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PRESENTER: Hello, and[br]welcome to Byte Size Med.

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This video is on[br]the breathing cycle.

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The breathing cycle[br]involves air going

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into the lungs during[br]inspiration and air

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leaving the lungs[br]during expiration.

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During this cycle, there are[br]pressure and volume changes.

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In this video, we're going to[br]put pressure and volume together

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and see what happens during[br]one cycle of respiration.

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There are three phases.

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There's rest where there's[br]no airflow, inspiration

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where air enters, and[br]expiration where air leaves.

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Now, we're going to[br]use this schematic lung

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to try and understand it.

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The lungs are surrounded[br]by pleural cavities

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lined by pleura.

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There's an inner[br]visceral layer, which

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is sort of attached[br]to the lungs,

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and the outer parietal layer,[br]which is towards the chest wall.

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Now, the pleural cavity[br]is filled with fluid.

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This fluid acts like a lubricant[br]and helps the lungs move

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during respiration.

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The pressure in[br]the pleural space,

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that's the intrapleural[br]pressure, or just simply

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pleural pressure.

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Now, the air is going to[br]enter through the airways

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into the alveoli.

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The pressure in the alveoli[br]is the alveolar pressure.

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So we've got the pleural[br]pressure and the alveolar

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pressure.

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Now, the difference between[br]these two pressures--

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that is the pressure[br]across the organ.

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That's the transmural pressure.

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Since we're talking[br]about the lungs,

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it's the transpulmonary[br]pressure.

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The pressures are in[br]centimeters of water

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and are in relation to[br]atmospheric pressure.

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To understand it, we consider[br]atmospheric pressure to be 0.

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So that's our[br]reference pressure.

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Air moves along a pressure[br]gradient from high pressure

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to low pressure.

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So that's what drives[br]the air to move

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between the lungs[br]and the atmosphere.

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First, let's look[br]at the volumes.

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There are four lung volumes--

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the inspiratory reserve[br]volume, the tidal volume,

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the expiratory reserve volume,[br]and the residual volume.

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The air that enters or[br]leaves the lungs just

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while quietly breathing[br]in and breathing out,

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that's the tidal volume, which[br]is around 500 milliliters.

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So what's left behind after[br]the tidal volume leaves?

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The expiratory reserve volume[br]and the residual volume,

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which together form the[br]functional residual capacity,

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the FRC.

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So that is the air[br]that gets left behind

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after quietly[br]breathing out, and it's

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the air that's in the[br]lungs in a state of rest.

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So at rest, the[br]volume in the lungs

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is at functional[br]residual capacity.

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During inspiration, 500 mL[br]of air enters the lungs,

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and during expiration, that[br]500 mL leaves the lungs.

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For this to happen,[br]pressures have to change.

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And now we're going to[br]add in the pressures.

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So let's start at rest.

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The volume is at FRC,[br]like I said before.

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The chest wall, it[br]has a natural tendency

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to want to pull outwards,[br]and the lungs have a tendency

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to want to collapse inwards.

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At this point, these two forces,[br]they balance each other out.

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The pressure in the[br]pleural space at rest

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is slightly negative at[br]minus 5 centimeters of water.

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That keeps the lungs open.

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The pressure in[br]the alveoli is 0,

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equal to that of the atmosphere.

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Remember, we consider[br]the atmosphere to be 0,

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and that's our reference.

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So now there's no gradient.

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There's no airflow.

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And the system is[br]at equilibrium.

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So this is the[br]situation at rest.

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Now, when inspiration[br]begins, the diaphragm

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is going to contract.

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The lungs expand.

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And so the alveolar pressure[br]becomes slightly negative.

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It's going to go down to[br]minus 1 centimeters of water.

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So now there's a gradient[br]between the atmosphere

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and the alveoli.

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And the alveolar pressure is[br]lower than the atmosphere.

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So the air is going[br]to enter the lungs.

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The fact that the chest[br]wall is expanding,

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that makes the pleural[br]pressure more negative.

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So it goes down[br]from it's negative 5

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to minus 7.5[br]centimeters of water.

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At the end of inspiration,[br]the alveolar pressure

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goes back up to 0.

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So now 500 mL of air[br]has entered the lungs,

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and the volume in[br]the lungs would now

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include both the functional[br]residual capacity

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and the tidal volume.

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Now, unlike[br]inspiration, which was

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active from muscles contracting,[br]expiration is passive.

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It's from elastic recoil.

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So the alveolar[br]pressure now becomes

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slightly positive at plus[br]1 centimeters of water.

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Now you can see that the[br]gradient has reversed.

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So air is going to move[br]in the opposite direction.

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From high to low pressure,[br]it moves from the lungs

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to the atmosphere.

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So it leaves the[br]lungs, and what's

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left behind is now the[br]functional residual capacity

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again.

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And at the end of expiration,[br]the pleural pressure

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comes back up to minus[br]5 centimeters of water.

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So now we're at rest again.

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So we've completed[br]one breathing cycle.

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And the cycle is[br]going to repeat again.

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So this is rest, the[br]phase of inspiration,

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and the phase of expiration.

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What happened to the volume?

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The volume of air that[br]entered, or the volume change,

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was from 0 to 500 milliliters.

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And then that 500 mL left[br]and it came back to 0.

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We're talking about[br]a volume change.

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The actual volume at rest was[br]the functional residual capacity

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and not 0, but the change that[br]happened, that was by 500 mL.

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For that 500 mL to[br]enter, what happened

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to the alveolar pressure?

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It went from 0 at[br]rest down to minus 1,

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came back to 0 at the[br]end of inspiration.

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Then for the 500 mL to[br]leave, it went to plus 1

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and then came back to 0 again.

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The pleural pressure went from[br]minus 5 at rest to minus 7.5

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at the end of inspiration and[br]then came back up to minus 5

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again.

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But there's one more pressure,[br]the transpulmonary pressure.

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It's the alveolar pressure[br]minus the pleural pressure.

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So if we take rest,[br]end of inspiration,

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and end of expiration,[br]at rest you

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can see it's 0 minus of minus 5.

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That's plus 5 centimeters[br]of water at rest.

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By the end of inspiration,[br]it's plus 7.5.

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And then it comes back to plus 5[br]again at the end of expiration.

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So throughout the breathing[br]cycle, it's positive.

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As long as the transpulmonary[br]pressure remains positive,

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the airways stay open.

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When the transpulmonary[br]pressure becomes negative,

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airways collapse.

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And that's what happens during[br]one cycle of respiration.

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If this video helped[br]you, give it a thumbs up,

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and subscribe to my channel.

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Thanks for watching, and[br]I'll see you in the next one.