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

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

your heart pounds faster,

and sweat pours over your skin.

How does this substance
suddenly materialize

and what exactly is its purpose?

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

eating spicy foods,

nervousness,

and when we're sick.

But exercise is probably the most familiar
and common.

In that case, sweating happens 
as a response to movement

triggered deep inside your cells.

As you increase your pace,
your muscles work harder,

increasing their demand for energy.

A process called cellular respiration

consumes glucose 
and oxygen to form ATP,

the energy currency of the cell.

Much of this process takes place
in structures called mitochondria.

The more you move,

the harder mitochondria work
to supply your body with energy.

All this work comes at a cost, though.

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

The heat stimulates temperature sensors
throughout your body.

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

and communicate that information
to the hypothalamus,

which regulates body temperature.

The hypothalamus responds

by sending signals out through 
the sympathetic nervous system

to the sweat glands in your skin.

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.

When a sweat gland first receives
the signal,

the fluid surrounding the cells
in its coiled base

contains high amounts 
of sodium and chloride.

The cells pump these ions
into a hollow tube

that runs through the sweat gland.

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

water moves into the tube by osmosis.

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.

Before it seeps onto the skin,

cells lining the tube will reclaim
as much salt as possible

so the process can continue.

The water in sweat absorbs 
your body's heat energy

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

which in turn lowers your temperature.

This process, 
known as evaporative cooling,

was an important adaptation 
for our ancestors.

This cooling effect isn't only helpful
during exercise.

We sweat in many other scenarios, too.

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

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

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

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.

This happens because adrenaline
stimulates muscle activity

and causes blood vessels to widen,

two responses that increase heat
and trigger the sweating response.

And sweating also occurs 
when we get sick.

When we're feverish,
we sweat because infections

stimulate the hypothalamus 
to increase muscle activity,

which in turn releases 
more energy as heat.

That increases your overall temperature,

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

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

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

your temperature receptors 
sense the decrease in heat

and the hypothalamus brings 
your sweating response to an end.

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.

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

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

enabling you to go that extra mile.