As your legs pick up the pace, your breathing gets deeper, your heart pounds faster, and sweat pours over your skin.

當你的雙腿加快步伐時，你的呼吸越來越深，你的心跳加快，汗水流淌在你的皮膚上。

How does this substance suddenly materialize and what exactly is its purpose?

為何突然滴下汗水？流汗的目的究竟是什麼？

There are a number of scenarios that can make us sweat:

有很多場合會讓我們流汗：

eating spicy foods, nervousness, and when we're sick.

吃辛辣的食物、緊張、還有生病時。

But exercise is probably the most familiar and common.

但運動可能是最熟悉、最常見會流汗的情況。

In that case, sweating happens as a response to movement triggered deep inside your cells.

在那種情況下，運動會觸發細胞的深處，而以出汗回應。

As you increase your pace, your muscles work harder, increasing their demand for energy.

當你加快步伐時，你的肌肉會更努力工作，增加對能源的需求。

A process called cellular respiration consumes glucose and oxygen to form ATP, the energy currency of the cell.

這是細胞呼吸的過程，消耗葡萄糖和氧以形成 ATP （註：三磷酸腺苷），就是細胞的能量貨幣。

Much of this process takes place in structures called mitochondria.

大部分的共乘都發生在「粒線體」結構中。

The more you move, the harder mitochondria work to supply your body with energy.

你動的越多，粒線體就越賣力為你的身體提供能量。

All this work comes at a cost, though.

儘管如此，這些全都要付出代價。

As the cells break down the ATP, they release heat.

熱量隨著細胞分解 ATP 而釋出。

The heat stimulates temperature sensors throughout your body.

熱量刺激整個身體的溫度傳感器。

Those receptors detect the excess heat being produced by your muscle cells and communicate that information to the hypothalamus,

這些受體檢測肌肉細胞產生的多餘熱量，並將這些信息傳給「下視丘」，

which regulates body temperature.

其功能為調節體溫。

The hypothalamus responds by sending signals out through the sympathetic nervous system to the sweat glands in your skin.

下視丘將信號通過交感神經系統發送到到皮膚中的汗腺進行回應。

These are distributed all over the body with especially high concentrations on the palms of your hands, the soles of your feet, and on your head.

汗腺遍佈全身，特別集中在手掌、腳底，和頭部。

When a sweat gland first receives the signal, the fluid surrounding the cells in its coiled base contains high amounts of sodium and chloride.

當汗腺首次接收到信號時，其捲曲基底細胞周圍的液體，含有大量的鈉和氯化物。

The cells pump these ions into a hollow tube that runs through the sweat gland.

細胞將這些離子泵入穿過汗腺的中空管。

Then, because it's saltier inside the tube than outside,

由於管內的鹽分比管外更鹹，

water moves into the tube by osmosis.

水「滲透」入管內。

As what's called the primary secretion builds up in the bottom of the tube, water pressure pushes it up into the long straight part of the duct.

隨著「主要分泌物」在管底部積聚，水壓將其推上長長直直的管道。

Before it seeps onto the skin, cells lining the tube will reclaim as much salt as possible so the process can continue.

在滲透到皮膚表面以前，襯管內的細胞盡可能地回收鹽分，讓這過程得以繼續。

The water in sweat absorbs your body's heat energy and then evaporates off of you when it reaches the surface,

汗水中的水分吸收人體熱能，當它到達表面時會蒸發掉，

which in turn lowers your temperature.

從而降低體溫。

This process, known as evaporative cooling, was an important adaptation for our ancestors.

這個過程稱為「蒸發冷卻」，是我們祖先重要的進化。

This cooling effect isn't only helpful during exercise.

冷卻效應不僅作用在運動過程中。

We sweat in many other scenarios, too.

我們還在其他許多場合出汗。

Eating particularly spicy food makes some people sweat profusely from their faces.

吃特別辛辣的食物會讓一些人的臉部大量流汗。

That happens because spices trigger the same neural response in the brain that activates temperature receptors, which usually respond to increased heat.

這是因為香料在大腦中引發相同的神經反應，激發體溫感受器，其通常會增加熱量的產出。

Sweating is also part of the fight or flight response stimulated by stressful scenarios, like asking someone on a date or interviewing for a job.

出汗也是「戰鬥或逃跑」反應的一部分，來自壓力情境的刺激，像是邀請某人約會或面試工作。

This happens because adrenaline stimulates muscle activity and causes blood vessels to widen,

這是因為腎上腺素刺激肌肉活動導致血管擴張，

two responses that increase heat and trigger the sweating response.

兩種反應增加熱量並引發出汗反應。

And sweating also occurs when we get sick.

我們生病時也會出汗。

When we're feverish, we sweat because infections stimulate the hypothalamus to increase muscle activity,

我們發燒時會出汗是因為感染會刺激下視丘，增加肌肉活動，

which in turn releases more energy as heat.

從而釋放出更多的熱量。

That increases your overall temperature,

這會增加你的整體溫度，

a protective mechanism that makes your body less habitable for infectious agents.

是一種保護機制使你的身體不適合傳染源生存。

Like with running, sweating helps your body vent that heat.

就像跑步一樣，出汗有助於身體排出熱量。

When the fever's over or you've won your race,

退燒後，或你贏了比賽後，

your temperature receptors sense the decrease in heat and the hypothalamus brings your sweating response to an end.

你的體溫感受器感受到的熱量減少，下視丘就會結束你的出汗反應。

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.

在某些情況下，如跑步後，下視丘也會向你的身體發出信號，提醒你需要補充流失的水分。

So, when you're pushing yourself to reach that next goal,

所以當你盡力達到下一個目標時，

you can think of sweat as your body's very own calibrator, enabling you to go that extra mile.

你可以將汗水看成是自己的校準器，使你能夠更努力一些。