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The finish line's in sight
and you put on an extra burst of speed.

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As your legs pick up the pace,
your breathing gets deeper,

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your heart pounds faster,

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and sweat pours over your skin.

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How does this substance
suddenly materialize

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and what exactly is its purpose?

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

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eating spicy foods,

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nervousness,

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and when we're sick.

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But exercise is probably the most familiar
and common.

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In that case, sweating happens 
as a response to movement

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triggered deep inside your cells.

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As you increase your pace,
your muscles work harder,

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increasing their demand for energy.

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A process called cellular respiration

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consumes glucose 
and oxygen to form ATP,

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the energy currency of the cell.

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Much of this process takes place
in structures called mitochondria.

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The more you move,

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the harder mitochondria work
to supply your body with energy.

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All this work comes at a cost, though.

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

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The heat stimulates temperature sensors
throughout your body.

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Those receptors detect the excess heat
being produced by your muscle cells

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and communicate that information
to the hypothalamus,

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which regulates body temperature.

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The hypothalamus responds

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by sending signals out through 
the sympathetic nervous system

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to the sweat glands in your skin.

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These are distributed all over the body

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with especially high concentrations
on the palms of your hands,

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the soles of your feet,

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and on your head.

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When a sweat gland first receives
the signal,

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the fluid surrounding the cells
in its coiled base

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contains high amounts 
of sodium and chloride.

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The cells pump these ions
into a hollow tube

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that runs through the sweat gland.

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

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water moves into the tube by osmosis.

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As what's called the primary secretion
builds up in the bottom of the tube,

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water pressure pushes it up
into the long straight part of the duct.

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Before it seeps onto the skin,

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cells lining the tube will reclaim
as much salt as possible

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so the process can continue.

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The water in sweat absorbs 
your body's heat energy

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and then evaporates off of you
when it reaches the surface,

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which in turn lowers your temperature.

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This process, 
known as evaporative cooling,

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was an important adaptation 
for our ancestors.

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This cooling effect isn't only helpful
during exercise.

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We sweat in many other scenarios, too.

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

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That happens because spices trigger
the same neural response in the brain

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that activates temperature receptors,
which usually respond to increased heat.

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Sweating is also part of 
the fight or flight response

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stimulated by stressful scenarios,
like asking someone on a date

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or interviewing for a job.

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This happens because adrenaline
stimulates muscle activity

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and causes blood vessels to widen,

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two responses that increase heat
and trigger the sweating response.

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And sweating also occurs 
when we get sick.

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When we're feverish,
we sweat because infections

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stimulate the hypothalamus 
to increase muscle activity,

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which in turn releases 
more energy as heat.

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That increases your overall temperature,

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

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

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

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your temperature receptors 
sense the decrease in heat

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and the hypothalamus brings 
your sweating response to an end.

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In some cases, like after a run,

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the hypothalamus also signals
to your body

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that you need to replenish the water
that you've oozed out.

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

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you can think of sweat as your body's
very own calibrator,

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enabling you to go that extra mile.