When the diaphragm relaxes, it regains its former domed shape. This squeezes the lungs slightly, forcing the air out again. When the chest is used for deep breathing, muscles raise the rib cage to increase its volume. This causes air to rush into the lungs. Air is forced out when the muscles relax.
Breathing is automatic.
A part of the brain which controls all of our important body functions automatically sends nerve impulses down the spinal cord to the diaphragm and the intercoastal muscles, instructing them to contract regularly. We can override these instructions, but only for a short while. The rate of breathing is controlled chemically. The control center in the brain can detect increases in carbon dioxide and step up the rate of breathing. Together Blood carrying carbon dioxide is pumped from the heart into the capillaries of the air sacs.