Human Reproduction

Reproduction is the formation of new organisms of the same type.

Sexual reproduction is the formation of new individuals of the same type by the use of gametes. Sexual reproduction is the only method of reproduction in the human species.

Gametes are specialised sex cells, female egg cell (ovum) and male (sperm), which fuse at fertilisation forming a zygote cell from which a new individual grows.

The growth of a new individual from a zygote involves an increase in cell number by cell division and differentiation of the new cells into specialised tissues, organs and systems.

The female sex cells are produced in the ovaries. The male sex cells are produced by the testes.

Male Reproductive System

Scrotum: an external sac that supports and protects the testes outside the body allowing normal development and storage of sperm at a temperature about 3°C lower than body temperature.

Testes: produce the sperm and testosterone that is the male hormone.

Sperm Duct: delivers the sperm to the urethra in the penis.

Prostate Gland: produces a fluid that protects and activates the sperm.

Penis: transfers the semen, that contains the sperm, into the vagina of the female. Semen is the fluid ejaculated by the male. It contains the sperm and the secretions of the seminal vesicles and prostate gland. The sperm make up about 1% of the semen.

Female Reproductive System

Ovaries: produces and releases the egg cells; secretes oestrogen and progesterone hormones, the release of the egg cell from the surface of the ovary is called ovulation.

Fallopian Tubes: collect the egg cell from the ovary, site of fertilisation, transfers the zygote to the uterus.

Uterus: site of implantation – the early developing embryo embeds into the lining of the uterus, formation of the placenta, development of the embryo until birth; the uterus is also known as the womb.

Cervix: the neck of the uterus, the sperm travel from the vagina to the uterus through the cervix; the cervix dilates during birth to allow the child to pass safely from the uterus into the vagina.

Vagina: site of insemination – the semen is delivered into the vagina by the penis, the baby passes out along the vagina during birth.

The Menstrual Cycle is a regularly occurring sequence of changes that take place in a fertile non-pregnant female’s body by which an egg cell is released from the ovary approximately every 28 days.

Each cycle a new lining forms on the wall of the uterus to receive and nourish that cycle’s egg cell should it be fertilised. If it is not fertilised then the lining breaks down and is discharged by way of the vagina.

Menstruation: the discharge of blood and fragments of the disintegrating lining from the vagina at intervals of about one month of non-pregnant females of childbearing age.

day 1 menstruation occurs at the start of the menstrual cycle. Menstruation takes three to five days. A new lining is made after menstruation. Menstruation indicates that fertilization did not happen in the previous cycle. If menstruation does not take place, then this is often taken as a sign that pregnancy has started.

Ovulation is not guaranteed to take place at a given time – it can vary from cycle to cycle. Insemination at any time in the cycle can lead to pregnancy. There is no such thing as a ‘non-fertile time’

The Most Fertile Period

This is the time in the menstrual cycle when a female is more likely to become pregnant if sexual intercourse takes place without the use of contraceptive methods. Sperm may live inside the female’s body for 5 days and the egg cell for at least a day. Therefore the fertile period extends from seven days before and two days after ovulation – from day 9 to day 16 of the menstrual cycle if ovulation occurs on day 14. But the day of ovulation cannot be predicted so the fertile period may vary from cycle to cycle.

Sexual Intercourse is also known as copulation. During sexual arousal the penis of the male become enlarged, rigid and erect for its insertion and delivery of semen into the vagina of the female. The vagina also is prepared during sexual arousal to receive the penis and semen.

Fertilisation normally occurs high up in the Fallopian tube. Once fertilisation has taken place development of a new individual begins immediately but it takes about 6 days before the growing structure reaches the uterus and becomes embedded in the lining of the uterus where it is receives nourishment from the mother’s blood. This embedding is known as implantation. After implantation the developing embryo releases a chemical into the mother’s blood preventing menstruation and so failure to menstruate is often the first sign of pregnancy.

Pregnancy is the condition of the female’s body while a baby is developing inside her uterus. It normally takes 40 weeks from the last menstruation or 38 weeks from the time of fertilisation. During the first eight weeks the new tissues and organs are forming. For the remainder of the pregnancy, the newly formed structures are maturing to allow the child to survive outside the mother.

The baby develops in a bag of water, amniotic fluid.

The placenta is a special organ jointly made by the developing baby and mother and its major function is to pass food, oxygen and water from the mother's blood to the baby's blood and remove wastes (carbon dioxide, urea) from the baby’s blood to the mother’s for excretion by the mother.

The placenta also protects the developing baby from the mother’s higher blood pressure and from disease-causing organisms. It is important to note that the baby’s and mother’s blood do not mix in the placenta – they are separated by the very thin barrier.

Birth There are three stages: dilation, delivery and placental.

1. Dilation

The cervix is forcibly opened to a width of about 10 cm. This is to allow the baby’s head to pass safely through from the uterus to the vagina. The force is generated by the contraction of the muscles in the uterus wall. Just before the baby is born the bag of liquid protecting the baby bursts and is discharged from the vagina – the ‘breaking of the waters’.

2. Delivery

The baby is passed through the cervix, then along the vagina (birth canal) to the outside.

The umbilical cord is clamped closed and cut on the side away from the baby.

This is not painful as there are no receptors in the umbilical cord.

3. Placental

About fifteen minutes after the baby is delivered, the placenta or afterbirth is discharged.

Growth is a non-stop process throughout childhood and adolescence.

Growth is extremely fast in the first two years and slows during the middle years of childhood.

There is a growth spurt at puberty which is then followed by no further increase in height when adult height is reached.

Rapid brain growth occurs in the first year of life; this increase reduces in the second year and after that brain growth is very gradual. Another spurt in brain growth occurs at the start of puberty.

Puberty is the period of life, often called adolescence and usually between the ages of 10 and 15, when the reproductive organs grow to their adult size and become functional. Generally puberty begins about 2 years earlier in females than in males but the age in which puberty begins varies widely.

Puberty is also the time when the secondary sexual characteristics develop distinguishing the sexually mature individual from the immature. In females the breasts enlarge, the hips widen, pubic hair grows and extra fat is deposited below the skin.

In males facial and pubic hair grows, the voice breaks, shoulders widen and there is an increase in musculature.

Contraception is taking deliberate action to prevent an unwanted pregnancy by inhibiting fertilisation or implantation.

Family planning is a decisive plan of action to influence the number of children and the timing of their birth.

Methods of contraception and failure rates

Failure rate: the number of females out of 100 who will become pregnant in a year by using that method

http://www.bbc.co.uk/bitesize/ks3/science/organisms_behaviour_health/reproduction/revision/1/