We breathe a lot—roughly 10 times a minute! Have you ever wondered how the process of breathing works so smoothly? Our lungs allow us to inhale the oxygen our body needs, but they do much, much more. They also allow us to get rid of carbon dioxide, the waste product created in the body, and they play a vital role in singing, shouting and even giggling. In this activity you will make a model of a lung and use it to discover how air flows in and out of the lungs with ease.
All cells in our body need oxygen to create energy efficiently. When the cells create energy, however, they make carbon dioxide. We get oxygen by breathing in fresh air, and we remove carbon dioxide from the body by breathing out stale air. But how does the breathing mechanism work?
Air flows in via our mouth or nose. The air then follows the windpipe, which splits first into two bronchi: one for each lung. The bronchi then split into smaller and smaller tubes that have tiny air sacs at their end called alveoli. We have millions of alveoli in our lungs! These sacs have thin walls—so thin that oxygen and carbon dioxide can pass through them and enter or leave our blood. The blood transports oxygen to almost every part of the body. The blood also gives the carbon dioxide a ride back to the lungs.
Lungs take up most of the space in the chest. The 12 pairs of ribs in our ribcage protect the lungs and other organs in our chest cavity, such as our heart.
Relaxed breathing is a reflex; we do not have to think to breathe. During this unforced inhalation our diaphragm—the dome-shaped muscle between the chest and the abdominal cavity—flattens. This expands the chest cavity and as a result air is drawn in. During exhalation the diaphragm relaxes and the lungs naturally recoil, and air is gently pushed out.
We can also breathe more forcefully. When we exercise, sing loudly or otherwise need or want more air or oxygen we can exert force to breathe more deeply. We use various muscles to increase chest volume more dramatically. In the same way as in relaxed breathing the expansion of the chest cavity draws air in so the lungs fill up. The relaxation of the chest cavity pushes air out. Muscles can also force the chest cavity to contract even further, pushing even more air out. Because the expansions and contractions are larger in this case a bigger volume of air flows in and out of our lungs, and our body gets a larger supply of oxygen or we have more air to create sound.
When you pulled the knot back, the space inside the bottle increased and your balloon probably filled up with air. In the same way, when the diaphragm in our body pulls back, the chest cavity increases and air flows into our lungs, and we inhale.
When you pushed the knot in, the space inside the bottle decreased, and the balloon probably deflated. In the same way, when the diaphragm relaxes the chest cavity decreases, and air is pushed out of the lungs, and we exhale.
When you pulled and pushed the knot further the balloon inflated and deflated more. This mirrors what happens when a bigger volume of air is displaced when we breathe more deeply.
This dynamic works because of air pressure, a measure of how hard air presses against objects. Air pressure increases when you decrease the amount of space the air has—and decreases when you give air more space. Close a flimsy empty plastic bottle and try to compress it. It is difficult! The air inside pushes back. Open the bottle and try to compress the bottle again. It is much easier. The air presses back with a much reduced force. Unless something blocks the movement, air will move from areas of high pressure to areas where the pressure is lower, and this is what happens when air rushes in or out of the lungs. When the chest cavity expands there is more space around your lungs. In this condition the lungs can expand, making it a low-pressure area, and air rushes in to balance out the difference in pressure. Then to breathe out the chest cavity and lungs shrink. This raises the air pressure in your lungs, and the air rushes back out.
This experiment is from Scientific American’s “Bring Science Home” series and features 3M scientists and partners. Reproduced with permission of Scientific American, “How Do We Breathe” by Science Buddies and Sabine De Branbandere, ©2019. For more activities please go to: Bring Science Home.