1
00:00:07,420 --> 00:00:12,350
The finish line's in sight
and you put on an extra burst of speed.

2
00:00:12,350 --> 00:00:15,480
As your legs pick up the pace,
your breathing gets deeper,

3
00:00:15,480 --> 00:00:17,021
your heart pounds faster,

4
00:00:17,021 --> 00:00:20,526
and sweat pours over your skin.

5
00:00:20,526 --> 00:00:23,162
How does this substance
suddenly materialize

6
00:00:23,162 --> 00:00:26,460
and what exactly is its purpose?

7
00:00:26,460 --> 00:00:29,782
There are a number of scenarios
that can make us sweat:

8
00:00:29,782 --> 00:00:30,896
eating spicy foods,

9
00:00:30,896 --> 00:00:31,871
nervousness,

10
00:00:31,871 --> 00:00:33,589
and when we're sick.

11
00:00:33,589 --> 00:00:37,892
But exercise is probably the most familiar
and common.

12
00:00:37,892 --> 00:00:41,403
In that case, sweating happens 
as a response to movement

13
00:00:41,403 --> 00:00:43,902
triggered deep inside your cells.

14
00:00:43,902 --> 00:00:47,093
As you increase your pace,
your muscles work harder,

15
00:00:47,093 --> 00:00:49,851
increasing their demand for energy.

16
00:00:49,851 --> 00:00:52,693
A process called cellular respiration

17
00:00:52,693 --> 00:00:56,913
consumes glucose 
and oxygen to form ATP,

18
00:00:56,913 --> 00:01:00,545
the energy currency of the cell.

19
00:01:00,545 --> 00:01:05,841
Much of this process takes place
in structures called mitochondria.

20
00:01:05,841 --> 00:01:07,275
The more you move,

21
00:01:07,275 --> 00:01:11,713
the harder mitochondria work
to supply your body with energy.

22
00:01:11,713 --> 00:01:14,335
All this work comes at a cost, though.

23
00:01:14,335 --> 00:01:18,568
As the cells break down the ATP,
they release heat.

24
00:01:18,568 --> 00:01:23,002
The heat stimulates temperature sensors
throughout your body.

25
00:01:23,002 --> 00:01:27,005
Those receptors detect the excess heat
being produced by your muscle cells

26
00:01:27,005 --> 00:01:30,713
and communicate that information
to the hypothalamus,

27
00:01:30,713 --> 00:01:32,991
which regulates body temperature.

28
00:01:32,991 --> 00:01:34,475
The hypothalamus responds

29
00:01:34,475 --> 00:01:37,825
by sending signals out through 
the sympathetic nervous system

30
00:01:37,825 --> 00:01:40,695
to the sweat glands in your skin.

31
00:01:40,695 --> 00:01:42,896
These are distributed all over the body

32
00:01:42,896 --> 00:01:46,786
with especially high concentrations
on the palms of your hands,

33
00:01:46,786 --> 00:01:48,426
the soles of your feet,

34
00:01:48,426 --> 00:01:50,025
and on your head.

35
00:01:50,025 --> 00:01:52,845
When a sweat gland first receives
the signal,

36
00:01:52,845 --> 00:01:56,121
the fluid surrounding the cells
in its coiled base

37
00:01:56,121 --> 00:01:59,986
contains high amounts 
of sodium and chloride.

38
00:01:59,986 --> 00:02:03,531
The cells pump these ions
into a hollow tube

39
00:02:03,531 --> 00:02:06,176
that runs through the sweat gland.

40
00:02:06,176 --> 00:02:09,757
Then, because it's saltier inside
the tube than outside,

41
00:02:09,757 --> 00:02:13,555
water moves into the tube by osmosis.

42
00:02:13,555 --> 00:02:18,268
As what's called the primary secretion
builds up in the bottom of the tube,

43
00:02:18,268 --> 00:02:24,057
water pressure pushes it up
into the long straight part of the duct.

44
00:02:24,057 --> 00:02:25,818
Before it seeps onto the skin,

45
00:02:25,818 --> 00:02:29,968
cells lining the tube will reclaim
as much salt as possible

46
00:02:29,968 --> 00:02:32,057
so the process can continue.

47
00:02:32,057 --> 00:02:35,147
The water in sweat absorbs 
your body's heat energy

48
00:02:35,147 --> 00:02:39,027
and then evaporates off of you
when it reaches the surface,

49
00:02:39,027 --> 00:02:43,498
which in turn lowers your temperature.

50
00:02:43,498 --> 00:02:46,494
This process, 
known as evaporative cooling,

51
00:02:46,494 --> 00:02:49,479
was an important adaptation 
for our ancestors.

52
00:02:49,479 --> 00:02:53,199
This cooling effect isn't only helpful
during exercise.

53
00:02:53,199 --> 00:02:56,796
We sweat in many other scenarios, too.

54
00:02:56,796 --> 00:03:02,269
Eating particularly spicy food makes some
people sweat profusely from their faces.

55
00:03:02,269 --> 00:03:06,771
That happens because spices trigger
the same neural response in the brain

56
00:03:06,771 --> 00:03:12,600
that activates temperature receptors,
which usually respond to increased heat.

57
00:03:12,600 --> 00:03:15,935
Sweating is also part of 
the fight or flight response

58
00:03:15,935 --> 00:03:19,991
stimulated by stressful scenarios,
like asking someone on a date

59
00:03:19,991 --> 00:03:22,290
or interviewing for a job.

60
00:03:22,290 --> 00:03:25,821
This happens because adrenaline
stimulates muscle activity

61
00:03:25,821 --> 00:03:28,751
and causes blood vessels to widen,

62
00:03:28,751 --> 00:03:33,481
two responses that increase heat
and trigger the sweating response.

63
00:03:33,481 --> 00:03:36,780
And sweating also occurs 
when we get sick.

64
00:03:36,780 --> 00:03:39,521
When we're feverish,
we sweat because infections

65
00:03:39,521 --> 00:03:43,281
stimulate the hypothalamus 
to increase muscle activity,

66
00:03:43,281 --> 00:03:46,951
which in turn releases 
more energy as heat.

67
00:03:46,951 --> 00:03:49,021
That increases your overall temperature,

68
00:03:49,021 --> 00:03:54,482
a protective mechanism that makes your
body less habitable for infectious agents.

69
00:03:54,482 --> 00:03:58,163
Like with running, sweating helps
your body vent that heat.

70
00:03:58,163 --> 00:04:00,561
When the fever's over 
or you've won your race,

71
00:04:00,561 --> 00:04:03,960
your temperature receptors 
sense the decrease in heat

72
00:04:03,960 --> 00:04:08,473
and the hypothalamus brings 
your sweating response to an end.

73
00:04:08,473 --> 00:04:10,352
In some cases, like after a run,

74
00:04:10,352 --> 00:04:13,004
the hypothalamus also signals
to your body

75
00:04:13,004 --> 00:04:16,452
that you need to replenish the water
that you've oozed out.

76
00:04:16,452 --> 00:04:19,411
So, when you're pushing yourself
to reach that next goal,

77
00:04:19,411 --> 00:04:23,386
you can think of sweat as your body's
very own calibrator,

78
00:04:23,386 --> 00:04:26,193
enabling you to go that extra mile.