Male and Female Human Reproductive Systems

The human reproductive systems produce gametes (sperm in males, eggs in females), and in females they also support the development of an embryo; the male system is based on the testes, and the female system on the ovaries, uterus and vagina. In brief: the testes make sperm and the hormone testosterone, and the ovaries make eggs and the hormones estrogen and progesterone. Puberty is the stage of life, usually between about ages 8 and 14, when these organs become functional and secondary sexual characteristics develop.

This guide describes the structure and function of each reproductive system in a factual, school-level way. It names the organs and the path of the gametes, explains the hormones that control them, covers puberty, compares the two systems in a table, works through a calculation with sperm and egg numbers and corrects common mistakes.

What do reproductive systems do?

Reproductive systems make sexual reproduction possible. Their functions are to produce gametes, to bring the gametes together for fertilization, and, in females, to protect and nourish the developing offspring until birth and then to feed it with milk. They also produce sex hormones that control the development of the body and sexual behavior.

The reproductive organs are called primary sex organs (gonads) if they make the gametes. The other structures are accessory organs.

What is the male reproductive system?

The main parts of the male system, and their functions, are:

• Testes (singular: testis): two oval organs that make sperm and testosterone. They hang in a skin sac called the scrotum outside the abdomen, where the temperature is about 2 to 3 °C lower than the body temperature, which is better for sperm production.
• Seminiferous tubules: coiled tubes inside each testis where sperm are produced by meiosis.
• Epididymis: a coiled tube on each testis where sperm mature and are stored.
• Sperm duct (vas deferens): a tube that carries sperm from the epididymis toward the urethra.
• Seminal vesicles, prostate gland and other glands: add fluids that nourish and protect sperm. The mixture of sperm and fluids is called semen.
• Urethra: the tube through the penis that carries semen out of the body. It also carries urine, but not at the same time.
• Penis: the organ that delivers sperm into the female body during sexual intercourse.

Sperm production is continuous from puberty and usually continues throughout adult life. A typical ejaculation contains between 15 million and several hundred million sperm per milliliter, in a volume of a few milliliters.

What is the female reproductive system?

• Ovaries: two almond-sized organs on each side of the lower abdomen. They contain follicles in which eggs develop, and they make the hormones estrogen and progesterone.
• Oviducts (fallopian tubes): two tubes with finger-like ends that catch the egg released by an ovary. Cilia and muscle contractions move the egg along. Fertilization usually happens here.
• Uterus (womb): a muscular, pear-shaped organ where the embryo develops. Its lining, the endometrium, thickens each month to prepare for a possible pregnancy.
• Cervix: the narrow lower end of the uterus that opens into the vagina and is closed by mucus except during birth and menstruation.
• Vagina: a muscular tube that receives sperm and serves as the birth canal.

The breasts (mammary glands) are accessory organs that produce milk after a baby is born. Unlike the male system, a girl is born with all the immature eggs she will have, in the ovaries, about one to two million at birth, falling to about 300 000 to 400 000 at puberty. Only roughly 400 to 500 are released during her lifetime.

How do sperm and egg cells differ?

A sperm cell is very small, about 50 micrometers long including the tail. It has a head containing the nucleus with 23 chromosomes and an acrosome, a vesicle containing enzymes that help it to penetrate the egg. The middle piece is packed with mitochondria that supply energy, and the tail (flagellum) propels the cell.

An egg cell (ovum) is one of the largest human cells, about 100 to 120 micrometers wide, just visible to the naked eye as a speck. It has a nucleus with 23 chromosomes, a store of nutrients in the cytoplasm and a jelly-like coat around the cell membrane. After fertilization, the coat changes to prevent other sperm from entering. The egg cannot move on its own and is carried along the oviduct.

What is puberty?

Puberty is the period during which the body becomes capable of reproduction. It is started by hormones from the pituitary gland, which stimulate the testes or ovaries to produce more sex hormones. It usually begins earlier in girls, between about ages 8 and 13, than in boys, between about 9 and 14, but the timing varies widely and is normal over a wide range.

• In boys, testosterone causes the testes and penis to grow, sperm production to start, the voice to deepen, hair to grow on the face and body, and muscles to develop.
• In girls, estrogen causes the breasts to develop, hips to widen, hair to grow under the arms and in the pubic region, and the menstrual cycle to begin.

Emotional changes also occur and are part of normal development. Puberty ends when a person has reached adult height and reproductive maturity.

The two systems compared

Worked example: gamete numbers over a lifetime

Compare how many gametes a man and a woman can produce. Assume that a man produces sperm at a rate of 100 million per day from age 15 to age 65, and a woman releases one egg in each menstrual cycle of 28 days from age 13 to age 51.

1. Sperm: 50 years × 365 days = 18 250 days. 18 250 × 100 million = 1 825 000 million, which is 1.8 × 10¹² sperm, about 1.8 trillion.
2. Eggs: 38 years × 365 days = 13 870 days. 13 870 ÷ 28 ≈ 495 cycles, so about 495 eggs. Pregnancy and use of hormonal contraception reduce this number.
3. Ratio: 1.8 × 10¹² ÷ 495 ≈ 3.7 × 10⁹, so a man makes about 3.7 billion sperm for each egg that a woman releases.

The large difference reflects the different strategies: sperm are small, cheap and numerous, since only a few reach the egg, whereas each egg is large, nutrient-rich and valuable. The numbers are estimates and vary between individuals.

How were the reproductive systems studied?

Antonie van Leeuwenhoek and his assistant Johan Ham first saw sperm cells under a microscope in 1677. Regnier de Graaf described the ovarian follicles in 1672, and Karl Ernst von Baer discovered the mammalian egg cell in 1827. The sex hormones were isolated in the 1920s and 1930s, and testosterone was first synthesized in 1935 by Leopold Ružička and others.

Which mistakes are common?

Mistake 1: Saying that the testes make semen

They make sperm. Most of the fluid in semen comes from glands such as the seminal vesicles and the prostate.

Mistake 2: Mixing up the oviduct and the uterus

The oviduct carries the egg and is where fertilization usually occurs. The uterus is where the embryo develops.

Mistake 3: Saying that females make new eggs throughout life

The follicles with immature eggs are formed before birth. They are used up over time.

Mistake 4: Using “ovum” and “ovary” interchangeably

The ovary is the organ. The ovum is the egg cell.