The  opening entries in the LIFE CYCLE section of the

Macquarie Home Guide to Health and Medicine

 

 

LIFE CYCLE

 

SEX AND REPRODUCTION

 

    The ability to reproduce is one of the most fascinating and mysterious features of life. Generally speaking, the lowest  forms of single cell life simply divide by themselves without the need for a partner. In most animals and plants, however, reproduction is sexual, meaning that a male and a female are involved, each with particular cells designed specifically to reproduce the species. These cells are called gametes. In a woman they are the ova or eggs, and in a man they are the sperm ejaculated during intercourse. If an egg is fertilised with a sperm, a new human life is begun. For this to happen an amazingly complex series of processes must occur.

 

Sexual intercourse

    The biological purpose of sexual intercourse is to reproduce the human race. However, human beings also have sex (make love) because it feels good. In the Western world, people reproduce on average no more than three or four times in their lives, but they will probably engage in sexual intercourse thousands of times. Intercourse consists of the man inserting his penis into the woman's vagina. Before he does this, ideally, each will become sexually aroused. In the man this means that the penis will become filled with blood so that it is larger, stiff and erect. In the woman, the vagina lengthens, and glands in the vagina produce a lubricating fluid which enables the man's penis to slide in easily. Thrusting movements by both partners stimulate the penis and vagina and produce pleasurable sensations that increase in intensity until a climax or orgasm is reached. In a woman an orgasm consists of contractions of the vagina, and the man will have an ejaculation of semen, which is a mix of seminal fluid and sperm.

    Positions for intercourse vary. The commonest is the so-called missionary position in which the woman lies on her back with the man on top of her. Penetration is generally deepest in this position.

    In a man, orgasm and ejaculation go together. One usually does not occur without the other. Consequently a man has to have an orgasm before he can father a child. The situation is different in women. A woman does not need to have an orgasm to conceive and some women have active sex lives rarely or never achieving orgasm.

85% of menstruating women will fall pregnant within one year if undertaking sexual intercourse at least once a week.

    The frequency of sexual intercourse varies significantly between couples and with age. The frequency of sex in no way determines the affection a couple have for each other.  In Australia, the average married couple of 30 to 35 years of age will have sex 106 times a year, while the average for couples between 50 and 55 years drops to 41 times a year. Social circumstances, pregnancy, children and individual preferences will result in couples varying significantly from these averages.

 

Masturbation

    Masturbation is sexual stimulation of oneself. Many people masturbate, especially during adolescence and at times of their lives when they do not have a sexual partner. Some religions have frowned on the practice and insisted on their adherents regarding it as sinful. Dire threats have sometimes been made that unpleasant physical consequences such as blindness will result. This is nonsense. Masturbation is harmless, and if it provides pleasure and sexual relief there is no reason not to engage in it.

 

Conception

    Conception occurs when, usually as a result of sexual intercourse, a female egg is fertilised by a male sperm. Once a month, generally 14 days before the beginning of the next menstrual period, an egg that is so small it is invisible to the human eye, is released from one of a woman’s ovaries, and travels down the Fallopian tube towards the womb (uterus). During this journey, the egg may encounter sperm released by the woman’s male partner during intercourse. If one sperm penetrates the egg, the egg is fertilised, in a process called conception, and the woman falls pregnant. Once an egg has been fertilised by one sperm, it immediately becomes impenetrable to other sperm, even though millions of sperm are deposited as a result of any single ejaculation.

    The new cell is called a zygote. The zygote divides quickly into two cells and then into four, eight, 16, 32 and so on, until all the cells required for a new human being are in existence - six million by the end of pregnancy.

    The ball of cells at the beginning of the division process is called a blastocyst. The blastocyst travels down the Fallopian tube to the uterus where it implants in the wall, seven days after fertilisation.

    If, perchance, two eggs are released and fertilised, there will be two babies or twins. Twins produced from two separate eggs each have their own separate set of inherited characteristics and are not identical. Occasionally a single egg divides into two separate cells which then produce two individuals. The result is identical twins, each with the same genetic make-up, i.e. they will be the same sex, have the same colour eyes and hair, and have similar features. It is not known why this occurs.

    Triplets, quads and quins usually result from both the production of more than one egg and the division of a single fertilised egg, so some of the babies will look alike and some will be different. Nowadays multiple pregnancies not infrequently result from treatment for infertility with hormones which stimulate the production of several eggs.

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