Sexual reproduction in humans

Biology · Development of organisms and continuity of life. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The testes make sperm and the hormone testosterone. Sperm travel along the sperm ducts, the prostate gland adds fluid, and the urethra carries the semen out through the penis. The ovaries release eggs and make oestrogen and progesterone. An egg moves along the oviduct, where fertilisation happens: the nucleus of a sperm fuses with the nucleus of an egg to form a zygote.

The zygote divides into a ball of cells, the embryo, which implants in the lining of the uterus. The placenta and umbilical cord then exchange substances between the mother's blood and the fetus's blood.

The menstrual cycle lasts about 28 days and is controlled by four hormones.

Rules to remember

Sperm: flagellum for swimming, many mitochondria to release energy, acrosome with enzymes to digest a way into the egg. Small, motile, made in millions.
Egg: large, cannot move itself, has energy stores, and a jelly coat that changes after one sperm enters so that no more can enter. Usually one is released each month.
FSH (from the pituitary gland) makes an egg mature in the ovary and makes the ovary release oestrogen.
Oestrogen repairs and thickens the lining of the uterus. LH causes ovulation, at about day 14 of a 28-day cycle.
Progesterone keeps the lining thick. If there is no pregnancy, progesterone falls and the lining breaks down (menstruation).
Amniotic sac holds amniotic fluid, which protects the fetus from knocks and supports it.
Placenta: oxygen, glucose and amino acids diffuse to the fetus; carbon dioxide and urea diffuse to the mother. The two bloods do not mix. Some viruses, such as rubella, can cross.

Common mistake

Many students say the mother's blood flows into the fetus. It does not: the two blood supplies stay separate and substances pass between them by diffusion across the placenta.

Worked example

Both oviducts of a woman are blocked. Her ovaries and uterus are normal. Which events still happen: ovulation, fertilisation, menstruation?

Ovulation is controlled by hormones in the blood (LH), not by the oviducts, so eggs are still released.
Sperm and egg meet in the oviduct. A blockage stops them meeting, so fertilisation cannot happen.
With no pregnancy, progesterone falls each cycle and the lining of the uterus breaks down as normal.

Answer: Ovulation and menstruation still happen; fertilisation does not.

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Reproduction in Humans

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