1
00:00:01,202 --> 00:00:02,486
- [Lecturer] When we start thinking about

2
00:00:02,486 --> 00:00:06,738
the heart and blood vessels
we have to think of,

3
00:00:06,738 --> 00:00:08,028
so we start to look at it you can see

4
00:00:08,028 --> 00:00:09,326
that the blood leaves the heart,

5
00:00:09,326 --> 00:00:12,016
this major vessel, the aorta,

6
00:00:12,016 --> 00:00:13,266
but heads on down to smaller

7
00:00:13,266 --> 00:00:16,016
and smaller and smaller arteries.

8
00:00:18,403 --> 00:00:20,451
Now these arteries can be compared

9
00:00:20,451 --> 00:00:21,863
to veins on the other side.

10
00:00:21,863 --> 00:00:24,542
The blue ones are veins.

11
00:00:24,542 --> 00:00:26,445
The veins are pretty big in the heart,

12
00:00:26,445 --> 00:00:30,800
but get smaller and smaller
as you go away from the heart.

13
00:00:30,800 --> 00:00:34,285
Right where the arteries and veins meet

14
00:00:34,285 --> 00:00:35,952
are a capillary bed.

15
00:00:37,257 --> 00:00:38,542
Now you can see at the very bottom here,

16
00:00:38,542 --> 00:00:42,877
capillaries are usually
only a single layer thick.

17
00:00:42,877 --> 00:00:45,844
Very thin, very permeable.

18
00:00:45,844 --> 00:00:49,200
This allows the gases
oxygen and carbon dioxide

19
00:00:49,200 --> 00:00:50,861
to flow back and forth.

20
00:00:50,861 --> 00:00:55,451
It allows the nutrients
to get into the tissues.

21
00:00:55,451 --> 00:00:57,883
But before they get to the capillaries,

22
00:00:57,883 --> 00:01:00,157
if you go back up that arterial side,

23
00:01:00,157 --> 00:01:02,770
back up that red side a little bit,

24
00:01:02,770 --> 00:01:05,531
you get what's called an arteriole.

25
00:01:05,531 --> 00:01:07,638
A little thicker than the capillary

26
00:01:07,638 --> 00:01:11,106
because we have some extra
smooth muscle around it.

27
00:01:11,106 --> 00:01:12,175
It's meant to give a little support

28
00:01:12,175 --> 00:01:15,678
and help out a little
bit with blood pressure.

29
00:01:15,678 --> 00:01:17,596
But if you keep going
back up towards the heart,

30
00:01:17,596 --> 00:01:20,555
on the red arterial side, you'll notice

31
00:01:20,555 --> 00:01:24,368
you still have the inner
layer, the endothelium.

32
00:01:24,368 --> 00:01:27,750
You'll have a thicker
smooth muscle layer now

33
00:01:27,750 --> 00:01:30,166
and it'll have an outer connective tissue

34
00:01:30,166 --> 00:01:34,968
to really give support,
strength, and structure.

35
00:01:34,968 --> 00:01:37,164
But when you compare
those to the other side,

36
00:01:37,164 --> 00:01:41,331
the full fledged artery
compares to a full fledged vein.

37
00:01:42,439 --> 00:01:44,325
But look at the inside of them.

38
00:01:44,325 --> 00:01:46,325
The vein is much larger.

39
00:01:47,194 --> 00:01:50,166
Still has the same basic three layers,

40
00:01:50,166 --> 00:01:54,575
but a much larger inside
or much larger lumen.

41
00:01:54,575 --> 00:01:58,012
What's happening here is the larger lumen

42
00:01:58,012 --> 00:02:00,899
allows for a lower resistance.

43
00:02:00,899 --> 00:02:04,319
Helps the blood to get
back to the heart easier.

44
00:02:04,319 --> 00:02:07,137
venule matches up with arteriole.

45
00:02:07,137 --> 00:02:11,456
Again venules are larger,
easier for blood flow.

46
00:02:11,456 --> 00:02:13,133
Reason being is as you go from your heart

47
00:02:13,133 --> 00:02:16,802
all the way down toward the capillaries,

48
00:02:16,802 --> 00:02:19,540
your blood pressure steadily declines,

49
00:02:19,540 --> 00:02:21,123
declines, declines.

50
00:02:22,736 --> 00:02:25,352
Well, down in the tissues there's no way

51
00:02:25,352 --> 00:02:28,163
of bringing the pressure back up again.

52
00:02:28,163 --> 00:02:30,500
So you have to have a low pressure

53
00:02:30,500 --> 00:02:32,666
or a low resistance system to get blood

54
00:02:32,666 --> 00:02:34,499
back up to your heart.

55
00:02:35,891 --> 00:02:37,514
So one thing we say about all arteries

56
00:02:37,514 --> 00:02:40,905
that carry blood away from the heart.

57
00:02:40,905 --> 00:02:43,755
They're under higher pressure.

58
00:02:43,755 --> 00:02:46,755
You cannot say all arteries are red,

59
00:02:48,096 --> 00:02:51,683
or all arteries carry oxygen rich blood.

60
00:02:51,683 --> 00:02:53,554
Those don't hold up.

61
00:02:53,554 --> 00:02:56,918
So arteries carry blood
away from the heart

62
00:02:56,918 --> 00:02:59,835
and under a higher pressure system.

63
00:03:01,245 --> 00:03:04,191
Capillaries, they're meant to exchange.

64
00:03:04,191 --> 00:03:07,218
Water, solutes like salts, gases,

65
00:03:07,218 --> 00:03:09,062
are all going back and forth.

66
00:03:09,062 --> 00:03:12,629
The bi tissues to the capillaries.

67
00:03:12,629 --> 00:03:16,796
Veins, their simple task is
returning blood to the heart.

68
00:03:17,638 --> 00:03:21,048
They'll handle larger
openings, so larger lumens.

69
00:03:21,048 --> 00:03:23,256
They're meant to bring
blood back from the tissues

70
00:03:23,256 --> 00:03:25,173
back towards the heart.

71
00:03:27,369 --> 00:03:30,355
Let's go back to these
arteries for a second now.

72
00:03:30,355 --> 00:03:33,824
We already saw they have
three different layers.

73
00:03:33,824 --> 00:03:37,087
There's this inner
layer, middle, and outer.

74
00:03:37,087 --> 00:03:39,955
Well the inner layer is endothelium.

75
00:03:39,955 --> 00:03:42,354
That's epithelial tissue.

76
00:03:42,354 --> 00:03:46,007
It's meant to allow for
a very smooth surface.

77
00:03:46,007 --> 00:03:48,619
That way the blood can't stick to it.

78
00:03:48,619 --> 00:03:50,886
Middle, smooth muscle.

79
00:03:50,886 --> 00:03:52,757
That's giving you the strength to create

80
00:03:52,757 --> 00:03:55,357
your blood pressure, assuming you create

81
00:03:55,357 --> 00:03:57,902
and maintain blood pressure.

82
00:03:57,902 --> 00:04:01,190
While the outer layer
is connective tissue,

83
00:04:01,190 --> 00:04:04,779
which this is meant to give
a little bit of support

84
00:04:04,779 --> 00:04:07,112
and structure to the artery.

85
00:04:08,326 --> 00:04:11,484
So we said arteries carry
blood away from the heart.

86
00:04:11,484 --> 00:04:12,707
Simple as that.

87
00:04:12,707 --> 00:04:15,730
They're pressurized, and they're typically

88
00:04:15,730 --> 00:04:17,563
or mostly oxygen rich.

89
00:04:18,779 --> 00:04:21,181
But the key thing is,
there's a few arteries

90
00:04:21,181 --> 00:04:24,605
in your body which are oxygen poor.

91
00:04:24,605 --> 00:04:29,096
So it's not always true to
say they're oxygen rich.

92
00:04:29,096 --> 00:04:32,115
Arterioles, well they're smaller arteries.

93
00:04:32,115 --> 00:04:34,855
The same basic structure, same basic idea,

94
00:04:34,855 --> 00:04:36,433
just they're smaller because they're

95
00:04:36,433 --> 00:04:38,766
further away from the heart.

96
00:04:39,622 --> 00:04:42,680
We have these precapillary sphincters.

97
00:04:42,680 --> 00:04:44,010
The idea here is they're gonna be

98
00:04:44,010 --> 00:04:47,152
at the end of the arterioles.

99
00:04:47,152 --> 00:04:50,735
Now a sphincter is just
a musculature ring.

100
00:04:52,455 --> 00:04:55,920
It's a ring of muscle
that can open and close

101
00:04:55,920 --> 00:04:58,647
to either allow more blood through,

102
00:04:58,647 --> 00:05:01,557
or less blood through, in this case.

103
00:05:01,557 --> 00:05:05,576
And precapillary means
before the capillaries.

104
00:05:05,576 --> 00:05:07,656
So it's helping to control the

105
00:05:07,656 --> 00:05:10,212
blood flow into the capillaries.

106
00:05:10,212 --> 00:05:12,731
We could have one of two things occur.

107
00:05:12,731 --> 00:05:17,227
We can have vasodilation,
or vasoconstriction.

108
00:05:17,227 --> 00:05:21,170
Well vaso refers to vessel, in this case,

109
00:05:21,170 --> 00:05:23,980
the arterioles, the capillaries.

110
00:05:23,980 --> 00:05:27,317
Dilation means opening up, you're dilating

111
00:05:27,317 --> 00:05:29,411
something, you're opening up

112
00:05:29,411 --> 00:05:33,996
while constriction means
closing down, getting smaller.

113
00:05:33,996 --> 00:05:37,873
So vasodilation, you
open the vessel larger,

114
00:05:37,873 --> 00:05:40,358
means more blood flow.

115
00:05:40,358 --> 00:05:44,866
Vasoconstriction, well you're
making the vessel smaller,

116
00:05:44,866 --> 00:05:48,789
you're gonna decrease or
cut down on blood flow.

117
00:05:48,789 --> 00:05:51,622
So vasodilation, vasoconstriction.

118
00:05:53,947 --> 00:05:55,445
In this image here you
can see how on the top

119
00:05:55,445 --> 00:05:56,695
it's arteriole.

120
00:05:58,105 --> 00:06:01,212
Those round discs, they're going around

121
00:06:01,212 --> 00:06:03,644
the opening to the capillaries.

122
00:06:03,644 --> 00:06:07,642
Those are your precapillary sphincters.

123
00:06:07,642 --> 00:06:10,272
If they're opened, they're vasodilated.

124
00:06:10,272 --> 00:06:11,464
You can see how blood is flowing

125
00:06:11,464 --> 00:06:13,622
through with the capillaries, no problem.

126
00:06:13,622 --> 00:06:16,649
Heading down to the venule.

127
00:06:16,649 --> 00:06:20,543
But if those precapillary
sphincters are closed

128
00:06:20,543 --> 00:06:24,067
or constricted, as the two on the right,

129
00:06:24,067 --> 00:06:25,067
you'll notice there's really

130
00:06:25,067 --> 00:06:27,947
not any blood flowing through.

131
00:06:27,947 --> 00:06:28,946
So this is how you can regulate

132
00:06:28,946 --> 00:06:32,935
how much blood is passing
through the capillaries.

133
00:06:32,935 --> 00:06:37,102
Either constrict or relax
the precapillary sphincters.

134
00:06:40,663 --> 00:06:43,178
Well if we open up the
precapillary sphincters,

135
00:06:43,178 --> 00:06:46,321
obviously we get blood
flowing into the capillaries.

136
00:06:46,321 --> 00:06:50,488
These capillaries are the
smallest of all blood vessels.

137
00:06:51,697 --> 00:06:54,681
One cell layer thick, very thin walled,

138
00:06:54,681 --> 00:06:56,396
they're porous.

139
00:06:56,396 --> 00:06:58,593
You can have different gases diffusing,

140
00:06:58,593 --> 00:07:00,479
different nutrients diffusing,

141
00:07:00,479 --> 00:07:03,204
different ions diffusing.

142
00:07:03,204 --> 00:07:04,491
There's a lot of different particles

143
00:07:04,491 --> 00:07:07,431
going back and forth between the blood

144
00:07:07,431 --> 00:07:11,598
in the capillaries and the
fluid around the capillaries.

145
00:07:14,883 --> 00:07:17,236
So typically we refer to
these as capillary beds

146
00:07:17,236 --> 00:07:18,291
and there's a whole network,

147
00:07:18,291 --> 00:07:22,995
a bunch of different
capillaries all woven together.

148
00:07:22,995 --> 00:07:25,249
The idea here is by having this whole

149
00:07:25,249 --> 00:07:29,314
network or web of
capillaries, you've increased

150
00:07:29,314 --> 00:07:31,447
the surface area.

151
00:07:31,447 --> 00:07:32,735
There's more tissue that can get

152
00:07:32,735 --> 00:07:36,071
nutrients now from these capillaries.

153
00:07:36,071 --> 00:07:38,622
The main rule, as I said, is selective

154
00:07:38,622 --> 00:07:42,019
exchange of substances, allowing certain

155
00:07:42,019 --> 00:07:44,574
small particles, gases, liquid,

156
00:07:44,574 --> 00:07:47,241
into and out of the capillaries.

157
00:07:48,985 --> 00:07:52,312
Can see the diagram
here, oxygen, nutrients,

158
00:07:52,312 --> 00:07:54,911
are heading into the tissues

159
00:07:54,911 --> 00:07:57,060
while the waste products, as well as

160
00:07:57,060 --> 00:07:59,995
the metabolic waste, carbon dioxide,

161
00:07:59,995 --> 00:08:01,828
is leaving the tissue.

162
00:08:04,498 --> 00:08:07,749
Let's start our return
trip back up to the heart.

163
00:08:07,749 --> 00:08:10,306
The veins, which are larger structures,

164
00:08:10,306 --> 00:08:13,109
and the venules, which are smaller,

165
00:08:13,109 --> 00:08:16,917
looking at a three layered,
thin-walled structure.

166
00:08:16,917 --> 00:08:21,388
So you still have the same
basic structure as an artery,

167
00:08:21,388 --> 00:08:24,281
but now it's much thinner.

168
00:08:24,281 --> 00:08:26,788
But even though the walls are thinner,

169
00:08:26,788 --> 00:08:29,680
the lumen is much larger.

170
00:08:29,680 --> 00:08:33,955
Lumen is the open space
the blood travels through.

171
00:08:33,955 --> 00:08:35,944
By having a larger lumen, there's

172
00:08:35,944 --> 00:08:39,033
less resistance, less pressure.

173
00:08:39,033 --> 00:08:42,969
That's why arteries are
the pressurized system.

174
00:08:42,969 --> 00:08:45,302
Veins are very low pressure.

175
00:08:46,416 --> 00:08:48,140
But these veins can also stretch.

176
00:08:48,140 --> 00:08:52,326
They also referred to
as high distensibility.

177
00:08:52,326 --> 00:08:54,368
High distensibility allows the veins

178
00:08:54,368 --> 00:08:57,451
to stretch to accommodate blood flow.

179
00:08:58,560 --> 00:08:59,786
So what are they doing?

180
00:08:59,786 --> 00:09:02,320
They're carrying blood towards the heart.

181
00:09:02,320 --> 00:09:03,520
That's the main rule.

182
00:09:03,520 --> 00:09:06,100
Carry blood back to the heart.

183
00:09:06,100 --> 00:09:09,433
But they also serve as a blood reservoir

184
00:09:11,328 --> 00:09:14,137
because veins will contain the

185
00:09:14,137 --> 00:09:17,053
majority of blood in your body.

186
00:09:17,053 --> 00:09:19,438
More often than not, they have over 60%

187
00:09:19,438 --> 00:09:23,229
of your body's blood within your veins.

188
00:09:23,229 --> 00:09:26,278
That means the heart's blood,

189
00:09:26,278 --> 00:09:28,695
it means the arteries' blood,

190
00:09:29,642 --> 00:09:32,913
it means the capillaries' blood.

191
00:09:32,913 --> 00:09:35,585
All three of those together

192
00:09:35,585 --> 00:09:39,752
do not have as much blood
in them as your veins would.

193
00:09:41,400 --> 00:09:43,623
So how do you get the
venous return to the heart?

194
00:09:43,623 --> 00:09:45,443
I mean you don't have a pump.

195
00:09:45,443 --> 00:09:46,849
The heart's the pump.

196
00:09:46,849 --> 00:09:50,572
Once it goes to the capillaries,
they're very low pressure.

197
00:09:50,572 --> 00:09:54,194
So your body has different
mechanisms to help assist

198
00:09:54,194 --> 00:09:56,981
in the return of blood to the heart.

199
00:09:56,981 --> 00:10:00,198
We have contraction of skeletal muscle,

200
00:10:00,198 --> 00:10:01,430
so for instance look at the picture here

201
00:10:01,430 --> 00:10:03,920
of the back leg, calf muscle.

202
00:10:03,920 --> 00:10:08,491
You have a major vein right
in the middle of that muscle.

203
00:10:08,491 --> 00:10:11,716
You can see other valves inside that vein.

204
00:10:11,716 --> 00:10:13,572
They're one way valves.

205
00:10:13,572 --> 00:10:15,872
Blood can go up toward the body,

206
00:10:15,872 --> 00:10:18,178
but cannot come back.

207
00:10:18,178 --> 00:10:20,210
So when your muscles contract,

208
00:10:20,210 --> 00:10:23,543
they shorten, then get wider or thicker.

209
00:10:24,524 --> 00:10:28,691
This widening or thickening
ends up pinching the vein.

210
00:10:30,304 --> 00:10:33,751
When you pinch a tube the
fluid has to go somewhere.

211
00:10:33,751 --> 00:10:37,473
Well in this case the
fluid cannot go back down.

212
00:10:37,473 --> 00:10:39,916
There's a valve preventing it.

213
00:10:39,916 --> 00:10:43,761
That means the fluid must
go up towards the body,

214
00:10:43,761 --> 00:10:45,341
going through that valve, which

215
00:10:45,341 --> 00:10:48,729
prevents it from coming back down.

216
00:10:48,729 --> 00:10:51,285
So that pinching right
there, pushes the blood

217
00:10:51,285 --> 00:10:53,035
up through the valve.

218
00:10:54,511 --> 00:10:58,045
The valve below that
area, or towards the heel,

219
00:10:58,045 --> 00:11:02,689
prevents blood from
traveling down to the foot.

220
00:11:02,689 --> 00:11:05,491
So these one-way valves
are pretty important.

221
00:11:05,491 --> 00:11:07,941
By using the skeletal muscle contraction,

222
00:11:07,941 --> 00:11:12,253
and the one way valves, it
helps move the blood up.

223
00:11:12,253 --> 00:11:13,437
Now maybe you heard if you're in a long

224
00:11:13,437 --> 00:11:15,293
car ride or on an airplane you're supposed

225
00:11:15,293 --> 00:11:17,805
to move your legs, get up and move around?

226
00:11:17,805 --> 00:11:19,414
This is why.

227
00:11:19,414 --> 00:11:22,254
By bouncing your legs or walking around,

228
00:11:22,254 --> 00:11:24,115
you're using these muscles

229
00:11:24,115 --> 00:11:28,458
to help bring the blood back
up from your lower extremities.

230
00:11:28,458 --> 00:11:30,417
Moving the blood up through your leg,

231
00:11:30,417 --> 00:11:33,834
back up towards your body and your heart.

232
00:11:34,841 --> 00:11:37,077
And then your lungs.

233
00:11:37,077 --> 00:11:38,985
Whenever you take a breath in,

234
00:11:38,985 --> 00:11:41,712
shallow breath, deep
breath, doesn't matter.

235
00:11:41,712 --> 00:11:44,030
Whenever you breathe in, you're changing

236
00:11:44,030 --> 00:11:47,491
the pressure in your thoracic cavity.

237
00:11:47,491 --> 00:11:50,566
So pretty much your whole
chest is a sealed cavity.

238
00:11:50,566 --> 00:11:53,778
When you breathe, you
change that pressure.

239
00:11:53,778 --> 00:11:57,571
That pressure change allows the blood

240
00:11:57,571 --> 00:11:59,860
to kind of sucked up or pulled up

241
00:11:59,860 --> 00:12:03,303
a little bit whenever it changes.

242
00:12:03,303 --> 00:12:05,319
Kind of think of a drinking straw.

243
00:12:05,319 --> 00:12:07,174
How did they get the liquid up the straw?

244
00:12:07,174 --> 00:12:08,752
It's a pressure change.

245
00:12:08,752 --> 00:12:12,595
You create a lower pressure
by sucking on the straw,

246
00:12:12,595 --> 00:12:14,687
pulling the liquid up.

247
00:12:14,687 --> 00:12:16,343
So there are three different ways

248
00:12:16,343 --> 00:12:19,598
that your body helps return
blood back to the heart.