The type of muscles that control the size of the bronchioles in the lungs are known as smooth muscles. Smooth muscles are found in both walls of the bronchioles and arterioles located in both lungs. They happen to be the only kind of muscle found in the lungs.
These smooth muscles are under the control of the sympathetic and parasympathetic divisions of the nervous system. Stimulation from the sympathetic division causes the smooth muscles to relax, causing dilation of the bronchial branches. More air passes to the lungs as a result. The opposite happens in the event of the stimulation of the parasympathetic division of the nervous system.