1.3 Mechanics of inhalation and expiration

Movement of the diaphragm and intercostal muscles acts to expand and decrease the size of the thoracic cavity, creating pressure gradients that draw air into and force air out of the lungs, as described in Video 4.

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Transcript: Video 4 Mechanics of inhalation and expiration.

NARRATOR

During inhalation, the diaphragm contracts, causing it to flatten out and move downwards. At the same time, the ribs are moved upwards and outwards by contraction of the external intercostal muscles. Contraction of the diaphragm and the external intercostal muscles increases the volume within the ribcage so that there is a larger area for the lungs to expand into. This creates a pressure gradient within the thoracic cavity that draws air into the lungs.

End transcript: Video 4 Mechanics of inhalation and expiration.

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Video 4 Mechanics of inhalation and expiration.

Expiration is generally a passive event brought about by relaxation of the diaphragm and external intercostal muscles. The ribcage, diaphragm and lung tissue itself return by elastic recoil to their original pre-inspiratory positions. The consequent retraction of the chest wall forces air out of the lungs. Forced expiration is mainly achieved by contraction of the internal intercostal muscles, aided to some extent by contraction of the abdominal muscles.

Most of the time, you will be unaware of the contraction and relaxation of the muscles that control respiration. They become much more noticeable when you cough or develop a bout of hiccups. In fact, hiccups are caused by a spasm of the diaphragm and intercostal muscles in response to increased activity of the phrenic nerve and vagus nerve (which innervates the muscles of the abdomen). The spasms cause the floor of the thoracic cavity to drop suddenly, which pulls air quickly and forcefully into the airways. Movement of the air past the closed vocal cords creates the characteristic ‘hic’ sound.

Activity 3 Lung in a bottle

Timing: Allow about 1 hour

You can explore the relationship between movement of the diaphragm and lung volume directly by making your own ‘lung in a bottle’ as shown in Video 5.

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Transcript: Video 5 ‘Lung in a bottle’ experiment.

CHERYL HAWKES

Today I’m going to show you how you can make your own lung in a bottle to better understand how changes in pressure drive air into and out of the lungs. For this activity, you will need a clear plastic bottle, about 500 millilitres, two balloons, some Blu-Tack with a hole in the middle, two small straws, or one large straw, and one pair of scissors. The first thing to do is to cut your bottle in half with the scissors, which can be a little bit fiddly, so I’ve made one already. And we’re going to use the top of it to make our model.

And next, take one of your balloons and cut the top of it off. I’m going to take this part of the balloon and wrap it around the base of the bottle. Next, we’re going to take the second balloon and insert it into the bottle.

And then wrap the lip of the balloon around the top of the bottle. Then, you take the two straws and cut them about a quarter of the way down. Next, we’ll insert the straws into the hole in the Blu-Tack, and put both of these into the mouth of the balloon. And then squeeze the Blu-Tack down over the straws and the lip of the bottle just to make sure that you seal off the bottle completely.

So here we have the model of the lung in the bottle where the straws represent the trachea, the balloon inside represents the lungs, the plastic represents the ribs and intercostal muscles, and the balloon at the bottom represents the diaphragm. Watch what happens when I pull up and down on the diaphragm-- you see that the balloon lung inside inflates as I pull down on the diaphragm. And that’s because when you do that the air pressure inside the lung is lower than the air pressure outside. And so the air flows down into the lungs.

When you press the diaphragm inwards, the balloon collapses, because the pressure inside the ribs is higher than the pressure outside. And this drives the air out of the lungs. So there you have it, your very own model of a lung in a bottle.

End transcript: Video 5 ‘Lung in a bottle’ experiment.

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Video 5 ‘Lung in a bottle’ experiment.

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According to the video, what factors are responsible for the inflation and deflation of the balloon ‘lung’ inside the bottle?

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Answer

Pulling down on the blue balloon ‘diaphragm’ caused the air pressure in the ribcage to drop lower than the air pressure in the atmosphere. This drove air to flow down its pressure gradient into the balloon, causing it to inflate. Pressing up on the diaphragm increased the air pressure in the ribcage, driving air out of the balloon lung and causing it to collapse.