Human Life Cycle

Human Life Cycle

The secondary sexual characteristics are traits that distinguish between the two sexes of a species, but that are not directly part of the reproductive system. These characteristics begin to appear during puberty as a result of sex hormones that are produced in the ovaries and testes. The males hormones are called androgens (the main one is called testosterone), and the females hormones are called oestrogens. These hormones are stimulated by the gonad-stimulating hormones from the pituitary gland found at the base of the brain.

The secondary sexual characteristics that appear in females are: breasts begin to enlarge; pubic hair starts to appear; hips grow wider than the shoulders; the menarche (first period) and fatty tissue starts to appear in and around the breasts, shoulders and hips.

The secondary sexual characteristics that appear in males are: penis, testes and scrotum begin to enlarge; facial, body and pubic hair starts to grow; voice deepens (breaks) due to larynx getting one third larger and their muscles grow and strengthen. Teenage boys also start to experience wet dreams.

1) Spermatogenesis is the process in which sperm is formed, while oogenesis is the process in which the egg cells, or ova, are formed. While both of these processes occur through meiosis, there are many differences between them.

In spermatogenesis diploid cells known as spermatogonium, which are found in the seminiferous tubules of the male testes, eventually mature to become sperm. Through the process of meiotic cell division each diploid cell is produced into four haploid sperm cells, which contain 23 single chromosomes each. Spermatogenesis begins at puberty and continues throughout the rest of life and literally millions of sperm can be produced every day.

Oogenesis takes place in the female's ovaries and like spermatogenesis haploid cells are produced from an original diploid cell, known as a primary oocyte, through meiosis. A female produces one egg each menstrual cycle, which is usually every 28 days. The majority of the cytoplasm is placed into this egg and the other cells, known as polar bodies, do not develop and disintegrate or divide again. The polar bodies dispose of any unnecessary chromosomes while retaining most of the cytoplasm in the egg. A single egg cell is normally only produced once a month, from puberty through to menopause.

There are two major differences between spermatogenesis and oogenesis with the first being that, in oogenesis only one final ovum, or egg cell, is produced from each diploid cell (in contrast to the four sperm that are generated from every spermatogonium). The second is that, there is an unequal division of the cytoplasm during meiotic division, which results in polar bodies that dispose of any unnecessary chromosomes. Another interesting difference between spermatogenesis and oogenesis is that oogenesis is not completed until after fertilization occurs.

2) The cycle starts with menstruation, which is why it is known as the menstrual cycle. During menstruation the lining of the uterus breaks down and a small amount of blood passes out through the vagina. This cycle usually lasts about 28 days and is controlled by the interaction of various hormones. The events of the menstrual cycle can be separated into three stages: The follicular phase; ovulatory phase and the luteal phase.

The first stage is known as the follicular phase, this is where one or more follicles begin to develop into a mature female gamete. The follicle cells surround the developing egg cell (oocyte), and produce hormones which trigger other responses.

The second stage, the ovulatory phase, is when the oocyte is released from the ovary and passes down the fallopian tube towards the uterus. The remaining follicle cells stay in the ovary.

The third and final stage is the luteal phase, this is where the remaining follicle cells in the ovary continue to develop and form a structure called the corpus luteum, and as a result more hormones are produced.

During the first phase of the menstrual cycle, the pituitary gland secretes follicle-stimulating hormone (FSH). FSH triggers one or more of the follicle cells in the ovary to develop and as these cells grow, oestrogen is then secreted. The secretion of this oestrogen slows down further production of FSH and stimulates the pituitary gland to secrete luteinising hormone (LH). It also stimulates the growth and repair of the uterine lining (endometrium).

As the follicular stage progresses, the developing follicle enlarges and becomes a mature follicle and the oestrogen levels begin to increase rapidly. This triggers a further release of LH and also a sudden release of FSH for the final development of the follicle. This LH surge triggers ovulation, which is when the oocyte leaves the ovary and passes into the fallopian tube. The female is now fertile.

The high concentration of LH that brings about ovulation has an effect on the follicle cells that remain in the ovary. These residual follicles are converted into the corpus luteum, which secretes oestrogen and a large amount of progestogen. The progestogen stimulates the mammary glands and uterus in anticipation of pregnancy, if the Oocyte is not fertilised within 36 hours of this process, it dies. After roughly 28 days, if there is a lack of progestogen the menstruation cycle will begin again.

High concentrations of oestrogen and progestogen inhibit production of FSH and LH. Without FSH and LH the cells of the corpus luteum get smaller – and in turn less progestogen and oestrogen are secreted.

Lower concentrations of oestrogen and progestogen, means that the FSH is no longer inhibited, and the cycle starts again.

3) Human chorionic gonadotrophin (HCG) is a hormone that is produced in the placenta and is secreted early on in the pregnancy. Its role is to prevent the disintegration of the corpus luteum which in turn, continues to secrete the hormones progestogen and oestrogen which prevents menstruation and helps maintain pregnancy once the foetus begins to develop. Measurement of the HCG hormone is used to detect if an individual is pregnant.

4) There are many different methods of contraception, but the two main types are the barrier and hormonal methods. Other types of contraception include: natural family planning methods such as only having sex at certain times of the month to prevent pregnancy; sterilization, which is a permanent surgical procedure; the intrauterine device (IUD) and intrauterine system (IUS), also known as 'the coil', which is a small plastic and copper device that is fitted into the uterus.

The barrier method physically prevents sperm from swimming into the uterus and fertilising the egg. The types of barrier method contraceptives are: the male condom; the female condom; the diaphragm or cap; sponge and spermicides in the form of foam or gels.

The male condom is the only method of contraception that boys can use. It's a thin sheath, usually made out of latex, which is rolled onto an erect penis before sexual contact. It's sealed at one of the ends so it can stop the sperm entering the uterus. Condoms can protect the users from sexually transmitted diseases.

The female condom (Femidom) is a thin, soft polyurethane pouch, which is fitted inside the vagina before sex. It has an inner ring that goes into the upper part of the vagina, and an outer one, which should be visible. It works the same as the male condom, except it is fitted inside the vagina.

The diaphragm and cap thin, soft rubber devices which are fitted into the upper part of the vagina to cover the cervix. Caps are smaller than diaphragms, but both are available in several types and sizes. They need to be used with a spermicidal cream or pessary, and should be left in place for at least six hours after sex.

A sponge is literally a small sponge which has been impregnated with a spermicidal gel or cream. It is moistened with water before use, and then inserted high into the vagina to cover the cervix. It needs to be left in place for at least six hours after sex, and can be left for up to 30 hours, although there is a risk of infection if left for longer than that.

Spermicides are creams, gels, foam or pessaries that contain a chemical that kills sperm and thus stops sperm from travelling up into the cervix. Spermicides do not provide effective protection on their own, but are great when combined with other barrier methods of contraception, such as the male condom.

The hormonal method alters the female's hormonal cycle to prevent fertilisation. There are three main types of hormonal contraceptive, these include: the contraceptive pill, the injectable hormonal contraceptive and the contraceptive implant.

The pill contains chemicals called hormones. There are two types of pill, the combined pill and the progestogen only pill. The combined pill contains both Oestrogen and Progestogen and stops the release of an egg every month – but doesn't stop the menstruation. The Progestogen only pill only contains Progestogen. It works by altering the mucous lining of the vagina to make it thicker so the sperm cannot get through, and is therefore unable to fertilize the egg.

The injection contains Progestogen and works in the same way as the Progestogen only pill, but does have the advantage of not having to remember to take a pill every day.

The implant works in a similar way to the contraceptive pill, but instead of taking a pill every day, hormones are steadily released into the female's body.

Type of Contraceptive

Effectiveness (if done correctly)

Male Condom


Female Condom


Diaphragm or Cap


Oral Contraceptives






Natural Family Planning







See pages 8 - 14

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Biology 2 – Level 3


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The blood from the placenta is enriched with oxygen and nutrients and while in the womb, a foetus ‘breathes' by exchanging oxygen and carbon dioxide through the mother's circulation via the placenta. This blood does not enter through the lungs instead it travels through the baby's heart and flows throughout its body.

When the infant is born, its lungs are not inflated and are filled with a fluid, called amniotic fluid. After about 10 seconds of being born, the baby will take its first breath. This breath will sound more like a gasp, as the baby's central nervous system reacts to the sudden change in temperature and environment.

After the newborn has taken its first breath, a number of changes occur in the lungs and circulatory system, such as an increase of the oxygen in the lungs causes a decrease in the blood flow resistance to the lungs, subsequently the blood flow resistance increases in the baby's blood vessels. The amniotic fluid will then drain or be absorbed from the respiratory system, which causes the lungs to inflate and to start working on their own, thus enabling oxygen to move into the bloodstream and carbon dioxide to be removed through exhalation.

Whilst the baby is still a foetus inside the womb, the liver's main function is to store sugar (glycogen) and iron, but when the baby is born, the liver has various functions, such as producing substances that help the blood to clot. It also starts to break down waste products such as excess red blood cells and it produces a protein that helps break down bilirubin, which if not correctly broken down it can cause the baby to suffer from jaundice.

These changes that an infant's body undergoes after birth are vital as they enable the infant to be able to survive outside the womb and to enable the infant's body to adapt to the change in environment.


Fertility and sexual function declines with age due to a decline in sex hormones. Both men and women, around 50 years old, go through a period of physical and psychological change, this transmission period is known as the climacteric. Women also experience menopause at about the same time.

The changes a female goes through are signified by menopause. During this process the ovaries stop releasing eggs and menstrual periods stop. Fertility varies depending on hormone levels prior to menopause and after menopause the women can no longer reproduce. Some women experience a reduced sex drive and their sexual responses may also change. Some common problems that a female will experience caused by aging include: pubic muscles lose tone; the vaginal, uterine or bladder prolapsed; vaginal walls become thinner and dryer and may become irritated.

Unlike women, men do not experience a sudden change in fertility instead changes occur gradually during a process called andropause. Aging changes in the male reproductive system occur primarily in the testes. Tissue mass in the testes decreases and some men begin to experience problems with erectile function. The tubes that carry sperm may become elastic and the rate of sperm cell production slows down. Fertility varies from man to man and is not determined by age, in fact some men can and do father children at a fairly old age. Some men experience a gradual decrease in their sex drive and their sexual responses may become slower and less intense.


Baker, M., Indge, B., Rowland, M., (2001) Further Studies in Human Biology. UK: Hodder & Stoughton

MacKean, D., (2000) Life Study – A Textbook of Biology. UK: John Murray (Publishers) Ltd.

Roberts, M., (1986) Biology for Life. 2nd ed. UK: Thomas Nelson & Sons Ltd.

Waugh, A., Grant, A., (2006) Anatomy and Physiology in Health and Illness. 10th ed. USA: Elsevier (2009) Preventing Pregnancy [online]. Available from: [Accessed: 4 January 2010].

NHS Choices (2009) How effective is contraception at preventing pregnancy? [online]. Available from: [Accessed: 4 January 2010]

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