3. fluid-filled chambers at the end of the tubes act as a respiratory surface 4. One layer covers the lung, the other lines the chest cavity. Fluid between the two allow for free movement. Pharynx - throat 2. G. Alveoli 1.
Breathing is regulated by the concentration of carbon dioxide in blood.
Respiratory center at the base of the brain stem monitors carbon dioxide in the blood. When carbon dioxide reaches certain levels, you breath to get rid of carbon dioxide.