Homeostasis
A-Level Biology Topic 14 9:42 English narration · English + 中文 subtitles burned in
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Look at a day's worth of blood sugar.
看一整天的血糖。
Three big meals go in — and the level hardly stirs.
三顿大餐吃进去——可血糖几乎不动。
It wanders a little, then comes straight back.
它稍微飘一下，然后马上回来。
That is not luck, and it is not because nothing happened.
这不是运气，也不是因为什么都没发生。
It is because something detected each change and actively corrected it, within minutes, without you noticing.
而是因为有东西察觉到了每一次变化， 并在几分钟之内主动把它纠正回来，你完全没有察觉。
The same is true of your temperature and your water content.
你的体温和水分含量也是一样。
In this lesson we look at how the body does that, and at the one pattern that nearly every exam answer follows.
这节课我们就来看身体是怎么做到的，以及几乎每道考题答案都遵循的那一个模式。
Homeostasis means keeping the conditions inside the body steady, even when the outside changes.
稳态的意思是让身体内部的条件保持稳定，即使外界在变。
Keep temperature, water and glucose steady, and your enzymes and cells can work properly all the time.
让体温、水分和血糖保持稳定，你的酶和细胞就能一直正常工作。
Nearly all of it follows the same plan.
几乎所有的稳态都遵循同一套流程。
First a stimulus — a change, inside or outside.
首先是刺激——一个变化，来自体内或体外。
That change is detected by a receptor.
这个变化被受体察觉。
The message is then carried by one of two systems: the nervous system, using nerve signals, or the endocrine system, using hormones.
消息接着由两个系统之一传递：神经系统用神经信号，内分泌系统用激素。
The message reaches an effector, which is a muscle or a gland, and the effector makes the response that corrects the change.
消息到达效应器，效应器是肌肉或腺体，由它做出纠正这个变化的反应。
And because the response pushes the level back towards normal, we call it negative feedback.
而因为这个反应把水平推回正常，我们把它叫做负反馈。
Watch the loop run.
看这个回路运转。
The level drifts above the normal range, a receptor picks that up, and a correction is triggered that brings it down again.
水平漂到正常范围以上，受体察觉到了， 于是触发一个纠正把它降下来。
Then it drifts below, and the opposite correction pushes it back up.
接着它又漂到下面，相反的纠正又把它推上去。
Notice that the system never sits perfectly still.
注意这个系统从来不会完全静止。
It is always overshooting slightly and being pulled back, which is exactly what a feedback loop looks like from the outside.
它总是略微冲过头，然后被拉回来， 从外面看，反馈回路就是这个样子。
Start with a waste problem.
先说一个废物问题。
Your body cannot store extra amino acids.
你的身体不能储存多余的氨基酸。
So in the liver, a process called deamination strips the amino group off them, and that group is turned into urea.
所以在肝脏里，一个叫脱氨基作用的过程把氨基从它们身上剥下来，再把它变成尿素。
The urea is carried in the blood to the kidneys to be removed.
尿素随血液被送到肾脏排出。
A kidney has an outer fibrous capsule holding it together, and inside that, two regions you must name. The outer one is the cortex and the inner one is the medulla, and between them they do the actual cleaning.
肾脏外面有一层纤维囊把它包住，里面有两个你必须说得出名字的区域： 外面的是皮质，里面的是髓质，清洁工作就由它们完成。
In the centre is a space called the renal pelvis, which collects the urine before it leaves down the ureter to the bladder.
中央是一个叫肾盂的空腔，尿液在这里汇集，然后沿输尿管流到膀胱。
Blood arrives through the renal artery and leaves through the renal vein.
血液由肾动脉进入，由肾静脉离开。
The cleaning is done by about a million tiny tubes in each kidney, called nephrons.
清洁工作由每个肾里大约一百万条细小的管子完成，它们叫肾单位。
Learn the parts in order.
按顺序记住各部分。
It begins with the glomerulus, which is a knot of capillaries, sitting inside a cup called the Bowman's capsule.
它从肾小球开始，肾小球是一团毛细血管，坐落在一个叫鲍曼囊的杯状结构里。
From there the fluid runs into the proximal convoluted tubule.
液体从那里流进近曲小管。
Then down and back up through the loop of Henle.
然后往下再往上穿过亨利环。
Then into the distal convoluted tubule.
接着进入远曲小管。
And finally into the collecting duct, which carries what is left away as urine.
最后进入集合管，由它把剩下的东西作为尿液带走。
Urine is made in two steps, and each one has its own address.
尿液分两步形成，每一步都有自己的部位。
Ultrafiltration happens in the Bowman's capsule.
超滤发生在鲍曼囊里。
The blood in the glomerulus is under high pressure, so water and small molecules — glucose, ions and urea — are pushed out through the capillary wall into the capsule.
肾小球中的血液压力很高，所以水和小分子——葡萄糖、离子和尿素—— 被挤过毛细血管壁进入囊中。
What comes out is called the filtrate.
出来的东西叫滤液。
Blood cells and large proteins are too big to pass, so they stay behind in the blood.
血细胞和大蛋白太大过不去，所以留在血液里。
Selective reabsorption happens next, in the proximal convoluted tubule.
接下来是选择性重吸收，发生在近曲小管。
Here the useful things are taken back.
有用的东西在这里被取回。
All of the glucose is reabsorbed, along with much of the water and the ions.
葡萄糖被全部重吸收，还有大部分的水和离子。
And the wall of that tubule is beautifully suited to the job: its cells carry microvilli, which give a large surface area, and they are packed with mitochondria, which supply the ATP for active transport.
而那段小管的管壁非常适合这项工作：细胞上带着微绒毛，提供很大的表面积， 细胞里塞满了线粒体，为主动运输提供 ATP。
Osmoregulation controls the water content of the blood, and it is run by the brain.
渗透调节控制血液的水分含量，由大脑来管理。
The hypothalamus monitors the water potential of the blood.
下丘脑监测血液的水势。
Suppose the blood becomes too concentrated.
假设血液变得太浓。
The hypothalamus detects it, and the pituitary gland releases antidiuretic hormone, which everyone calls ADH.
下丘脑察觉到了，垂体就释放抗利尿激素，大家都叫它 ADH。
ADH travels to the kidney and makes the collecting ducts more permeable to water, by inserting water channels called aquaporins into their membranes.
ADH 到达肾脏，通过在集合管的膜上插入叫水通道蛋白的水通道，让集合管对水更通透。
With those channels in place, more water is reabsorbed back into the blood.
有了这些通道，更多的水被重吸收回血液。
So the urine that comes out is small in volume and more concentrated, and the blood's water content is restored.
于是排出的尿量小而且更浓，血液的水分含量就恢复了。
That is negative feedback again.
这又是一次负反馈。
Now for cell signalling — how a hormone actually gets its message into a cell.
现在看激素究竟怎样把消息送进细胞。
When blood glucose falls, glucagon is released, and here is what happens at the liver cell.
当血糖下降时，胰高血糖素被释放， 在肝细胞这里发生的事是这样的。
Glucagon binds to a receptor on the cell surface, and that makes the receptor change shape — a conformational change.
胰高血糖素结合到细胞表面的受体上， 使受体改变形状——这叫构象变化。
The changed receptor activates a G-protein, which switches on an enzyme called adenylyl cyclase.
变了形的受体激活一个 G 蛋白， G 蛋白又打开一种叫腺苷酸环化酶的酶。
That enzyme makes cyclic AMP inside the cell, and cyclic AMP is the second messenger — the molecule that carries the message onwards on the inside.
这种酶在细胞内制造环腺苷酸， 而环腺苷酸就是第二信使——在细胞内部把消息继续传下去的分子。
Cyclic AMP then activates protein kinase A, which starts an enzyme cascade: one enzyme switches on the next by phosphorylation.
环腺苷酸接着激活蛋白激酶 A，由它启动一串酶级联反应： 一种酶通过磷酸化打开下一种酶。
And because each enzyme switches on many of the next, the signal is hugely amplified.
而因为每一种酶都能打开许多个下一种，信号被极大地放大。
The final enzyme breaks glycogen down into glucose, and the blood glucose level rises.
最后那种酶把糖原分解成葡萄糖，血糖水平就升上去了。
Blood glucose is held steady by two hormones pulling in opposite directions.
血糖由两种方向相反的激素维持稳定。
When glucose is too high, insulin is released, and it makes muscle and liver cells take glucose in and store it as glycogen — so the level comes down.
血糖太高时，胰岛素被释放， 它让肌肉细胞和肝细胞把葡萄糖吸收进去，以糖原的形式储存起来——于是水平下降。
When glucose is too low, glucagon is released, and it makes liver cells break glycogen back down into glucose — so the level goes up.
血糖太低时，胰高血糖素被释放，它让肝细胞把糖原重新分解成葡萄糖——于是水平上升。
Between them they hold the level in a narrow band.
两者合起来把血糖维持在一个很窄的范围里。
And to measure it, test strips and biosensors use two enzymes, glucose oxidase and peroxidase, which react with glucose to give either a colour change or an electric signal.
至于测量，试纸和生物传感器用两种酶——葡萄糖氧化酶和过氧化物酶—— 它们与葡萄糖反应，产生颜色变化或电信号。
Here is the classic question.
这是经典的题目。
A person drinks a large volume of water.
一个人喝了大量的水。
Trace the response.
追踪整个反应过程。
First, the blood's water potential rises — it becomes less negative.
首先，血液的水势上升——变得没那么负。
Osmoreceptors in the hypothalamus detect that.
下丘脑里的渗透压感受器察觉到这一点。
So the pituitary releases less ADH.
于是垂体释放较少的 ADH。
With less ADH, fewer aquaporins are inserted into the collecting duct membranes, so the duct becomes less permeable to water.
ADH 少了，插进集合管膜上的水通道蛋白就少了， 于是集合管对水的通透性下降。
Less water is reabsorbed, so the person produces a large volume of dilute urine, and the blood's water potential falls back towards normal.
被重吸收的水变少， 所以这个人产生大量稀释的尿，血液的水势就回落到正常。
That return to the set point is what makes it negative feedback.
回到设定点，这就是它之所以是负反馈的原因。
And notice how many named steps there are. An answer that jumps from "drinks water" straight to "more urine" skips every marking point in between.
还要注意这里有多少个要点名的步骤： 一个从"喝水"直接跳到"尿更多"的答案，把中间每一个得分点都跳过去了。
Plants do this too.
植物也做同样的事。
Stomata are the pores in a leaf, and the plant opens and closes them to balance two needs that pull against each other: letting carbon dioxide in for photosynthesis, and limiting water loss by transpiration.
气孔是叶子上的小孔，植物开合它们，是为了平衡两个互相拉扯的需要： 让二氧化碳进来进行光合作用，同时限制蒸腾作用造成的失水。
Each pore is worked by two guard cells.
每个孔由两个保卫细胞控制。
To open it, the guard cells pump ions in, which lowers their water potential, so water enters by osmosis.
要打开它，保卫细胞把离子泵进来，水势降低，于是水靠渗透进入。
They swell up, become turgid, and bend apart, leaving a pore between them.
它们膨胀起来，变得饱满，向两边弯开，中间就留出一个孔。
To close it, the ions leave, the water follows, the guard cells go floppy and the pore shuts.
要关上它，离子出去，水跟着出去，保卫细胞变软塌，孔就闭合。
And when the plant is short of water — water stress — a hormone called abscisic acid is released, which makes the guard cells lose their ions and close the stomata.
而当植物缺水时（水分胁迫），一种叫脱落酸的激素被释放出来， 它让保卫细胞失去离子，把气孔关上。
Inside those guard cells, calcium ions act as the second messenger.
在这些保卫细胞内部，钙离子充当第二信使。
Four things the examiner keeps asking for.
考官反复要的四点。
First, frame the answer as negative feedback and name all three parts: the receptor, the coordinator, and the effector.
第一，把答案写成负反馈的形式，并且三个部分都要点名： 受体、传导系统、效应器。
Second, be sure which way round the glucose hormones go. Insulin lowers the level, glucagon raises it.
第二，两种血糖激素的方向不要弄反： 胰岛素降低血糖，胰高血糖素升高血糖。
Third, ultrafiltration and selective reabsorption happen in a different place each — the Bowman's capsule and the proximal tubule.
第三，超滤和选择性重吸收各自发生在不同的部位——鲍曼囊和近曲小管。 要说清楚是哪个。
Say which. Fourth, stomata open when the guard cells become turgid, because potassium ions enter and water follows.
第四，气孔在保卫细胞变得饱满时打开，因为钾离子进入，水随之而来。
Five leftover facts.
还剩五件事实。
First, the liver cannot store extra amino acids: deamination removes the amino group and turns it into urea, which the blood carries to the kidneys.
第一，肝脏不能储存多余氨基酸：脱氨基去掉氨基并变成尿素，血液把它送到肾脏。
Second, urine collects in the renal pelvis and leaves down the ureter; the fibrous capsule, cortex, medulla, renal artery and renal vein complete the structure.
第二，尿液在肾盂汇集，沿输尿管离开；纤维囊、皮质、髓质、肾动脉和肾静脉补全结构。
Third, the proximal tubule's cells have microvilli for surface area and many mitochondria to power active transport.
第三，近曲小管细胞有微绒毛增大面积，还有许多线粒体为主动运输供能。
Fourth, test strips and biosensors measure glucose with glucose oxidase and peroxidase.
第四，试纸和生物传感器用葡萄糖氧化酶和过氧化物酶测葡萄糖。
Fifth, stomata usually open by day and close at night, balancing carbon dioxide in against water lost by transpiration.
第五，气孔通常白天开、夜里关，在二氧化碳进入和蒸腾失水之间取得平衡。
So there it is.
就是这样了。
One plan — detect, carry, correct — running in your kidneys, your blood and even in the pores of a leaf.
同一套流程——察觉、传递、纠正—— 运行在你的肾脏里、血液里，甚至叶子的气孔里。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。