0:00:07.420,0:00:12.350
The finish line's in sight[br]and you put on an extra burst of speed.

0:00:12.350,0:00:15.480
As your legs pick up the pace,[br]your breathing gets deeper,

0:00:15.480,0:00:17.021
your heart pounds faster,

0:00:17.021,0:00:20.526
and sweat pours over your skin.

0:00:20.526,0:00:23.162
How does this substance[br]suddenly materialize

0:00:23.162,0:00:26.460
and what exactly is its purpose?

0:00:26.460,0:00:29.782
There are a number of scenarios[br]that can make us sweat:

0:00:29.782,0:00:30.896
eating spicy foods,

0:00:30.896,0:00:31.871
nervousness,

0:00:31.871,0:00:33.589
and when we're sick.

0:00:33.589,0:00:37.892
But exercise is probably the most familiar[br]and common.

0:00:37.892,0:00:41.403
In that case, sweating happens [br]as a response to movement

0:00:41.403,0:00:43.902
triggered deep inside your cells.

0:00:43.902,0:00:47.093
As you increase your pace,[br]your muscles work harder,

0:00:47.093,0:00:49.851
increasing their demand for energy.

0:00:49.851,0:00:52.693
A process called cellular respiration

0:00:52.693,0:00:56.913
consumes glucose [br]and oxygen to form ATP,

0:00:56.913,0:01:00.545
the energy currency of the cell.

0:01:00.545,0:01:05.841
Much of this process takes place[br]in structures called mitochondria.

0:01:05.841,0:01:07.275
The more you move,

0:01:07.275,0:01:11.713
the harder mitochondria work[br]to supply your body with energy.

0:01:11.713,0:01:14.335
All this work comes at a cost, though.

0:01:14.335,0:01:18.568
As the cells break down the ATP,[br]they release heat.

0:01:18.568,0:01:23.002
The heat stimulates temperature sensors[br]throughout your body.

0:01:23.002,0:01:27.005
Those receptors detect the excess heat[br]being produced by your muscle cells

0:01:27.005,0:01:30.713
and communicate that information[br]to the hypothalamus,

0:01:30.713,0:01:32.991
which regulates body temperature.

0:01:32.991,0:01:34.475
The hypothalamus responds

0:01:34.475,0:01:37.825
by sending signals out through [br]the sympathetic nervous system

0:01:37.825,0:01:40.695
to the sweat glands in your skin.

0:01:40.695,0:01:42.896
These are distributed all over the body

0:01:42.896,0:01:46.786
with especially high concentrations[br]on the palms of your hands,

0:01:46.786,0:01:48.426
the soles of your feet,

0:01:48.426,0:01:50.025
and on your head.

0:01:50.025,0:01:52.845
When a sweat gland first receives[br]the signal,

0:01:52.845,0:01:56.121
the fluid surrounding the cells[br]in its coiled base

0:01:56.121,0:01:59.986
contains high amounts [br]of sodium and chloride.

0:01:59.986,0:02:03.531
The cells pump these ions[br]into a hollow tube

0:02:03.531,0:02:06.176
that runs through the sweat gland.

0:02:06.176,0:02:09.757
Then, because it's saltier inside[br]the tube than outside,

0:02:09.757,0:02:13.555
water moves into the tube by osmosis.

0:02:13.555,0:02:18.268
As what's called the primary secretion[br]builds up in the bottom of the tube,

0:02:18.268,0:02:24.057
water pressure pushes it up[br]into the long straight part of the duct.

0:02:24.057,0:02:25.818
Before it seeps onto the skin,

0:02:25.818,0:02:29.968
cells lining the tube will reclaim[br]as much salt as possible

0:02:29.968,0:02:32.057
so the process can continue.

0:02:32.057,0:02:35.147
The water in sweat absorbs [br]your body's heat energy

0:02:35.147,0:02:39.027
and then evaporates off of you[br]when it reaches the surface,

0:02:39.027,0:02:43.498
which in turn lowers your temperature.

0:02:43.498,0:02:46.494
This process, [br]known as evaporative cooling,

0:02:46.494,0:02:49.479
was an important adaptation [br]for our ancestors.

0:02:49.479,0:02:53.199
This cooling effect isn't only helpful[br]during exercise.

0:02:53.199,0:02:56.796
We sweat in many other scenarios, too.

0:02:56.796,0:03:02.269
Eating particularly spicy food makes some[br]people sweat profusely from their faces.

0:03:02.269,0:03:06.771
That happens because spices trigger[br]the same neural response in the brain

0:03:06.771,0:03:12.600
that activates temperature receptors,[br]which usually respond to increased heat.

0:03:12.600,0:03:15.935
Sweating is also part of [br]the fight or flight response

0:03:15.935,0:03:19.991
stimulated by stressful scenarios,[br]like asking someone on a date

0:03:19.991,0:03:22.290
or interviewing for a job.

0:03:22.290,0:03:25.821
This happens because adrenaline[br]stimulates muscle activity

0:03:25.821,0:03:28.751
and causes blood vessels to widen,

0:03:28.751,0:03:33.481
two responses that increase heat[br]and trigger the sweating response.

0:03:33.481,0:03:36.780
And sweating also occurs [br]when we get sick.

0:03:36.780,0:03:39.521
When we're feverish,[br]we sweat because infections

0:03:39.521,0:03:43.281
stimulate the hypothalamus [br]to increase muscle activity,

0:03:43.281,0:03:46.951
which in turn releases [br]more energy as heat.

0:03:46.951,0:03:49.021
That increases your overall temperature,

0:03:49.021,0:03:54.482
a protective mechanism that makes your[br]body less habitable for infectious agents.

0:03:54.482,0:03:58.163
Like with running, sweating helps[br]your body vent that heat.

0:03:58.163,0:04:00.561
When the fever's over [br]or you've won your race,

0:04:00.561,0:04:03.960
your temperature receptors [br]sense the decrease in heat

0:04:03.960,0:04:08.473
and the hypothalamus brings [br]your sweating response to an end.

0:04:08.473,0:04:10.352
In some cases, like after a run,

0:04:10.352,0:04:13.004
the hypothalamus also signals[br]to your body

0:04:13.004,0:04:16.452
that you need to replenish the water[br]that you've oozed out.

0:04:16.452,0:04:19.411
So, when you're pushing yourself[br]to reach that next goal,

0:04:19.411,0:04:23.386
you can think of sweat as your body's[br]very own calibrator,

0:04:23.386,0:04:26.193
enabling you to go that extra mile.