We take breathing for granted. In, out. It happens so naturally and without any thought that we hardly notice our normal breathing patterns. In fact, most of us breath very shallowly because we don't pay attention to our breath. This leads to both physical and psychological stress. The reality is we need to learn to breath properly.
The diaphragm, the muscle that surrounds the base of the ribcage, is designed to move the lungs up and down for respiration. It is the only muscle that originates on itself and attaches on the inside of the ribcage. Since the lungs can't move on their own, the diaphragm is necessary to move them.
However, many people chest breath and don't use the diaphragm. Chest breathing creates feelings of anxiety, doesn't fully expand the lungs and doesn't allow for ample oxygen to flow in. In chest breathing the diaphragm is used as a lid to hold down emotions. Notice the next time you are stressed where your breathing from and how tight your diaphragm feels. Simply moving the breath down to the diaphragm can create a calming sensation.
In a diaphragmatic breath, the base of the ribcage expands out and the lower ribs flare to the sides. The chest and the lower abdomen remain still. In the three part breath, taught in most yoga classes, this is also true with the exception that the top of the chest lifts. Because students are often unfamiliar with diaphragmatic breathing, many teachers, including myself, will say "let you stomach expand out when you breath in." This is also referred to as belly breathing. It's actually inaccurate. It's the diaphragm that should expand not the stomach. However, to keep things simple we say stomach.
Sometimes when we become aware that we need to relax we try taking a deep breath. This is a good idea in theory but most likely we inhale too deeply and forget to exhale fully and slowly. The breath comes out more like a sigh. Deep inhalations are stimulating. It's the exhalation that calms the mind and the body. The slower the exhalation the more the body move away from the flight or fight response.
The breath should also flow in and out through the nose. Mouth breathing can create panic, feelings of heaviness in the body and dehydration. Some forms of exercise do require exhaling from the mouth and that's fine, but the inhalation should come through the nose. The nose is designed to filter the air we breathe, not to mention to avoid swallowing things like bugs and germs!
The breath is a very powerful tool for changing both our emotional and physical response to stress and life events. It also has very profound spiritual implications too. I love that the spirit of God is sometimes referred to the breath of God. When God breathes on people they come to life both physically and spiritually as in the case of Adam. When we recognize that the breath is God-given, breathing becomes not only a physical act but a spiritual one as well.