Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 14 - Think and Explain: 65

Answer

We do not "suck" air into our lungs, but rely on creating a lower pressure region there so that atmospheric pressure pushes air in. (Your favorite search engine can refer you to how the diaphragm works to expand the lungs.) When you are submerged, your body is compressed, along with your lungs and the air within them. This makes it difficult to expand the lungs to create that low-pressure region, and for humans, the difficulty begins at about 1m of depth.
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