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女人为什么会有生理周期 – 译学馆
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女人为什么会有生理周期

Why do women have periods?

地球上有一小部分物种拥有貌似神秘的特性:
A handful of species on Earthshare a seemingly mysterious trait:
生理周期
a menstrual cycle.
人类也是被选中的少数之一
We’re one of the select few.
猴子 猿 蝙蝠 人类 可能还包括象鼩
Monkeys, apes, bats, humans,and possibly elephant shrews
是地球上仅存的有生理周期的哺乳动物
are the only mammals on Earththat menstruate.
我们的生理周期也比其他动物更频繁
We also do it more than any other animal,
尽管这很浪费营养物质 也可能给身体带来不适
even though its a waste of nutrients and can be a physical inconvenience.
这不寻常的生理过程意义何在?
So where’s the sense in thisuncommon biological process?
答案要从怀孕开始说起
The answer begins with pregnancy.
怀孕期间 母体的资源被聪明地
During this process, the body’s resourcesare cleverly used to shape
用于为胎儿营造一个适宜的环境
a suitable environment for a fetus,
为母亲创造一个内部港湾以养育她正在成长的孩子
creating an internal haven for a mother to nurture her growing child.
从这点来说 怀孕值得人们敬畏
In this respect,pregnancy is awe-inspiring,
但这只是故事的一半
but that’s only half the story.
故事的另一半显示怀孕会使母亲和胎儿处于不一致的状态
The other half reveals that pregnancy places a mother and her child at odds.
对所有生物而言
As for all living creatures,
人类的身体发展到提升自身基因速度的程度
the human body evolved to promote the spread of its genes.
对母亲来说 则意味着她应努力
For the mother, that meansshe should try to provide equally
为所有后代提供同等条件
for all her offspring.
但母亲和胎儿之间分享的并不是完全一样的基因
But a mother and her fetus don’t share exactly the same genes.
因为胎儿也从其父亲那继承基因
The fetus inherits genesfrom its father, as well,
这些基因通过从母亲那吸收超过彼此平等分享的资源
and those genes can promote their ownsurvival by extracting
可以提高自身的存活率
more than their fair shareof resources from the mother.
这种进化带来的利益冲突将母亲
This evolutionary conflict of interests places a woman
和她还未出生的孩子置于一场发生在子宫内的
and her unborn child in a biological tug-of-war
生物上的拔河比赛中
that plays out inside the womb.
加剧这种内部争斗的一个因素
One factor contributingto this internal tussle
就是胎盘
is the placenta,
即与母体血液供给相连接
the fetal organ that connects to the mother’s blood supply
并滋养胎儿生长的胎体器官
and nourishes the fetus while it grows.
在大多数哺乳动物中 胎盘被限定在母体细胞形成的屏障之后
In most mammals, the placenta is confined behind a barrier of maternal cells.
这一屏障使母亲得以控制对胎儿营养的供给
This barrier lets the mother control the supply of nutrients to the fetus.
但在人类和某些其他物种之中
But in humans and a few other species,
胎盘却直接穿入到母体的循环系统中
the placenta actually penetrates rightinto the mother’s circulatory system
直接与母体的血流接触
to directly access her blood stream.
通过胎盘 胎儿通过分泌激素从母体的动脉中泵血
Through its placenta, the fetuspumps the mother’s arteries with hormones
这些激素使动脉扩张 从而持续提供营养物丰富的血流
that keep them open to provide a permanent flow of nutrient-rich blood.
这种可以无限制接触母体血液的胎儿会合成激素
A fetus with such unrestricted accesscan manufacture hormones
使得母体血糖升高 动脉扩张
to increase the mother’s blood sugar,dilate her arteries,
并使血压升高
and inflate her blood pressure.
大多数雌性哺乳动物在需要时都能清除或重新吸收胚胎
Most mammal mothers can expelor reabsorb embryos if required,
但对人类而言 一旦胎儿与母体血液供给连接起来
but in humans, once the fetus is connected to the blood supply,
切断这一连接就可能导致大出血
severing that connectioncan result in hemorrhage.
如果胎儿发育的不好或死亡
If the fetus develops poorly or dies,
母亲的健康就会受到威胁
the mother’s health is endangered.
随着胎儿的成长 对资源的持续需求
As it grows, a fetus’s ongoing need
会给母亲带来强烈的疲劳感
for resources can cause intense fatigue,
高血压 以及糖尿病和子痫前期等状况
high blood pressure, and conditionslike diabetes and preeclampsia.
因为这些风险 怀孕一直是一个巨大的
Because of these risks, pregnancy is always a huge,
有时也是危险的投资
and sometimes dangerous, investment.
所以母体仔细筛选胚胎
So it makes sense that the bodyshould screen embryos carefully
找出值得冒险的那个就说的过去了
to find out which onesare worth the challenge.
这就是生理周期的来源
This is where menstruation fits in.
怀孕以一种叫做着床的过程而开始
Pregnancy starts with a processcalled implantation,
这一过程中 胚胎植入子宫内膜 使子宫出现一条线
where the embryo embeds itself in the endometrium that lines the uterus.
子宫内膜已经进化到让着床变得困难
The endometrium evolved to makeimplantation difficult
以至于只有健康的胚胎才可以存活
so that only the healthy embryoscould survive.
但是这样做
But in doing so,
它也被最具活力的侵略性胚胎所选中
it also selected for the most vigorously invasive embryos,
因而形成一个不断进化的反馈循环
creating an evolutionary feedback loop.
胚胎参与了一个复杂的 时间极为准确的激素互动
The embryo engages in a complex,exquisitely timed hormonal dialogue
从而将子宫内膜改造为允许着床的状态
that transforms the endometriumto allow implantation.
如果胚胎在测试中失败会发生什么?
What happens whenan embryo fails the test?
它也许仍会尝试着床
It might still manage to attach,
或者只是部分穿过子宫内膜
or even get partlythrough the endometrium.
随着胚胎慢慢死去
As it slowly dies,
就会使得母体极易受到感染
it could leave its mother vulnerable to infection,
若持续下去
and all the time,
则会放出激素信号扰乱母体组织
it may be emitting hormonal signals that disrupt her tissues.
而母体通过简单地移除每个可能的风险而避免了这一问题
The body avoids this problemby simply removing every possible risk.
每次只要排卵没有导致健康怀孕
Each time ovulation doesn’t resultin a healthy pregnancy,
子宫就会摆脱掉子宫内膜上的线条
the womb gets ridof its endometrial lining,
连同任何没有受精的卵子 有病的 将死的 或已死的胚胎
along with any unfertilized eggs,sick, dying, or dead embryos.
这个保护的过程就叫做生理周期
That protective processis known as menstruation,
会导致出血
leading to the period.
这一生理特性 可能很奇怪
This biological trait,bizarre as it may be,
但却驱使着人类得以继续繁衍下去
sets us on coursefor the continuation of the human race.

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大姨妈是一种保护机制

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