Light microscopy typically shows neutrophils in bronchi, bronchioles and adjacent alveolar spaces. The respiratory bronchioles lead into alveolar ducts which are deeply lined with alveoli. The branching process forms the bronchi, bronchioles, and ultimately the alveoli. Crackles are often associated with inflammation or infection of the small bronchi, bronchioles, and alveoli. Bronchoconstriction is defined as the narrowing of the airways in the lungs (bronchi and bronchioles). There are oxygen sensors in the smaller bronchi and bronchioles. The bronchi divide many times before evolving into the bronchioles. Alveolar ducts are distal continuations of the respiratory bronchioles. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into alveolar ducts. Simple ciliated cuboidal cells are also present in the respiratory bronchioles. Alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. These airways carry the oxygen to the bronchioles inside the lungs. The receptors are less abundant in the distal airways, and absent beyond the respiratory bronchioles. The inhaled form may result in constriction of the bronchioles. The respiratory bronchioles and the alveolar ducts are responsible for 10% of the gas exchange.