Your body absorbs oxygen and gives off carbon dioxide as air moves in and out of your lungs. The structures that move air in and out of your lungs are the muscles of your ribcage and your diaphragm, a sheet of muscle that sits beneath your lungs and above your abdomen. Air is carried through your lungs from your mouth all the way down to small structures that are like hollow sacs. Each sac contains a mesh of blood vessels where oxygen can enter the bloodstream. When oxygen levels are too low, the brain sends signals to the muscles that control your breathing so that they will work harder. This means that people with breathing difficulties have to work harder to get enough oxygen. If the lungs are stiff and not flexible, the diaphragm also has to work harder. In addition to the muscles that directly control breathing, people with breathing problems often use other muscles to breathe, including the muscles of the neck and shoulders. All this effort can make breathing very tiring.