Human Reproduction, Hormones and STIs
The reproductive systems, gametes, pregnancy, sex hormones and sexually transmitted infections. Part B of Chapter 16.
The Male Reproductive System
You must be able to identify the parts of the male reproductive system on a diagram and state their functions.
| Part | Function |
|---|---|
| Testes | Produce sperm (and the hormone testosterone) |
| Scrotum | A bag of skin that holds the testes outside the body, keeping them cooler than body temperature for sperm production |
| Sperm ducts | Carry sperm from the testes to the urethra |
| Prostate gland | Adds fluid to the sperm |
| Urethra | Carries sperm (and urine) out through the penis |
| Penis | Places sperm inside the vagina |
Worked example (pathway of sperm): Name, in order, the structures that sperm pass through from where they are made to leaving the body. Step 1. Sperm are made in the testes. Step 2. They pass along the sperm ducts, then the urethra, and leave through the penis. Answer: testes; sperm duct; urethra; penis.
Worked example (identifying parts): Name the structure that adds fluid to the sperm, and the structure that carries both sperm and urine. Step 1. The prostate gland adds fluid to the sperm. Step 2. The urethra carries both sperm and urine out of the body. Answer: prostate gland; urethra.
The Female Reproductive System
You must also be able to identify the parts of the female reproductive system and state their functions.
| Part | Function |
|---|---|
| Ovaries | Produce egg cells (and the hormone oestrogen) |
| Oviducts | Carry the egg to the uterus; fertilisation usually happens here |
| Uterus | Where the fetus develops |
| Cervix | A ring of muscle at the lower end of the uterus, where it opens into the vagina |
| Vagina | Receives the penis and sperm |
Worked example (pathway of an egg): State where an egg cell is made, and which structures it passes through to reach the uterus. Step 1. An egg cell is made in an ovary. Step 2. It passes along an oviduct to the uterus. Answer: ovary; oviduct; uterus.
Worked example (identifying parts): Name the structure where the fetus develops, and the structure at its lower opening into the vagina. Step 1. The fetus develops in the uterus. Step 2. The ring of muscle at the lower end of the uterus is the cervix. Answer: uterus; cervix.
Gametes and Fertilisation
Fertilisation is the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg cell).
| Feature | How it helps |
|---|---|
| Flagellum (sperm) | Allows the sperm to swim towards the egg |
| Mitochondria (sperm) | Release energy for swimming |
| Enzymes in the acrosome (sperm) | Digest the egg’s outer layers so the sperm can enter |
| Energy stores (egg) | Supply energy for the early embryo |
| Jelly coat (egg) | Changes at fertilisation to stop more sperm entering |
| Compare | Sperm | Egg cell |
|---|---|---|
| Size | Very small | Much larger |
| Structure | Tail (flagellum), mitochondria, acrosome with enzymes | Large cytoplasm with energy stores; jelly coat |
| Motility | Swims | Cannot swim; carried along the oviduct |
| Numbers | Millions produced | Far fewer; usually one released each cycle |
Worked example (adaptation): Explain why a sperm cell contains many mitochondria. Step 1. Mitochondria release energy by respiration. Step 2. The sperm needs this energy to swim to the egg using its flagellum. Answer: energy for swimming.
Pregnancy and the Fetus
After fertilisation the zygote forms an embryo, which is a ball of cells. The embryo implants into the lining of the uterus.
| Structure | Function |
|---|---|
| Placenta | Attaches to the wall of the uterus and links the fetus to the mother’s blood supply, via the cord |
| Umbilical cord | Connects the fetus to the placenta |
| Amniotic sac | The membrane that surrounds the fetus and holds the fluid |
| Amniotic fluid | Protects the fetus from knocks and shocks |
ExtendedThe placenta and the umbilical cord
Dissolved nutrients, oxygen and other gases, and excretory products are exchanged between the blood of the mother and the blood of the fetus through the placenta. Oxygen and nutrients pass to the fetus; carbon dioxide and urea pass to the mother. The umbilical cord carries the fetus’s blood to and from the placenta. Some pathogens and toxins can also pass across the placenta and affect the fetus.
Puberty and the Menstrual Cycle
During puberty, sex hormones cause the development of secondary sexual characteristics. Testosterone and oestrogen also help to regulate these features.
| Hormone | Secondary sexual characteristics |
|---|---|
| Testosterone (boys) | Deeper voice, facial hair, more muscle |
| Oestrogen (girls) | Breast development, wider hips; the menstrual cycle begins |
| Both sexes | Pubic and underarm hair |
The menstrual cycle
In the ovaries, an egg cell develops and is released about once a month (ovulation; in a typical 28-day cycle, about day 14). In the uterus, the lining thickens ready for an embryo. If the egg is not fertilised, the lining breaks down and is lost through the vagina: this is menstruation.
Worked example (the cycle): Describe what happens to the lining of the uterus if the egg released from the ovary is not fertilised. Step 1. The lining of the uterus is no longer needed. Step 2. It breaks down and is lost through the vagina (menstruation). Answer: lining breaks down; menstruation.
Hormones in the Menstrual Cycle
ExtendedWhere the hormones are made
In the menstrual cycle, oestrogen and progesterone are produced in the ovaries. In pregnancy, they are produced by the placenta.
ExtendedWhat each hormone does
| Hormone | Role |
|---|---|
| FSH | Causes an egg cell to mature in the ovary |
| LH | Triggers ovulation (release of the egg) |
| Oestrogen | Repairs and thickens the lining of the uterus |
| Progesterone | Maintains the lining of the uterus; during pregnancy it keeps the lining in place |
Worked example (hormones in pregnancy): Explain why menstruation does not happen during pregnancy. Step 1. The placenta produces progesterone (and oestrogen). Step 2. Progesterone maintains the lining of the uterus, so it is not lost. Answer: progesterone maintains the lining.
Sexually Transmitted Infections
A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact. HIV (human immunodeficiency virus) is a pathogen that causes an STI. HIV infection may lead to AIDS.
How HIV is transmitted
| Method | Example |
|---|---|
| Sexual contact | Sex with an infected person |
| Infected blood | Sharing needles; transfusion of infected blood |
| From mother to child | During pregnancy, at birth or in breast milk |
Controlling the spread of STIs
| Control measure | How it limits the spread |
|---|---|
| Using a condom | Acts as a barrier that stops body fluids passing between partners, so it reduces the risk of infection |
| Avoiding sexual contact with an infected person | Removes the route of transmission |
| Testing and treating infected people | Infected people can be identified and treated, so fewer pass it on |
| Having fewer sexual partners | Reduces the chance of meeting an infected person |
| Not sharing needles; screening donated blood | Stops transmission through infected blood |
Antibiotics kill bacteria but do not affect viruses, so they can treat bacterial STIs but not HIV.
Worked example (controlling HIV): State two ways in which the spread of HIV can be reduced. Step 1. Using a condom reduces the chance of the virus passing between partners. Step 2. Not sharing needles stops the virus passing in infected blood. Answer: condoms; not sharing needles.
Exam advice
Common mistakes
Model answer
Recall checklist
- Label the male and female systems and state functions.
- Describe fertilisation.
- Explain sperm and egg adaptations; compare them.
- Describe implantation and the fetal structures.
- Describe puberty and the menstrual cycle.
- Describe HIV transmission and the control of STIs.
- Explain exchange at the placenta (Extended)
- Explain the roles of FSH, LH, oestrogen, progesterone (Extended)
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