How menstruation works
http://ed.ted.com/lessons/how-menstruation-works-emma-bryce
This might seem hard to
believe, but right now,
300 million women across
the planet are experiencing the same thing:
a period.
The monthly menstrual
cycle that leads to the period
is a reality most women
on Earth will go through in their lives.
But why is this cycle so
universal?
And what makes it a
cycle in the first place?
Periods last anywhere
between two and seven days,
arising once within in a
28-day rotation.
That whole system occurs
on repeat,
happening approximately
450 times during a woman's life.
Behind the scenes are a
series of hormonal controls
that fine tune the
body's internal workings
to make menstruation
start or stop during those 28 days.
This inner machinery
includes two ovaries
stocked with thousands
of tiny sacks called follicles
that each contain one
oocyte, an unfertilized egg cell.
At puberty, ovaries hold
over 400 thousand egg cells,
but release only one
each month,
which results in
pregnancy or a period.
Here's how this cycle
unfolds.
Each month beginning
around puberty,
the hormone-producing
pituitary gland in the brain
starts releasing two
substances into the blood:
follicle stimulating
hormone and luteinizing hormone.
When they reach the
ovaries,
they encourage the
internal egg cells to grow and mature.
The follicles respond by
pumping out estrogen.
The egg cells grow and
estrogen levels peak,
inhibiting the
production of FSH,
and telling the
pituitary to pump out more LH.
That causes only the
most mature egg cell from one of the ovaries
to burst out of the
follicle and through the ovary wall.
This is called
ovulation,
and it usually happens
ten to sixteen days before the start of a period.
The tiny oocyte moves
along the fallopian tube.
A pregnancy can only
occur if the egg is fertilized by a sperm cell
within the next 24
hours.
Otherwise, the egg's
escapade ends,
and the window for
pregnancy closes for that month.
Meanwhile, the now empty
follicle begins to release progesterone,
another hormone that
tells the womb's lining
to plump up with blood
and nutrients
in preparation for a
fertilized egg that may embed there and grow.
If it doesn't embed,
a few days later, the
body's progesterone and estrogen levels plummet,
meaning the womb stops
padding out and starts to degenerate,
eventually falling away.
Blood and tissue leave
the body, forming the period.
The womb can take up to
a week to clear out its unused contents,
after which, the cycle
begins anew.
Soon afterwards, the
ovaries begin to secrete estrogen again,
and the womb lining
thickens,
getting ready to
accommodate a fertilized egg
or be shed.
Hormones continually
control these activities
by circulating in ideal
amounts delivered at just the right time.
The cycle keeps on
turning,
transforming each day
and each week into a milestone along its course
towards pregnancy or a
period.
Although this cycle
appears to move by clockwork,
there's room for
variation.
Women and their bodies
are unique, after all.
Menstrual cycles occur
at diffferent times in the month,
ovulation comes at various
points in the cycle,
and some periods last
longer than others.
Menstruation even begins
and ends at different times in life
for different women,
too.
In other words,
variations between periods are normal.
Appreciating these
differences and learning about this monthly process
can empower women,
giving them the tools to
understand and take charge of their own bodies.
That way, they're able
to factor this small cycle
into a much larger cycle of life.
这听上去令人难以置信，但是现在全球近3亿的女性正在经历着同样的事情：一段特殊的时期。
这就是每月一次的月经期，是一个全球妇女在她们的一生中必然要面对的现实。
但为什么这个周期如此普遍呢？最开始的周期是怎样形成的呢？
月经会持续2-7天，每28天循环发生。
月经现象会重复不断地出现，
女人的一生大概会有450次的月经。
表象之下，这一切都是由人体激素进行调控的，
激素精密地控制着身体内部的运作，
在这28天里控制经期的起始。
这套内部机制包含两个卵巢，卵巢内存有成千上万的微小袋子，叫做卵泡，
每个卵泡内都含有一个卵母细胞，即未受精卵细胞。
青春期时期，卵巢内含有超过40万的卵细胞，
但每个月只会释放出一个卵细胞，
正是这个卵细胞决定着女性是受孕还是月经。
接下来我们来看看这个循环过程是如何产生的。
青春期的每个月初时候，大脑中的脑下垂体会分泌激素
向血液中释放两种物质：促卵泡激素和黄体化激素。
当它们到达卵巢后，会促进子卵巢内卵泡的发育和成熟。
卵泡的成熟，会进一步促进雌性激素的分泌。
伴随着卵细胞的成长，雌性激素的分泌也将达到峰值，
同时抑制促卵泡激素，
促进垂体释放更多的黄体化激素。
结果就是，只有其中一个卵巢中发育最为成熟的卵细胞，
才能冲出卵泡，穿越子房壁。
这就是排卵现象，排卵通常发生在月经前的10到16天。
微小的卵母细胞沿输卵管运动。
只有当卵母细胞（卵子）和精子结合后女性才会受孕，
这需要在排卵后24小时之内发生。
否则，意味着卵子使命的终结，女性在该月受孕的窗口也会关闭。
与此同时，空虚的卵泡开始释放孕酮，另一种激素以刺激子宫内膜
产生血液和营养物质，为受精卵的着床和生长做准备。
如果没有受精卵着床，几天后，身体的孕酮和雌性激素骤降，
刺激子宫停止补充养分，内膜开始退化坏死，渐渐脱落。
血液和脱落的子宫内膜离开身体，形成月经。
子宫清理内部无用物质的过程常常会持续一周，
之后，一个新的周期又开始了。
很快，卵巢又开始分泌雌性激素，
子宫内膜开始变厚，为受精卵着床最准备或再一次脱落。
激素长期控制着整个活动过程，通过在精确的时间输送理想的量。
随着周期的循环前行，令每一天，每个星期，
都是这个受孕或经期过程中的一个里程碑。
虽然这个过程看上去如时钟般精准，但它仍然会有区别。
毕竟每位女性及其身体都是独一无二的。
每个月，月经产生的时间因人而异；
排卵期出现的节点不同；甚至经期持续的时间也各有长短。
另外，对于不同的女性来说，一生中初潮和绝经的时间也不尽相同。
换言之，不同个体的经期有差别是正常的。
欣赏这些不同并学习关于生理周期的过程，
能让女性更加的从容，自主，让女性了解并掌控自己的身体。
这样，她们能够将从这个小周期领会到的东西，
运用在更加复杂的生活周期之中。
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