My Science School

Can I breathe out all the air in my lungs?

Normally you only breathe out about 10 percent of the air in your lungs. If you are running and panting very hard you may use up to 60 percent of the air, but at least 20 percent will always be permanently trapped in the alveoli. You breathe faster and deeper when you exercise in order to get more oxygen into your body. This helps to break down sugar and provide energy for your muscles to work.

How is air passed in and out of the lungs?

Most of the airflow in the lungs is caused by the contraction of a sheet of muscle called the diaphragm. It is a curved muscle sheet that separates the contents of the chest from the abdomen. As the diaphragm contracts and flattens, the volume of the chest is increased. This causes the pressure inside the lungs to drop, so air rushes in. When the diaphragm relaxes it becomes curved again and forces air out of the lungs. The chest also helps in breathing when you take vigorous exercise, and the ribs are moved in and out by muscles that run between them.

How long can you hold your breath?

Most healthy people can hold their breath for about 90 seconds, but it soon becomes very uncomfortable. You normally breathe automatically, and never have to think about it. This is because there is an area in the brain that controls breathing. Normally the brain detects the build-up of waste carbon dioxide if you exercise. It speeds up breathing to get rid of it quickly by flushing it out of the lungs. If you try to hold your breath you can override this mechanism for a while. However, the brain will not let carbon dioxide builds up too much and will soon force you to breathe.

How can people breathe underwater?

            You can breathe through a snorkel if you are swimming on the surface. It would not be possible to suck air down a snorkel if you were 1 m under water. The pressure of the water would prevent your lungs from expanding enough to draw the air in. divers use compressed air to breathe. It is fed to them by a valve that automatically provides air at exactly the same pressure as the surrounding water, no matter how deep they dive, so they can breathe easily.

            Some divers who need to work at great depths have to breathe a special mixture of gases, because nitrogen and oxygen become poisonous at very high pressure. The mixture of gases is adjusted automatically to provide the correct amounts.