Ventilation: How Breathing Works

0:22The chest cross-section

Your lungs sit inside a space called the thoracic cavity. The walls of that space are the rib cage at the sides, and the diaphragm, a sheet of muscle, across the bottom. The lungs have no muscle of their own. They cannot pull air in by themselves. So the muscles change the volume of the cavity, and that changes the pressure.

Volume and pressure. Everything in this lesson comes down to those two words.

0:49Volume up, pressure down

Here is the rule. If the volume of a space goes up, the pressure inside it goes down. If the volume goes down, the pressure goes up. And air always moves from a region of higher pressure to a region of lower pressure. That is a pressure gradient, and it is the only thing that moves air.

1:09Inspiration

Inspiration, breathing in, is an active process. The external intercostal muscles, between the ribs, contract. That pulls the rib cage upwards and outwards. At the same time the diaphragm contracts and flattens, moving down. The lungs follow the walls of the cavity as it expands, so they expand too.

Contract is the word. The diaphragm contracts when you breathe in, it does not relax. Both movements increase the volume of the thoracic cavity. So the pressure in the lungs falls below atmospheric pressure. Now there is a pressure gradient. Air moves down it, from outside into the lungs.

1:51Normal expiration

Now expiration, breathing out, when you are resting. The external intercostal muscles relax, and the ribs fall downwards and inwards. The diaphragm relaxes and returns to its dome shape. The volume of the thoracic cavity decreases, so the pressure in the lungs rises above atmospheric pressure.

Air moves down the gradient, out of the lungs. Normal expiration is passive. No muscle has to contract to do it.

2:19The intercostal muscles

The two sets of intercostal muscles work as an antagonistic pair. That means when one contracts, the other relaxes, and they produce opposite movements of the ribs. The internal intercostal muscles contract during forced expiration, such as blowing out hard or exercise. They pull the ribs down and in, faster and further than gravity alone.

The abdominal muscles also contract, pushing the diaphragm up. So at rest, only breathing in uses muscle. During hard breathing, breathing out does too.

2:54Breathing in and out compared

Let's put the two side by side, because examiners love a comparison. Breathing in. Ribs up and out, diaphragm flat, volume up, pressure down, air moves in. Breathing out at rest. Ribs down and in, diaphragm domed, volume down, pressure up, air moves out. Notice the pressure is never just high or low. It is always higher or lower than atmospheric pressure, the pressure of the air outside.

Air only flows when there is a difference to flow along. When the two pressures are equal, the air stops moving. So in your answer, always say compared with what.

4:08Examiner trap and recap

The trap. Never say air is sucked in, and never say the diaphragm relaxes during inspiration. Always finish the chain: muscles, volume, pressure, then air moving down the gradient. Recap. Breathing in: muscles contract, volume rises, pressure falls, air flows in. Next time, we see what happens to gas exchange and ventilation when lungs are diseased.