Reproduction

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Male

Each male possesses two testes, consisting of two components: the tubules where the sperm cells are produced and the cells where the male sex hormone is secreted.

For testes to be actively functional they must be approximately 1.5 degrees centigrade cooler than the body. To allow the correct temperature, a sac called the scrotum, which hangs away from the body, stores each testis. After the sperm are produced in the testes they are transported to the storage organ called the epididymis.

The organ that places the sperm inside the female's reproductive system is called the penis.  After sufficient stimulation of the penis, the male experiences an orgasm, which results in the expulsion or ejaculation of the male's sperm into the female's reproductive system. Initially, an orgasm results in the contraction of the epididymis that forces the sperm along the transport duct called the sperm duct (vas deferens).  When the sperm reaches the urethra, seminal fluid is added to the sperm from the seminal vesicle, prostate and Cowper's gland to form semen, which serves three functions:

1.    a medium of transport for the sperm within the female;
2.    to lubricate the passage of the sperm;
3.    to provide nourishment for the sperm.

Following the addition of the seminal fluid, the semen pass along the length of the urethra, through the penis into the female's reproductive system.

Female

The female has two ovaries, which are situated within the lower section of the abdominal cavity. The ovaries not only produce the sex cells (gametes), but also secrete the sex hormones.  Every female is born with approximately 100,000 immature gametes (known as oocytes), of which only around 400 will ever mature and be released. The activation of these oocytes precedes the release of the mature gametes, known as ovum.  The release of the ovum is referred to as ovulation.  Following ovulation, the ovum passes along the oviduct (or Fallopian tube) towards the uterus by a wave-like movement of the cilia (i.e. hairs) on the surface of the oviduct. The uterus is specialised for the housing and nourishment of the developing embryo and has a very thick muscular wall that contains numerous blood vessels.

The part of the female reproductive tract that receives the gametes is called the vagina. The vagina is a muscular tube that functions as a receptacle for the penis during copulation and as a passageway for the baby during childbirth.  At one end, the vagina joins the uterus by a protruding muscular ring called the cervix, while at the other it joins the external genitalia, known collectively as the vulva. The external genitalia consist of the clitoris and two folds of skin, called the labia minor and labia major. The clitoris is an erectile tissue that is homologous in structure to the male penis and is the principal site of sexual stimulation in a female.

If an ovum is fertilised within the oviduct, the zygote becomes implanted in the wall of the uterus approximately eight to ten days later. Development of the zygote proceeds by mitotic division until birth, approximately 40 weeks (9 months) later. The developing embryo is supplied with oxygen and nutrients by the placenta, which is the structure that develops from the tissues of the mother and the embryo.

As the development of the embryo proceeds, a liquid called the amniotic fluid forms around the embryo which maintains a constant temperature and absorbs most of the bumps caused by the mother's movement.  When the baby is fully developed, the process of birth is triggered by the release of hormones, causing the uterus wall to begin contracting on the baby.  The contractions eventually result in the complete expulsion of the baby through the vagina. Directly following birth, the placenta and associated structures (commonly called the afterbirth) are also passed out through the vagina.

Menstrual Cycle

Menstruation is the periodic vaginal discharge in humans and other mammals, consisting of blood and cells shed from the endometrium, or lining of the uterus. Menstruation accompanies a woman's childbearing years, usually beginning between the ages of 10 and 16, at puberty, and most often ceasing between the ages of 45 and 50, at menopause. Menstruation is part of the process that prepares a woman for pregnancy. Each month the lining of the uterus thickens; if pregnancy does not occur, this lining breaks down and is discharged through the vagina. The three to seven days that menstruation lasts is called the menstrual period.

In most women the menstrual cycle is about 28 days, but it can vary considerably even from one month to another. The cycle is initiated by hormones in the blood that stimulate the ovaries (the two female organs that produce ova, or eggs). Each month, hormones cause an egg in one of the two ovaries to mature (to become capable of being fertilized and develop into a fetus). The ovaries also produce hormones of their own, primarily estrogen, which cause the endometrium to thicken. About midway through the menstrual cycle, 14 to 15 days before the next period, the ovary releases the mature egg in a process called ovulation. The egg passes through the Fallopian tube to the uterus. If the egg unites with a sperm on its way to the uterus, fertilization occurs and pregnancy ensues.

The three to five days the egg takes to reach the uterus after being released by the ovary is known as the woman's fertile period. If fertilization does occur, the fertilized egg attaches itself to the enriched uterine lining and pregnancy continues. Menstruation does not occur during pregnancy, and a missed period is often the first indication of pregnancy a woman notices. If fertilization does not occur, the lining of the uterus does not receive the hormones it needs to continue the thickening process. Thus, the uterine lining breaks down and is discharged from the body during menstruation.

Many women experience premenstrual discomfort. Tenderness of the breasts and a tendency to retain fluid (bloat) are common one to seven days before each period. Some women also experience premenstrual tension in the form of headache, irritability, nervousness, fatigue, crying spells, and depression with no apparent cause (premenstrual stress, or PMS). A few women also experience menstrual cramps during the first day or two of the period. Although premenstrual symptoms and discomfort during menstruation were once thought to be of psychological origin, research now indicates that hormonal and chemical changes are responsible. New medications are effective in treating these problems.

 

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Stand: 06. Juli 2005.