Bronchioles begin with rings and then slivers of cartilage which helps to keep them open. As they become smaller and smaller, the cartilage is replaced by smooth muscle. These smooth muscles regulate how much air comes into the lungs.
If the sympathetic nervous system stimulates cardiac muscles to increase the heart rate, this causes dilation of the bronchioles of the lungs (increasing oxygen intake) by relaxing the smooth muscle.
Stretch receptors in the walls of bronchi and bronchioles are activated when the lungs expand to their physical limit. These receptors signal the respiratory center to discontinue stimulation of the inspiratory muscles, allowing expiration to begin as the muscles relax and return to their original size. This response is called the inflation (Hering-Breuer) reflex.
Problems with relaxation of the smooth muscles can cause problems associated with emphysema and asthma.