Final answer:
The primary muscles used in quiet breathing are the diaphragm and the external intercostal muscles. The diaphragm contracts and flattens to pull air into the lungs, while the external intercostal muscles raise the rib cage to aid in inhalation. These muscles work without conscious effort and allow for passive exhalation when they relax.
Step-by-step explanation:
The primary muscles used in quiet breathing, also known as eupnea, are the diaphragm and the external intercostals.
The diaphragm is a large, dome-shaped muscle located below the lungs that separates the thoracic cavity from the abdominal cavity.
When this muscle contracts, it flattens, creating more space in the thoracic cavity and pulling air into the lungs for inhalation. As the diaphragm relaxes and returns to its dome shape, air is passively expelled from the lungs in a process known as diaphragmatic breathing.
In addition to the diaphragm, quiet breathing involves the contraction of the external intercostal muscles. These muscles are situated between the ribs and assist in breathing by elevating the rib cage when contracted, which increases the volume of the thoracic cavity and aids in inhalation.
During exhalation, the external intercostal muscles relax, and the rib cage lowers as air is passively released from the lungs in what is referred to as costal breathing. The action of these muscles during quiet breathing is typically not conscious and occurs at rest.