Excretion in Humans
IGCSE Biology Topic 13 4:50 English narration · English + 中文 subtitles burned in
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Your kidneys filter your entire blood supply about thirty times a day.
你的肾脏每天大约把你全身的血液过滤三十遍。
But here is the clever part: they do not pick out the rubbish.
但巧妙之处在于： 它们不是把垃圾挑出来。
They throw almost everything out first — water, sugar, salts, waste — and then take back what is worth keeping.
它们先把几乎所有东西都扔出去——水、糖、盐、废物—— 然后再把值得保留的拿回来。
Understand that two-step trick and this topic is easy.
理解这个两步的巧妙做法，这个专题就简单了。
Welcome to excretion in humans.
欢迎来到人体的排泄。
The urinary system, the nephron, and where urea comes from.
泌尿系统、肾单位，以及尿素从哪里来。
Let's begin.
让我们开始吧。
Excretion is the removal of the waste products of metabolism, and of substances the body has in excess.
排泄是排出新陈代谢的废物，以及身体过量的物质。
Learn that wording — waste products of metabolism, meaning wastes your own reactions made.
记住这个措辞——新陈代谢的废物，意思是你自己的反应产生的废物。
Two main wastes leave your body.
有两种主要的废物离开你的身体。
Carbon dioxide is made during respiration, and it is breathed out through the lungs.
二氧化碳在呼吸作用中产生，通过肺呼出。
Urea is made in the liver, and it is removed by the kidneys together with excess water and excess ions.
尿素在肝脏中产生，由肾脏连同过量的水和过量的离子一起排出。
Notice each waste has its own organ: carbon dioxide leaves through the lungs, urea through the kidneys.
注意每种废物都有自己的器官：二氧化碳通过肺离开，尿素通过肾脏离开。
The kidneys clean the blood and make urine.
肾脏清洁血液并产生尿液。
Then learn the path the urine takes, because the names are easy to muddle.
然后要记住尿液走的路径，因为这些名字容易混淆。
From each kidney the urine flows down a ureter — that is the tube from kidney to bladder.
尿液从每个肾脏沿着输尿管往下流——那是从肾脏到膀胱的管子。
The bladder stores it.
膀胱储存它。
And the urethra carries it out of the body.
尿道把它带出身体。
Ureter and urethra sound alike, so fix them by position: the ureter is above the bladder, the urethra is below it.
输尿管和尿道听起来很像，所以用位置来记： 输尿管在膀胱上方，尿道在膀胱下方。
If the kidneys stop working, a dialysis machine can do their job.
如果肾脏停止工作，透析机可以代替它们的工作。
The patient's blood flows out through the machine.
病人的血液流出并通过机器。
Inside, waste and excess water pass out of the blood across a thin membrane, and the clean blood returns to the body.
在机器里，废物和过量的水通过一层薄膜从血液中出去，干净的血液再回到身体。
That membrane is the whole point, and it is the same idea as diffusion from earlier in the course: waste moves from where it is concentrated, in the blood, to where it is not, in the fluid outside.
那层膜正是关键，它和课程前面讲的扩散是同一个概念： 废物从浓度高的地方，也就是血液里，移动到浓度低的地方，也就是外面的液体里。
Look inside.
看看里面。
Blood arrives under pressure, liquid is squeezed out into a tube, and then most of it is taken back.
血液在压力下到达，液体被挤进一根管子，然后大部分又被拿回去。
For the supplement paper, look at the structure.
补充部分要看结构。
A kidney has two regions: an outer cortex and an inner medulla.
肾脏有两个区域：外面的皮质和里面的髓质。
Inside are millions of tiny tubes called nephrons, and each one cleans the blood in two steps.
里面有数百万条叫做肾单位的细小管子，每一条都分两步清洁血液。
Step one is filtration.
第一步是过滤。
At the glomerulus, which is a knot of capillaries, high pressure forces water, glucose, urea and ions out of the blood and into the nephron.
在肾小球，也就是一团毛细血管， 高压把水、葡萄糖、尿素和离子从血液挤进肾单位。
Step two is reabsorption.
第二步是重吸收。
As that liquid flows along the nephron, the blood takes back all of the glucose, some of the ions, and most of the water.
当那些液体沿着肾单位流动时， 血液把全部的葡萄糖、一部分离子和大部分水拿回去。
What is left behind — urea, excess water and excess ions — becomes the urine.
剩下的东西——尿素、过量的水和过量的离子——就变成尿液。
Here is the question that separates the grades.
这是拉开分数的题目。
A healthy person's urine contains urea and ions but no glucose — even though glucose is forced out of the blood at the glomerulus.
健康人的尿液含有尿素和离子，但没有葡萄糖—— 尽管葡萄糖在肾小球被挤出血液。
Explain that.
请解释。
Filtration does not choose.
过滤不做选择。
High pressure pushes water, glucose, urea and ions into the nephron all together.
高压把水、葡萄糖、尿素和离子一起推进肾单位。
The choosing happens at the next step.
选择发生在下一步。
During reabsorption the blood takes back all of the glucose, so none is left to reach the urine.
在重吸收时，血液把全部葡萄糖拿回去，所以没有剩下的能到达尿液。
And glucose does appear in the urine of someone with untreated diabetes, because there is more of it than the nephron can reabsorb.
而未经治疗的糖尿病人的尿液里确实会出现葡萄糖， 因为葡萄糖的量超过了肾单位能重吸收的量。
Remember it as one line: filtration removes, reabsorption selects.
用一句话记住：过滤负责移除，重吸收负责选择。
Finally, where urea comes from.
最后，尿素从哪里来。
The liver carries out the assimilation of amino acids: it joins them together into the proteins your body needs.
肝脏进行氨基酸的同化： 它把氨基酸连接成你身体需要的蛋白质。
But the body cannot store extra amino acids, so the liver breaks the excess down by deamination.
但身体不能储存多余的氨基酸， 所以肝脏通过脱氨基作用把多余的分解掉。
Deamination means removing the nitrogen-containing part of each amino acid, and that part is then turned into urea.
脱氨基作用的意思是去掉每个氨基酸中含氮的部分，那部分再被变成尿素。
Now the last question: why must we excrete urea at all?
最后一个问题：为什么我们必须排出尿素？
Because it is toxic — poisonous — if it is allowed to build up in the blood.
因为它有毒—— 如果让它在血液中积累就是有害的。
Made in the liver, removed by the kidneys.
在肝脏中产生，由肾脏排出。
Three marks to lock in.
锁住三个分。
First: each waste has its own organ — carbon dioxide through the lungs, urea and excess water and ions through the kidneys.
第一：每种废物都有自己的器官——二氧化碳通过肺， 尿素、过量的水和离子通过肾脏。
Second: filtration removes, reabsorption selects — all the glucose is taken back, some ions, most of the water.
第二：过滤负责移除，重吸收负责选择——全部葡萄糖被拿回， 一部分离子，大部分水。
Third: urea is made in the liver from excess amino acids by deamination, and we excrete it because it is toxic.
第三：尿素是在肝脏中由多余的氨基酸通过脱氨基作用产生的， 我们排出它是因为它有毒。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。