Breathing in vertebrates is composed of cycles of inhalation and exhalation to get the air inside and outside of the lungs, this is an autonomic process and is controlled by the autonomic nervous system.
During inhalation, the chest cavity expands, which means the rib cage is pulled upward (A) and outward (B); while the diaphragm contracts, changing its shape from dome-shaped to flat (D), and moves downward (C)