The finish line's in sight and you put on an extra burst of speed.
终点线近在眼前了，你开始加速。
As your legs pick up the pace, your breathing gets deeper, your heart pounds faster, and sweat pours over your skin.
你的腿逐渐变快，呼吸也越来越深，你的心脏跳动得更快，你的汗如雨下。
How does this substance suddenly materialize and what exactly is its purpose?
汗水是如何突然之间出现的，它又有什么作用呢？
There are a number of scenarios that can make us sweat: eating spicy foods, nervousness, and when we're sick.
我们在很多情况下都会流汗：吃辣的东西，紧张，以及生病。
But exercise is probably the most familiar and common.
但是运动可能是其中最寻常的一种情况。
In that case, sweating happens as a response to movement triggered deep inside your cells.
在这种情况下，出汗是对于体内细胞运动的一种反应。
As you increase your pace, your muscles work harder, increasing their demand for energy.
当你的速度加快，你身上的肌肉更努力的工作，需要的能量也就越多。
A process called cellular respiration consumes glucose and oxygen to form ATP, the energy currency of the cell.
一种名为细胞呼吸的过程消耗葡萄糖和氧气来合成ATP，也就是细胞的“能量货币”。
Much of this process takes place in structures called mitochondria.
这一过程大部分发生在线粒体中。
The more you move, the harder mitochondria work to supply your body with energy.
你运动的越多，线粒体就越努力工作，以支持你身体所需的能量。
All this work comes at a cost, though. As the cells break down the ATP, they release heat.
但是，这些工作都是有代价的。在细胞分解ATP时，它们会同时散发热量。
The heat stimulates temperature sensors throughout your body.
这些热量刺激你体内的热量感应器。
Those receptors detect the excess heat being produced by your muscle cells
感应器接收了你肌肉细胞所产生的多余热量，
and communicate that information to the hypothalamus, which regulates body temperature.
并把这一信息传给负责调控体温的下丘脑。
The hypothalamus responds by sending signals out through the sympathetic nervous system to the sweat glands in your skin.
下丘脑对此作出反应，通过遍布人体的交感神经系统，向皮肤中的汗腺发出信号。
These are distributed all over the body
这些汗腺遍布你的全身，
with especially high concentrations on the palms of your hands, the soles of your feet, and on your head.
尤其是密集分布于手掌、脚掌和头部。
When a sweat gland first receives the signal, the fluid surrounding the cells in its coiled base contains high amounts of sodium and chloride.
当汗腺一开始接到信号时，在汗腺底部细胞周围的液体中含有大量的钠离子和氯离子。
The cells pump these ions into a hollow tube that runs through the sweat gland.
这些细胞将这些离子打进一个贯穿汗腺的小管。

Then, because it's saltier inside the tube than outside, water moves into the tube by osmosis.
紧接着，由于管内比管外盐度更高，渗透作用使水分进入管中。
As what's called the primary secretion builds up in the bottom of the tube,
随着这些主要分泌物在小管的底部越积越多，
water pressure pushes it up into the long straight part of the duct.
压力使水上升到小管长而直的部分中。
Before it seeps onto the skin, cells lining the tube will reclaim as much salt as possible so the process can continue.
在渗出皮肤之前，小管周围的细胞会尽可能多的吸收盐分以确保这一过程继续进行。
The water in sweat absorbs your body's heat energy and then evaporates off of you when it reaches the surface,
汗液中的水分能吸收你身体的热量在到达肌肤表层时蒸发，带走热量，
which in turn lowers your temperature.
以此来降低你的体温。
This process, known as evaporative cooling, was an important adaptation for our ancestors.
这一过程叫做蒸发冷却，是我们的祖先的一种重要的适应性变化。
This cooling effect isn't only helpful during exercise.
这种冷却效应不光在运动中发挥作用。
We sweat in many other scenarios, too.
我们也在其他情况下出汗。
Eating particularly spicy food makes some people sweat profusely from their faces.
吃辣的东西会让一些人脸部大量出汗。
That happens because spices trigger the same neural response in the brain
这是因为香辛料同样会刺激大脑的一些神经反应，
that activates temperature receptors, which usually respond to increased heat.
这些反应会激活感知热量增高的温度感应器。
Sweating is also part of the fight or flight response stimulated by stressful scenarios,
出汗也是“战斗/逃跑反应”的一部分，这种反应一般在焦虑的情况下会被激活，
like asking someone on a date or interviewing for a job.
例如邀请某人约会，或是工作面试。
This happens because adrenaline stimulates muscle activity and causes blood vessels to widen,
这是因为肾上腺素刺激肌肉运动，使血管变宽，
two responses that increase heat and trigger the sweating response.
这两者都会增加热量，从而触发流汗反应。
And sweating also occurs when we get sick.
我们在生病时也会出汗。
When we're feverish, we sweat because infections stimulate the hypothalamus to increase muscle activity,
当我们发烧时，我们会出汗是因为炎症使下丘脑增加肌肉运动，
which in turn releases more energy as heat.
并以散热的方式来释放热量。
That increases your overall temperature, a protective mechanism that makes your body less habitable for infectious agents.
这会让你的体温整体上升，这是一种保护机制，使得传染源在你体内不易生存。
Like with running, sweating helps your body vent that heat.
就像跑步时一样，流汗帮你带走热量。
When the fever's over or you've won your race,
当你退烧了或是赢了跑步比赛，
your temperature receptors sense the decrease in heat and the hypothalamus brings your sweating response to an end.
热量感应器会感受到热量下降，下丘脑也随之终止流汗反应。
In some cases, like after a run, the hypothalamus also signals to your body that you need to replenish the water that you've oozed out.
在一些情况下，例如跑完步后，下丘脑也会向你的身体发送信号，提醒你补充流失的水分。
So, when you're pushing yourself to reach that next goal,
所以，当你在尽力完成下一个目标时，
you can think of sweat as your body's very own calibrator, enabling you to go that extra mile.
你可以将汗水想成你身体的校准器，让你能够再多努力一点。