Trachea (IPA: ['treikiÉ™]) is a common biological term for an airway through which respiratory air transport takes place in organisms. In terrestrial vertebrates, such as birds and humans, the trachea lets air move from the throat to the lungs. In terrestrial invertebrates, such as onychophorans and beetles, tracheae conduct air from outside the organism directly to all internal tissues.
Vertebrate Trachea
The trachea, or windpipe, is a tube extending from the larynx to the bronchi in mammals, and from the pharynx to the syrinx in birds, carrying air to the lungs. It is lined with ciliated cells which push particles out, and cartilage rings which reinforce the trachea and prevent it from collapsing on itself during breathing. These numerous cartilaginous half-rings, located one above the other along the trachea, have open ends adjacent to the esophagus. The rings are connected by muscular and fibrous tissue, and they are lined inside with a ciliated mucous membrane.
In ill or injured persons, the natural airway formed by the trachea may be damaged or closed off. Intubation is the medical procedure of inserting an artificial tube into the trachea to permit breathing.
Many terrestrialarthropods have evolved a closed respiratory system composed of spiracles, tracheae, and tracheoles to transport metabolicgasses to and from tissue. Some terrestrial woodlice have evolved pseudotrachea, a system which is also called corpus alatum, and is made up of air tubes that delivers oxygen to their hemolymph; a similar system has been found in some caterpillars. The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have no more than one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometers up to 0.8mm. The smallest tubes, tracheoles, penetrate cells and serve as sites of diffusion for water, oxygen, and carbon dioxide. Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their hemolymph; this is one of the factors that may limit their size.
A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices.
In the head, thorax, or abdomen, tracheae may also be connected to air sacs. Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill. Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).
