They mainly occur during inhalation but can also happen during exhalation. Crackles occur as a result of small airways suddenly snapping open. They may indicate that a person’s lungs have fluid.

The word or phrase respiration refers to the metabolic processes whereby certain organisms obtain energy from organic molecules; processes that take place in the cells and tissues during which energy is released and carbon dioxide is produced and absorbed by the blood to be transported to the lungs, or the bodily process of inhalation and exhala.

Active inspiration involves the contraction of the accessory muscles of breathing (in addition to those of quiet inspiration, the diaphragm and external intercostals). All of these muscles act to increase the volume of the thoracic cavity: Scalenes – elevates the upper ribs. Sternocleidomastoid – elevates the sternum.