Menstruation refers to the periodic
discharge of blood, secretions and disintegrating mucous membrane from the
vagina that had lined the uterus. Menstrual cycle is the cycle of natural
changes that occurs in the uterus and ovary as an essential part of making
sexual reproduction possible. It is
essential for the production of eggs and for the preparation of the uterus for
pregnancy. The menstrual cycle lasts about 28 days, although there is some
variation. It involves interactions between the pituitary gland in the base of
the brain, the follicles in the ovary, and the lining of the uterus, (Beyene, 1989).
The cycle is normally counted from the
beginning of menstruation, due to the lining of the uterus peeling away with
associated loss of blood. After this stage, FSH (follicle stimulating
hormone) together with LH (luteinizing hormone), both released from the
pituitary, stimulate a follicle in the ovary, causing it to develop so that the
ovum (egg) within it matures. This also causes the ovary to release the hormone
oestrogen, (Strassmann,
1997).
The combination of FSH and LH, and
oestrogen, has a positive feedback effect, causing the release of more and more
oestrogen, FSH and LH. Oestrogen causes the muscle and lining layers (endometrium)
of the uterus to grow thicker, in preparation for the possible embryo.
The surge of LH on or about day 14 causes ovulation (release of an egg from the follicle). The ovum enters the oviduct (Fallopian tube) and travels towards the uterus. Along the way, it may
or may not meet up with a sperm cell and become fertilized, (Shannon, 2001).
The surge of LH on or about day 14 causes ovulation (release of an egg from the follicle). The ovum enters the oviduct (Fallopian tube) and travels towards the uterus. Along the way, it may
or may not meet up with a sperm cell and become fertilized, (Shannon, 2001).
The remaining section of the
follicle develops into the corpus luteum (yellow body), which secretes the
hormone progesterone for several
days. Progesterone has the effect of maintaining the lining of the uterus, and
developing more blood vessels. This would be necessary for the interchange of
materials with an embryo developing from a fertilized egg, if present, (Klump, 2013).
The combination of oestrogen and progesterone and has a negative feedback effect on the pituitary, stopping the release of FSH and LH,(Weschler,2002).
The combination of oestrogen and progesterone and has a negative feedback effect on the pituitary, stopping the release of FSH and LH,(Weschler,2002).
The diagram of menstrual cycle
If
the ovum is not fertilized
|
If
the ovum is fertilized
|
Eventual
deterioration of the corpus luteum causes progesterone production to stop.
Falling levels of progesterone and oestrogen cause the uterine lining to shrink and lose blood - menstruation. Increased FSH and LH production causes the cycle to repeat. |
The
implanted embryo (ball of cells) produces HCG (human chorionic gonadotrophin)
which passes into the mother's blood stream and maintains the corpus luteum
so that it continues to produce progesterone, causing steady conditions
during pregnancy.
|
REFERENCES
Beyene, Y. (1989). From Menarche to
Menopause: Reproductive Lives of Peasant Women in Two
Cultures. Albany, NY: State
University of New York Press.
Klump, K.L, et al ( 2013). "The interactive effects of estrogen and
progesterone on changes in
emotional eating across the menstrual cycle". USA
Losos, J. B.et al (2002). Biology. New York:
McGraw-Hill.
Shannon, M. M. (2001). Fertility, cycles
& nutrition : how your diet affects your menstrual cycles &
fertility (3rd ed.). Cincinnati, OH:
Couple to Couple League International.USA
Strassmann B. I. (1997). "The biology of menstruation in Homo sapiens: total lifetime
menses, fecundity,
and nonsynchrony in a natural fertility
population". USA
Weschler, T. (2002). Taking
Charge of Your Fertility (Revised ed.). New York: HarperCollins
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