between these tubes, called Leydig cells, make testosterone.
Sperm mixed with the fluids is called semen. A sperm has a head, a middle piece and a tail. The tail helps it swim. The sperm is among the smallest cells in the human body.
Testosterone is the main male hormone. It causes changes at puberty. These include a deeper voice, facial hair and stronger muscles.
3. The female reproductive system
| Organ | Job |
|---|---|
| Ovaries | Make the ovum and the hormones oestrogen and progesterone |
| Fallopian tubes (oviducts) | Carry the ovum to the uterus; fertilisation happens here |
| Uterus (womb) | The baby grows here |
| Cervix | The narrow lower end of the uterus |
| Vagina | The birth canal |
A woman has two ovaries. Usually, one ovum is released each month. This release is called ovulation.
The ovum is the largest cell in the human body. It can be seen with the naked eye as a tiny dot.
Oestrogen controls female changes at puberty. Progesterone prepares the uterus for pregnancy and helps keep it. It is often called the pregnancy hormone.
The uterus has a thick inner lining called the endometrium. It gets ready every month to hold a fertilised egg.
4. The menstrual cycle
The menstrual cycle is a monthly cycle in women. It lasts about 28 days on average. The first cycle at puberty is called menarche. The cycle stops for good around 45 to 55 years. This stop is called menopause.
The cycle has these main steps:
- Menstruation - days 1 to about 5. The old lining of the uterus breaks down and comes out with blood.
- Follicular phase - an egg grows inside the ovary. The uterus lining builds up again.
- Ovulation - around day 14, the ovum is released.
- Luteal phase - the empty follicle becomes the corpus luteum. It makes progesterone.
If the egg is not fertilised, progesterone falls. The lining breaks down, and the next cycle starts.
The cycle is controlled by hormones. The pituitary gland makes FSH (follicle stimulating hormone) and LH (luteinising hormone). The ovaries make oestrogen and progesterone. A sudden rise in LH causes ovulation.
5. Fertilisation, pregnancy and birth
Fertilisation is the joining of a sperm and an ovum. In humans, it is internal. It normally happens in the fallopian tube. Only one sperm enters the ovum.
The zygote divides again and again as it moves to the uterus. It then fixes itself to the uterus wall. This is called implantation.
A special organ called the placenta then forms. It connects the baby to the mother through the umbilical cord. The placenta passes food and oxygen to the baby. It also removes the baby's waste.
The placenta makes a hormone called hCG. Home pregnancy tests look for this hormone in urine.
The growing baby is called an embryo in the early weeks. Later it is called a foetus. A normal pregnancy lasts about 9 months, or about 280 days from the last period.
At birth, the hormone oxytocin makes the uterus muscles contract. After birth, prolactin helps the mother make milk. The first milk is called colostrum. It is rich in antibodies.
Twins are of two types:
- Identical twins - one zygote splits into two. They have the same genes and the same sex.
- Fraternal twins - two eggs are fertilised by two sperm. They can be of different sex.
6. How the sex of a child is decided
Humans have 23 pairs of chromosomes. One pair is the sex chromosomes. Females have XX. Males have XY.
Every ovum carries an X chromosome. A sperm carries either an X or a Y.
- Sperm with X + ovum with X = XX = girl.
- Sperm with Y + ovum with X = XY = boy.
So the father's sperm decides the sex of the child. The chance of a boy or a girl is about 50 percent each time.
7. Reproductive health
Contraception means methods that prevent pregnancy. They include barrier methods like condoms, hormonal pills, the copper-T placed in the uterus, and surgical methods. Vasectomy is the surgical method for men. Tubectomy is the surgical method for women.
Sexually transmitted diseases (STDs) spread through sexual contact. Examples are HIV/AIDS, gonorrhoea and syphilis. Condoms reduce the risk.
Some couples cannot have children naturally. IVF (in vitro fertilisation) can help. In IVF, the egg and sperm join outside the body, in a lab. The embryo is then placed in the uterus. Such a baby is often called a test-tube baby. The world's first test-tube baby, Louise Brown, was born in England in 1978.
In India, finding the sex of a foetus for sex selection is banned. The law is the PCPNDT Act, 1994. It was passed to stop female foeticide.
8. Previous year questions
- Where are sperm produced in the human body? – Testes (seminiferous tubules)
- Which is the largest cell in the human body? – Ovum
- Where does fertilisation normally take place in humans? – Fallopian tube (oviduct)
- Which hormone is known as the male sex hormone? – Testosterone
- Which hormone is called the pregnancy hormone? – Progesterone
- Oestrogen is produced by which organ? – Ovary
- Why are the testes located outside the body in the scrotum? – Sperm need a lower temperature
- What is the average length of the menstrual cycle? – About 28 days
- Around which day of the cycle does ovulation usually occur? – Day 14
- Which organ connects the foetus with the mother? – Placenta
- Which hormone is detected in a pregnancy test? – hCG (human chorionic gonadotropin)
- Which hormone causes contraction of the uterus during childbirth? – Oxytocin
- The sex of a child is determined by the chromosome of whom? – The father
- What are the sex chromosomes of a human male? – XY
- Vasectomy is a method of contraception for whom? – Men
9. How to revise this topic
Start with the two organ tables. Learn each organ and its job. Many questions come from them.
Next, make a small hormone list: testosterone, oestrogen, progesterone, FSH, LH, hCG, oxytocin and prolactin. Write where each is made and what it does.
Then learn the menstrual cycle steps in order, with day 14 for ovulation.
Finally, revise the "firsts and bests": largest cell, site of fertilisation, the XY rule, and the first test-tube baby.
10. Frequently Asked Questions
Where does fertilisation take place in humans?
Fertilisation normally takes place in the fallopian tube, also called the oviduct. The fertilised egg, or zygote, then moves to the uterus and fixes itself to the uterus wall.
Which is the largest cell in the human body?
The ovum, or female egg cell, is the largest cell in the human body. It can be seen with the naked eye as a tiny dot. The sperm is among the smallest cells.
Who decides the sex of the baby?
The father. Every ovum carries an X chromosome, while a sperm carries either X or Y. If an X sperm fertilises the ovum, the baby is a girl (XX). If a Y sperm does, the baby is a boy (XY).
What is the function of the placenta?
The placenta connects the baby to the mother through the umbilical cord. It passes food and oxygen to the baby, removes the baby's waste, and makes hormones such as hCG.
What is menopause?
Menopause is the permanent end of the menstrual cycle in women. It usually happens between about 45 and 55 years of age. The first menstrual cycle at puberty is called menarche.
For more Biology topics in simple language, see our Endocrine System and Hormones, Genetics and Heredity and Plant Reproduction posts, or browse the General Science section. Preparing for a specific exam? Start with the RRB NTPC guide or join the 100 Hour GS Course.