The Heart, Part 1 - Under Pressure: Crash Course Anatomy & Physiology #25 — Transcript

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1. 0:00Your heart, that throbbing, beating muscle, is probably the most iconic organ in your body.
2. 0:04No other organ gets its own holiday, or as much radio play. And you’re not likely to
3. 0:09get a love note decorated with a kidney or a spleen, or even a brain, which is really what rules the emotions.
4. 0:14Don’t get me wrong, the heart does some great things -- namely, it powers the entire
5. 0:18circulatory system, transporting nutrients, oxygen, waste, heat, hormones, and immune
6. 0:22cells throughout the body, over and over.
7. 0:24But in the end, the heart does not make you love. It doesn’t break apart if you get
8. 0:28dumped by your boo. And it’s not a lonely hunter.
9. 0:31The truth is, the heart is really just a pump -- a big, wet, muscley brute of a pump.
10. 0:37And it doesn’t care about poetry or chocolate, or why you’re crying.
11. 0:40The heart only has one concern: maintaining pressure.
12. 0:44If you’ve ever squeezed the trigger on a squirt gun or opened up a shaken can of soda,
13. 0:47you’ve seen how fluids flow from areas of high pressure -- like inside the gun or the
14. 0:52can -- to areas of low pressure, like outside.
15. 0:55The heart’s entire purpose is to maintain that same kind of pressure gradient, by generating high
16. 0:59hydrostatic pressure to pump blood out of the heart, while also creating low pressure to bring it back in.
17. 1:05That gradient of force is what we mean when we talk about blood pressure.
18. 1:08It’s basically a measure of the amount of strain your arteries feel as your heart moves
19. 1:11your blood around -- more than five liters of it -- at about 60 beats per minute.
20. 1:15That’s about 100,000 beats a day, 35 million a year, 2 to 3 billion heart beats in a lifetime,
21. 1:20the basic physiology of which you can easily feel, just by taking your own pulse.
22. 1:25I don’t have a watch
23. 1:26Now, that might not inspire a lot of poetry, but it turns out, it’s still a a pretty good story.
24. 1:42Let us begin with a little anatomy.
25. 1:44Unless you happen to be of the Grinch persuasion, the average adult human heart is about the
26. 1:47size of two fists clasped together -- one of the few bits of trivia you often hear about
27. 1:51human anatomy that is actually true.
28. 1:53The heart is hollow, vaguely cone-shaped, and only weighs about 250 to 350 grams -- a
29. 1:58pretty modest size for your body’s greatest workhorse.
30. 2:01And although Americans tend to put their right hand over their left breast while pledging
31. 2:04allegiance, the heart is actually situated pretty much in the center of your chest, snuggled
32. 2:09in the mediastinum cavity between your lungs.
33. 2:12It sits at an angle, though, with one end pointing inferiorly toward the left hip, and
34. 2:15the other toward the right shoulder. So most of its mass rests just a little bit left of the midsternal line.
35. 2:21The heart is nestled in a double-walled sac called the pericardium.
36. 2:24The tough outer layer, or fibrous pericardium, is made of dense connective tissue and helps
37. 2:28protect the heart while anchoring it to some of the surrounding structures, so it doesn’t
38. 2:32like bounce all over the place while beating.
39. 2:34Meanwhile, the inner serous pericardium consists of an inner visceral layer, or epicardium
40. 2:38-- which is actually part of the heart wall -- and an outer parietal layer.
41. 2:42These two layers are separated by a thick film of fluid that acts like a natural lubricant,
42. 2:46providing a slippery environment for the heart to move around in so it doesn’t create friction as it beats.
43. 2:50The wall of the heart itself is made of yet more layers, three of them: that epicardium
44. 2:55on the outside; the myocardium in the middle, which is mainly composed of cardiac muscle
45. 2:59tissue that does all the work of contracting; and the innermost endocardium, a thin white
46. 3:04layer of squamous epithelial tissue.
47. 3:06Deeper inside, the heart has a whole lot of moving pieces that I’m not going to pick
48. 3:09apart here, because the really big thing to understand is how the general system of chambers,
49. 3:13and valves, veins, and arteries all work together to circulate blood around your body.
50. 3:18Of course fluid likes to move from areas of high pressure to areas of low pressure, and
51. 3:21the heart creates those pressures.
52. 3:23Form once again following function.
53. 3:25Your heart is divided laterally into two sides by a thin inner partition called the septum.
54. 3:30This division creates four chambers -- two superior atria, which are the low pressure
55. 3:34areas, and two inferior ventricles that produce the high pressures.
56. 3:38Each chamber has a corresponding valve, which acts like -- like a bouncer at a club at closing
57. 3:42time -- like he’ll let you out, but not back in.
58. 3:44When a valve opens, blood flows in one direction into the next chamber. And when it closes,
59. 3:49that’s it -- no blood can just flow back into the chamber it just left.
60. 3:52So if you put your ear against someone’s chest -- and yeah, ask for permission first
61. 3:56-- you’ll hear a “lub-DUB, lub-DUB”.
62. 3:58What you’re really hearing there are the person’s heart valves opening and closing.
63. 4:02It’s a relatively simple, but quite elegant set up, really.
64. 4:05Functionally, those atria are the receiving chambers for the blood coming back to the
65. 4:09heart after circulating through the body.
66. 4:11The ventricles, meanwhile, are the discharging chambers that push the blood back out of the heart.
67. 4:16As a result, the atria are pretty thin-walled, because the blood flows back into the heart under
68. 4:20low pressure, and all those atria have to do is push it down into the relaxed ventricles,
69. 4:24which doesn’t take a whole lot of effort.
70. 4:25The ventricles are beastly by comparison. They’re the true pumps of the heart, and
71. 4:30they need big strong walls to shoot blood back out of the heart with every contraction.
72. 4:35And the whole thing is connected to the rest of your circulatory system by way of arteries
73. 4:39and veins. We’ll go into a whole lot more detail about these later, but the thing to
74. 4:43remember first, if you don’t already remember it, is that arteries carry blood away from
75. 4:47the heart, and veins carry it back toward the heart.
76. 4:50To differentiate the two, anatomy diagrams typically depict arteries in red, while veins
77. 4:54are drawn in blue, which, incidentally, is part of what has led to the common misconception
78. 4:58that your blood is actually blue at some point.
79. 5:00But, it isn’t. It is always red. It’s just a brighter red when there’s oxygen in it.
80. 5:04So let’s look at how this all comes together, starting with a big burst of blood flowing out of your heart.
81. 5:09The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary
82. 5:13trunk, which is just a big vessel that splits to form the left and right pulmonary arteries.
83. 5:17From there -- and this is the only time in your body where deoxygenated blood goes through
84. 5:21an artery -- the blood goes straight through the pulmonary artery into the lungs, where
85. 5:25it can pick up oxygen.
86. 5:26It finds its way into very small, thin-walled capillaries, which allow materials to move
87. 5:30in and out of the blood stream. In the case of the lungs, oxygen moves in, and carbon dioxide moves out.
88. 5:36The blood then circles back to the heart by way of four pulmonary veins, where it keeps
89. 5:39moving to the area of lowest pressure -- because that is what fluids do -- and in this case
90. 5:44that’s inside the relaxed left atrium.
91. 5:46Then the atrium contracts, which increases the pressure, so the blood passes down through
92. 5:50the mitral valve into the left ventricle.
93. 5:52So the thing that just happened here, where a wave of blood was pumped from the right
94. 5:56ventricle to the lungs and then followed the lowest pressure back to the left atrium?
95. 6:00There is a name for that, it is the pulmonary circulation loop.
96. 6:03It’s how your blood unloads its burden of carbon dioxide into the lungs, and trades
97. 6:07it in for a batch of fresh oxygen. It’s short, it’s simple -- at least in the way
98. 6:11I have time to describe it -- and it’s just delightfully effective.
99. 6:13Of all of the substances you need to continue existing, oxygen is the most urgent -- the
100. 6:18one without which you will die in minutes instead of hours, or days, or weeks.
101. 6:22But it’s pretty useless unless the oxygen can actually reach your cells. And that hasn’t happened yet.
102. 6:27For that, your newly oxygenated blood needs to travel through the rest of your
103. 6:30organ systems and share the wealth.
104. 6:32And that fantastic journey -- known as the systemic loop -- begins in the left ventricle,
105. 6:37when it flexes to increase pressure. Now the blood would like to flow into the nice low
106. 6:40pressure left atrium where it just came from, but the mitral valve slams shut, forcing it
107. 6:45through the aortic semilunar valve into your body’s largest artery -- nearly as big around
108. 6:50as a garden hose -- the aorta, which sends it to the rest of your body.
109. 6:54And after all your various greedy muscles, and neurons, and organs, and the heart itself have had
110. 6:58their oxygen feast at the capillary-bed buffet, that now-oxygen-poor blood loops back to the
111. 7:04heart, entering through the big superior and inferior vena cava veins, straight into the right atrium.
112. 7:09And when the right atrium contracts, the blood passes through the tricuspid valve, into the
113. 7:13relaxed right ventricle, and right back to where we started.
114. 7:16This whole double-loop cycle plays out like a giant figure eight -- heart to lung to heart
115. 7:20to body to heart again -- and runs off that constant high-pressure, low-pressure gradient
116. 7:25exchange regulated by the heart valves.
117. 7:27So the first “lub” that you hear in that lub-DUB is made by the mitral and tricuspid
118. 7:31valves closing. And they do that because your ventricles contract to build up pressure and
119. 7:35pump blood out of the heart. This high pressure caused by ventricular contraction is called systole.
120. 7:40Now, the “DUB” sound -- and, just to be clear, I am not talking about dubstep sounds
121. 7:45-- that’s the aortic and pulmonary semilunar valves closing at the start of diastole. That’s
122. 7:50when the ventricles relax, to receive the next volume of blood from the atria.
123. 7:53When those valves close, the high-pressure blood that’s leaving the heart tries to
124. 7:56rush back in, but runs into the valves.
125. 7:58So you know when you get your blood pressure measured, and the nurse gives you two numbers,
126. 8:01like, 120 over 80?
127. 8:03The first number is your systolic blood pressure -- essentially the peak pressure, produced
128. 8:06by the contracting ventricles that push blood out to all of your tissues.
129. 8:10The second reading is your diastolic blood pressure, which is the pressure in your arteries
130. 8:14when the ventricles are relaxed.
131. 8:16These two numbers give your nurse a sense of how your arteries and ventricles are doing,
132. 8:19when they’re experiencing both high pressure -- the systolic -- and low pressure -- the diastolic.
133. 8:24So if your systolic blood pressure is too low, that could mean that, say, the volume
134. 8:27of your blood is too low -- like, maybe because you’ve lost a lot of blood, or you’re dehydrated.
135. 8:31And if your diastolic is too high, that could mean that your blood pressure is high,
136. 8:34even when it’s supposed to be lower.
137. 8:36Considering how much we’ve talked about the importance of homeostasis, it should come
138. 8:38as no surprise that blood pressure that’s too high or too low, or anything that affects
139. 8:42your blood’s ability to move oxygen around can be dangerous.
140. 8:45Prolonged high blood pressure can damage arterial walls, mess with your circulation and ultimately
141. 8:50endanger your heart, your lungs, brain, kidneys, and nearly every part of you.
142. 8:54So I guess you could say the best way to break a heart is to mess with its pressure.
143. 8:57But good luck trying to write a song about that.
144. 8:59Today you learned how the heart’s ventricles, atria, and valves create a pump that maintains
145. 9:03both high and low pressure to circulate blood from the heart to the body through your arteries,
146. 9:08and bring it back to the heart through your veins. We also talked about what systolic
147. 9:12and diastolic blood pressure are, and how they’re measured.
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152. 9:28Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode
153. 9:32was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon
154. 9:36Jackson. It was directed by Nicholas Jenkins; the script supervisor and editor is Nicole
155. 9:40Sweeney; our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.