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The finish line's in sight and you put on an extra burst of speed.
终点线在望，你又加速了一阵
As your legs pick up the pace, your breathing gets deeper,
当你的双腿加快步伐时你的呼吸越来越深
your heart pounds faster,
你的心跳加快
and sweat pours over your skin.
汗水流淌在你的皮肤上
How does this substance suddenly materialize
为何突然滴下汗水？
and what exactly is its purpose?
流汗的目的究竟是什么？
There are a number of scenarios that can make us sweat:
有很多场合会让我们流汗：
eating spicy foods,
吃辛辣的食物
nervousness,
紧张
and when we're sick.
生病时
But exercise is probably the most familiar and common.
但运动可能最熟悉、最常见
In that case, sweating happens as a response to movement
在那种情况下运动会触发细胞的深处
triggered deep inside your cells.
而以出汗回应
As you increase your pace, your muscles work harder,
当你加快步伐时你的肌肉会更努力工作
increasing their demand for energy.
增加对能源的需求
A process called cellular respiration
这是个细胞呼吸的过程
consumes glucose and oxygen to form ATP,
消耗葡萄糖和氧以形成 ATP（注：三磷酸腺苷）
the energy currency of the cell.
细胞的能量货币
Much of this process takes place in structures called mitochondria.
过程大部分发生在“粒线体”结构中
The more you move,
你动的越多
the harder mitochondria work to supply your body with energy.
粒线体就越卖力为你的身体提供能量
All this work comes at a cost, though.
尽管如此，这些全都要付出代价
As the cells break down the ATP, they release heat.
热量随着细胞分解 ATP 而释出
The heat stimulates temperature sensors throughout your body.
热量刺激整个身体的温度传感器
Those receptors detect the excess heat being produced by your muscle cells
这些受体检测肌肉细胞产生的多余热量
and communicate that information to the hypothalamus,
并将这些信息传给调节体温的“下视丘”
which regulates body temperature.
来调节体温
The hypothalamus responds
下视丘回应
by sending signals out through the sympathetic nervous system
将信号通过交感神经系统
to the sweat glands in your skin.
发送到到皮肤中的汗腺
These are distributed all over the body
汗腺遍布全身
with especially high concentrations on the palms of your hands,
特别集中在手掌
the soles of your feet,
脚底
and on your head.
和头部
When a sweat gland first receives the signal,
当汗腺首次接收到信号时
the fluid surrounding the cells in its coiled base
其卷曲基底细胞周围的液体
contains high amounts of sodium and chloride.
含有大量的钠和氯化物
The cells pump these ions into a hollow tube
细胞将这些离子
that runs through the sweat gland.
泵入穿过汗腺的中空管
Then, because it's saltier inside the tube than outside,
由于管内的盐分比管外更咸
water moves into the tube by osmosis.
水“渗透”入管内
As what's called the primary secretion builds up in the bottom of the tube,
随着“主要分泌物”在管底部积聚
water pressure pushes it up into the long straight part of the duct.
水压将其推上长长直直的管道
Before it seeps onto the skin,
在渗透到皮肤表面以前
cells lining the tube will reclaim as much salt as possible
衬管内的细胞尽可能地回收盐分
so the process can continue.
让这过程得以继续
The water in sweat absorbs your body's heat energy
汗水中的水分吸收人体热能
and then evaporates off of you when it reaches the surface,
当它到达表面时会蒸发掉
which in turn lowers your temperature.
从而降低体温
This process, known as evaporative cooling,
这个过程称为“蒸发冷却”
was an important adaptation for our ancestors.
是我们祖先的重要的进化
This cooling effect isn't only helpful during exercise.
冷却效应不仅作用在运动过程中
We sweat in many other scenarios, too.
我们还在其他许多场合出汗
Eating particularly spicy food makes some people sweat profusely from their faces.
吃特别辛辣的食物会让一些人的脸部大量流汗
That happens because spices trigger the same neural response in the brain
这是因为香料在大脑中引发相同的神经反应
that activates temperature receptors, which usually respond to increased heat.
激发体温感受器通常会增加热量的产出
Sweating is also part of the fight or flight response
出汗也是“战斗或逃跑”反应的一部分
stimulated by stressful scenarios, like asking someone on a date
来自压力情境的刺激
or interviewing for a job.
像是邀请某人约会或面试工作
This happens because adrenaline stimulates muscle activity
这是因为肾上腺素刺激肌肉活动
and causes blood vessels to widen,
导致血管扩张
two responses that increase heat and trigger the sweating response.
两种反应增加热量并引发出汗反应
And sweating also occurs when we get sick.
我们生病时也会出汗
When we're feverish, we sweat because infections
我们发烧时会出汗
stimulate the hypothalamus to increase muscle activity,
因为感染会刺激下视丘增加肌肉活动
which in turn releases more energy as heat.
从而释放出更多的热量
That increases your overall temperature,
这会增加你的整体温度
a protective mechanism that makes your body less habitable for infectious agents.
是一种保护机制使你的身体不适合传染源生存
Like with running, sweating helps your body vent that heat.
就像跑步一样出汗有助于身体排出热量
When the fever's over or you've won your race,
退烧后，或你赢了比赛后
your temperature receptors sense the decrease in heat
你的体温感受器会感受到热量的减少
and the hypothalamus brings your sweating response to an end.
下视丘会结束你的出汗反应
In some cases, like after a run,
在某些情况下，如跑步后
the hypothalamus also signals to your body
下视丘也会向你的身体发出信号
that you need to replenish the water that you've oozed out.
提醒你需要补充流失的水分
So, when you're pushing yourself to reach that next goal,
所以当你尽力达到下一个目标时
you can think of sweat as your body's very own calibrator,
你可以将汗水看成是自己的校准器
enabling you to go that extra mile.
使你能够更努力一些
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