The trachea carries the air necessary for respiration for many types of animals. Both vertebrates and invertebrates possess tracheae, but in many invertebrates, the trachea is often a branching structure that carries gases directly to and from the animal's cells. In many vertebrates, the trachea is a structure that carries air to the lungs, organs that are dedicated specifically to gas exchange.
Not all animals possess a trachea. Fish, for instance, do not breathe air and conduct oxygen exchange in their gills. Only air-breathing animals have a trachea.
In humans, the trachea is about 6 inches long and about 1 inch in diameter. It serves not only to carry air but to moisten and clean the air on its way to the delicate lungs. To clean the air, the trachea has multiple hair-like cilia that secretes mucus and water droplets. These trap contaminants and gradually move them up and out of the airway. The human trachea, as well as that of most other air-breathing vertebrates, is reinforced with rings of cartilage to keep it from collapsing. Amphibians are the only exception with sparser cartilage reinforcements. This reinforcement structure of multiple rings allows the trachea to stretch as needed during breathing.