Reproduction in human beings

Growing up brings changes in the body and in feelings. Among them: the reproductive organs mature, and the body begins to make gametes, sperm in a male and eggs in a female.

Humans reproduce sexually, using a set of organs found only for this purpose (compare Fig. 11.18 and Fig. 11.19). Three names mark the journey of one new life: first a zygote, the single cell formed when a sperm and an egg fuse inside the mother; then an embryo; later a foetus, both growing in her uterus. The sections below follow each step: making gametes, their meeting, and the growth of a new life.

Two jobs define the male system: producing sperm, the male germ cells, and delivering them into the female body. Tap any part in the schematic below to see its role.

The two oval testes (one is a testis) make the sperm. They hang outside the body in a skin pouch, the scrotum, where it is a little cooler than inside, and sperm can only form at that slightly lower temperature. There is a second product too: a hormone, one of the body's chemical messengers. This one keeps sperm production running, and it is behind the way a boy's body changes at puberty.

Sperm travel out of each testis along a long tube, the vas deferens, which leads into the urethra. The urethra is a shared exit for urine and for sperm. Along the way, fluid is added by two glands, the seminal vesicles and the prostate. Think of it as food plus a swimming medium, so the sperm stay active.

Look at one sperm and you see a tiny swimmer in two parts: the head is the package of genetic material, the long tail beats to move it along.

Count the organs from top to bottom: an ovary and an oviduct (fallopian tube) on each side, then one uterus in the middle and the vagina below it. Tap the parts below.

Ovaries: they make the eggs, the female germ cells, and release hormones behind the changes girls go through at puberty. Oviducts: two tubes, the egg's pathway from ovary to uterus. Uterus: hollow and shaped like a bag; a growing foetus lives here. Cervix: the narrow neck at the lower end of the uterus, opening into the vagina.

The testes and the ovaries are where gametes are made, a process called gametogenesis. The bead game of Activity 11.4 showed the rule: meiosis makes every gamete carry half the chromosomes of a body cell. So a human body cell carries 46 chromosomes while a sperm or an egg carries 23, and when the two fuse, the zygote is back to 46, matching its parents.

The two kinds of gametes could hardly be more different. Males make huge numbers of tiny sperm that move actively. Females make a single large egg at a time. Compare them (Table 11.5).

This difference between male and female gametes in size, number and structure is found in most animals, not only in humans.

A baby girl is born with her lifetime store of eggs already in her ovaries: millions of them, all immature. Once puberty starts, one of the two ovaries normally releases a single mature egg each month; this release is ovulation. Step through what can happen next.

Put together: the lining of the uterus starts building up even before ovulation. The released egg moves into the oviduct. In sexual intercourse, sperm in their millions enter the female body; they swim on through the uterus, and a few may get as far as the egg in the oviduct. Should one sperm fuse with the egg, the result is a zygote (Fig. 11.20).

Meanwhile the lining keeps thickening and grows a rich supply of blood vessels. The zygote, dividing by mitosis again and again, drifts down to the uterus and settles into the lining to be nourished. Once it is attached, called implantation, the woman is pregnant.

Without a sperm, the released egg lasts only around one day before it breaks down. The thick, blood-rich lining, prepared to receive and feed a zygote, now has no job, so the body sheds it. Lining and a little blood flow out through the vagina: this is menstruation, also called a period, and it normally lasts 3 to 7 days.

Ovulation, the build-up of the lining and menstruation repeat as a cycle, typically every 21 to 35 days, often around 28. The cycle usually starts at puberty, between about 10 and 14 years of age, and continues until menopause, around age 50. Move round a typical 28-day cycle (Fig. 11.21).

The dial shows a typical 28-day cycle. Real cycles vary from person to person and month to month, anywhere in the 21 to 35 day range, so the day of ovulation is not fixed either. During pregnancy no new cycle begins: the lining stays in place to nourish the developing baby.

Think of a human pregnancy as roughly nine months cut into three blocks of about three months, each called a trimester. Slide through them below (Fig. 11.22).

All this time the uterus keeps the baby safe and fed. At birth the uterus wall, which is muscle, tightens again and again with great force until the baby passes out along the birth canal. Not every birth can go this way. When it would put mother or baby at risk, doctors step in with medicines or an operation to bring the baby out safely.

Newborns need careful looking after. Top of the list is breastfeeding. A mother's milk alone gives the baby all the food it needs, and it also helps shield the baby from many diseases. Warmth, vaccinations on schedule and gentle handling matter too. So does the mother: good food, plenty of rest, and no smoking, alcohol or medicines a doctor has not advised. The health of mother and baby is tied together.

How a mother lives during pregnancy shapes how safely her baby grows. Four things matter most:

Food: balanced meals with plenty of protein, vitamins and minerals. Check-ups: regular visits to a doctor or health centre. Activity and rest: light exercise and rest, as her doctor advises. Feelings: her emotional health counts as well, and a supportive family helps keep her well and calm.

Adolescence is a time of many changes. One of them is sexual maturation: step by step, as the rest of the body grows, it becomes able to reproduce. In boys this shows as the production of sperm, in girls as the start of menstrual cycles.

But a body that can reproduce does not make a person ready for adult responsibilities. Emotional maturity grows more slowly. It shows in how you manage your feelings, how clearly you express yourself and how carefully you think decisions through.

Being ready for sexual activity is not just a matter of a changed body. It also needs the ability to think things through and act responsibly. Careful decisions guard against two risks at once. One is a pregnancy nobody planned; the other is a sexually transmitted infection (STI). They also keep relationships respectful and healthy.

Close physical contact lets certain infections spread from someone who has them to someone who does not. Your book names five STIs: gonorrhoea, herpes, syphilis, genital warts and HIV. Over time, HIV can lead to AIDS. Not all of them can be cured yet. Of the methods in this lesson, only the condom works as a barrier to infection, and it is a birth control method as well.

Methods of preventing pregnancy are called contraceptive methods. The table sums up the methods the book describes.

Doctors can, in some cases, end an unwanted pregnancy by surgery (abortion). The window is usually the first trimester, while the embryo is still tiny. When it is chosen only because parents prefer a boy or a girl, it becomes a serious social problem, and it can tip the balance between the numbers of males and females, the sex ratio.

For this reason, Indian law strictly prohibits prenatal sex determination, that is, finding out the sex of a baby before birth. A balanced sex ratio matters for a healthy society that looks to the future.