Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes unique anatomical features such as holes on the undersides of leaves known as stomata.
For humans and mammals respiration is essential. In humans and mammals, the respiratory system can be subdivided into an upper respiratory tract and a lower respiratory tract based on anatomical features. The upper respiratory tract includes the nasal passages, pharynx and the larynx. While the trachea, the primary bronchi and lungs are parts of the lower respiratory tract. The respiratory system can also be divided into physiological, or functional, zones. These include the conducting zone (the region for gas transport from the outside atmosphere to just above the alveoli), the transitional zone, and the respiratory zone (the alveolar region where gas exchange occurs). (See also respiratory tract.)
The horse's respiratory system is divided into two sections, the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the nostrils, the nasal passages, pharynx, larynx and the trachea. The lower respiratory tract is made up of the bronchi, bronchioles, and alveoli, all of which reside within the lungs of the horse. See Respiratory system of the horse for a detailed description.
The elephant is the only mammal known to have no pleural space. Rather, the parietal and visceral pleura are both composed of dense connective tissue and joined to each other via loose connective tissue. This lack of a pleural space, along with an unusually thick diaphram, are thought to be evolutionary adaptations allowing the elephant to remain underwater for long periods of time while breathing through its trunk which emerges as a snorkle.
Skin is one of the important respiratory organs in amphibians. It is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. These properties aid rapid gas exchange.
In most fish the respiration takes place through gills. (See also aquatic respiration.) Lungfish, however, do possess one or two lungs. The labyrinth fishes have developed a special organ that allows them to take advantage of the oxygen of the air, but is not a true lung.
These animals lack specialized organs for gas exchange, instead taking in gases directly from the surrounding water.
Air enters the respiratory system of most insects through a series of external openings called spiracles. These external openings, which act as muscular valves in some insects, lead to the internal respiratory system, a densely-networked array of tubes called trachea. The tracheal system wihtin an individual is composed of interconnecting transverse and longitudinal tracheae which maintain equivalent pressure throughout the system. These tracheae branch repeatedly, eventually forming tracheoles, which are blind-ended, water-filled compartments only one micrometer in diameter 
It is at this level of the tracheoles that oxygen is delivered to the cells for respiration.
Insects were once believed to exchange gases with the environment continuously by the simple diffusion of gases into the tracheal system. More recently, however, large variation in insect ventilatory patterns have been documented and insect respiration appears to be highly variable. Some small insects do demonstrate continuous respiration and may lack muscular control of the spiracles. Others, however, utilize muscular contraction of the abdomen along with coordinated spiracle contraction and relaxation to generate cyclical gas exchange patterns. The most extreme form of these patterns is termed discontinuous gas exchange cycles (DGC) .
Inhalation is initiated by the diaphragm and supported by the external intercostal muscles. Normal resting respirations are 10 to 18 breaths per minute. Its time period is 2 seconds. During vigorous inhalation (at rates exceeding 35 breaths per minute), or in approaching respiratory failure, accessory muscles of respiration are recruited for support. These consist of sternocleidomastoid, platysma, and the scalene muscles of the neck.
Inhalation is driven primarily by the diaphragm. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume, which in turn causes a decrease in intrathoracic pressure. As the pressure in the chest falls, air moves into the conducting zone. Here, the air is filtered, warmed, and humidified as it flows to the lungs.
During forced inhalation, as when taking a deep breath, the external intercostal muscles and accessory muscles further expand the thoracic cavity.
The lungs have a natural elasticity; as they recoil from the stretch of inhalation, air flows back out until the pressures in the chest and the atmosphere reach equilibrium.
During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.
Upon inhalation, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometres). These walls are composed of a single layer of epithelial cells (type I and type II epithelial cells) in close proximity to the pulmonary capillaries which are composed of a single layer of endothelial cells. The close proximity of these two cell types allows permeability to gases and, hence, gas exchange.
Disorders of the respiratory system can be classified into four general areas:
The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.
Disorders of the respiratory system are usually treated internally by a pulmonologist or respiratory physician.
Plant respiration is limited by the process of diffusion. Plants take in carbon dioxide through holes on the undersides of their leaves known as stomata (sing:stoma). However, most plants require little air. Most plants have relatively few living cells outside of their surface because air (which is required for metabolic content) can penetrate only skin deep. However, most plants are not involved in highly aerobic activities, and thus have no need of these living cells.
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