Human reproduction
The female reproductive system:
Eggs (ova) are the female gametes, produced in the ovaries.
The ovaries also produce the hormones oestrogen and progesterone.
Oviducts (fallopian tubes) connect the ovaries to the uterus.
The uterus:
Has thick, muscular, and elastic walls.
Is about the size of a pear, but can stretch significantly during pregnancy.
Has a rich blood supply.
At the base of the uterus is the cervix — a narrow opening that connects to the vagina.
It’s controlled by a ring of muscle.
Produces watery mucus to help sperm travel into the uterus.
The vagina:
A muscular, flexible tube leading to the outside of the body.
Serves as the birth canal and receives the penis during intercourse.
Egg Production
Eggs are made and stored in the ovaries before birth.
Once puberty begins and the menstrual cycle starts:
One egg matures and is released into the oviduct each month.
The empty follicle turns into the corpus luteum, which helps support early pregnancy if fertilisation occurs.
The Egg Cell
Also known as the ovum (plural: ova).
It’s the largest human cell, measuring about 0.1 mm in diameter.
Contains a large nutrient supply to nourish the embryo before the placenta is fully developed.
The male reproductive system:
Sperm (male gametes) are produced in the testicles (testes).
Located outside the body in the scrotum, as sperm need a cooler temperature to develop properly.
The testes also produce testosterone, the male sex hormone.
Sperm travel from each testis through the sperm duct (vas deferens).
Seminal fluid, made by two glands and the prostate, mixes with sperm to form semen, which transports and nourishes the sperm.
Sperm Production
Testes contain thousands of coiled tubes where sperm are produced.
Sperm production begins at puberty and continues throughout life.
It is highly temperature-sensitive — if the tubules overheat, sperm production decreases.
Fertilisation and Implantation
Each month, one ovum is released into the oviduct.
Fertilisation usually occurs in the oviduct when the nucleus of a sperm fuses with the nucleus of the egg, forming a zygote.
As the zygote moves toward the uterus, it begins to divide, becoming an embryo.
The embryo then implants into the uterus lining, which has a rich blood supply to support early development.
Development of the Foetus
As the embryo grows, it forms a placenta — a special organ that allows exchange of substances between the mother’s and baby’s blood.
The umbilical cord connects the embryo to the placenta.
As development continues, the embryo becomes a foetus.
The foetus is enclosed in an amniotic sac filled with amniotic fluid, which cushions and protects it from bumps during pregnancy.
The process by which these particles move is called diffusion. Some harmful substances, such as viruses and alcohol, can cross from the mother to the baby.
Birth:
Uterine muscles contract, helping to open the cervix — this process is called labour.
Once the cervix is wide enough, strong muscle contractions push the baby out through the vagina.
Parental Care of the Baby:
During pregnancy, breast glands enlarge.
After birth, they begin to produce milk — this is known as lactation.
Advantages of Breastfeeding:
Breast milk contains nearly all nutrients a baby needs (except iron, which is stored during pregnancy).
Rich in antibodies, especially in the first milk (colostrum), helping protect the baby from disease.
Naturally served at the ideal temperature, with no need for warming.
Hormones
Hormones are chemical messengers transported in the bloodstream to control functions in the body.
A gland is an organ that secretes chemicals.
Endocrine glands produce hormones and have no ducts (unlike sweat or salivary glands).
Hormones enter the blood vessels running through these glands.
They travel to specific target organs, where they trigger responses.
Only target cells respond, as they have the correct receptors in their cell membranes.
Hormones:
ADH:
Where is it made?
pituitary
Function:
regulates blood concentration by controlling urine production
Adrenaline:
Where is it made?
Adrenal glands
Function:
readies the body for action (fight or flight)
increased heart rate
increased breathing rate
increases awareness
Insulin:
Where is it made?
Pancreas
Function:
Controls blood sugar
causes the liver to store glucose as glycogen
Puberty
Puberty is the process where the body rapidly changes to become sexually mature.
These changes are regulated by the female hormone oestrogen and the male hormone testosterone.
The resulting physical changes are known as secondary sexual characteristics.
Example:
In the male:
Growth of armpit, facial and pubic hair
broadening of shoulders
Growth in size of penis
In the female:
Growth of armpit and pubic hair
widening of hips
Development of breasts
The menstrual cycle:
During puberty the menstrual cycle starts. The 2 main events in this cycle are the period and the release of the egg (ovulation)
The cycle typically lasts about 28 days
Day 1-5
The uterus lining breaks down and passes out the vagina
In one of the ovaries, an egg starts to mature
The egg grows in a structure called the follicle
Day 6-13
The uterus starts to regrow and thicken
The egg continues to mature
Day 14
Ovulation occurs
Day 15-28
The uterus lining is maintained so that a fertilised egg can implant
Controlling the Menstrual Cycle
FSH released by the pituitary gland at the start of the cycle causes an egg to mature in the ovary.
As the egg develops, the ovary releases oestrogen, which stimulates the uterus lining to regrow.
Rising oestrogen levels inhibit FSH production — a form of negative feedback.
Around day 14, high oestrogen levels trigger the release of LH from the pituitary gland, and a small spike in FSH occurs (oestrogen levels drop).
LH stimulates ovulation.
Both progesterone and oestrogen maintain the uterus lining, inhibit FSH (preventing more eggs from maturing), and inhibit LH (preventing further ovulation).
As day 28 approaches, oestrogen and progesterone levels drop, triggering the start of menstruation.
Whole process of menstruation:
