The respiratory system is responsible for allowing the exchange of oxygen and carbon dioxide to occur within the body. First, oxygen-rich air enters the nostrils or the mouth; then it passes through the pharynx and then the larynx (also known as the voicebox). After that, in order, the air enters the trachea, bronchi, bronchioles, and then the alveoli, the final branchings of the lungs where the gas exchanges actually occur. As one can see, the process the human body takes to get air is not as simple as one would think.
Many muscles are involved in breathing: the lungs, diaphragm, abdominals, and intercostal muscles (inner and outer rib cage muscles). A good way to maintain respiratory health is to take good care of these muscles. Having a healthy posture gives the lungs enough room to work, and strengthening the diaphragm through exercise helps make its job easier. Working out all four sections of the abdominals (transverse, rectus, external oblique, and internal oblique) will also contribute to a better overall health.
In addition to exercise, some people recommend herbal remedies (i.e. certain teas and supplements) to maintain healthy lungs, improve air passages, and prevent respiratory infections like emphysema and pneumonia. Sometimes it is taken for granted that breathing comes so easy, but it really is such a blessing to be able to do so.