Menstuation (also known as
the menstrual cycle or the period )
refers to the monthly haemorrhage caused by the the shedding of the lining
of the uterus (termed the endometrium ). Althought
this cycle occurs in roughly monthly cycles, the actual number of days
in any individual woman's cycle can be anywhere between 20 and 40 days.
Menstruation reflects a woman's reproductive years and extends
from the menarche (first menstruation) Menstruation
begins in and ends with
. By definition, the
first day of bleeding is counted as the beginning of each menstrual cycle
(day 1). The cycle ends just before the next menstrual period. Menstrual
cycles range from about 21 to 40 days. Only 10 to 15% of cycles are exactly
The intervals between periods are generally longest in the years immediately
after menarche and before menopause.
The menstrual cycle is typically separated into three phases,
follicular , ovulatory ,
and luteal , which begin with the first day
of menses (bleeding).
- The follicular phase , is the phase
which begins with the first day of menses (Day 1). During the early
part of this phase, the pituitary gland increases release of Follicle
Stimulating Hormone (FSH) and Luteinising
Hormone (LH). FSH stimulates the growth of numerous follicles
in the ovary as well as the release of oestrogen from the ovary. The
oestrogen causes thickening of the endometrium. However, only one
of the follicles matures and gets to release the
(egg) contained within. LH is secreted in a sudden burst mid-cycle
(circa Day 14) and stimulates the release of the ovum from the mature
follicle in the ovary.
- The ovulatory phase is where the ovum
is released from the mature follicle in a process termed ovulation,
due to the increased presence of the hormone LH. Ovulation generally
occurs in the 16 to 32 hours following follicle exposure to increased
LH levels. The follicle ruptures and releases the ovum into the funnel-shaped
end of the .
The ovum then makes its way down the tube and into the uterus, hoping
to get fertilised on the way.
- The luteal phase is the period following
ovulation, which continues until the next menstrual period. Following
ovulation the follicle that ruptured, releasing the ovum, closes and
becomes a corpus luteum . The corpus luteum
produces increasing levels of oestrogen as well as progesterone .
Both hormones have the effect of preparing the endometrium for the
implantation of a fertilised egg. If pregnancy does not occur, the
corpus luteum regresses and the levels of progesterone and oestrogen
decline. This results in the breakdown of the endometrium and menstrual
bleeding occurs. If pregnancy does occur, the corpus luteum begins
to produce human chorionic gonadotropin (HCG). It is the top and middle
endometrium layers that are lost during the menstrual period. The
bottom layer remains and prepares once again for implantation in the
Menstrual function may be disturbed in a number of ways.
The main types of menstrual dysfunction can be seen .
The information in this page is presented in summarised form and has been taken
from the following source(s):
Introduction to Human Physiology , 2nd Edition (1981), M. Griffins, Macmillan Publishing
Co. Inc, New York
Child Development , 6th Edition (1994), J. W. Santrock, Wm. C. Brown
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