respiratory system

respiratory system

respiratory system: see respiration.
In living organisms, a respiratory system functions to allow gas exchange. The gases that are exchanged, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on the organism. In humans and other mammals, for example, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs.

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.

Anatomy of Respiratory System in Vertebrates


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.)

Unique Aspects of Comparative Anatomy/Physiology in Mammals

Marine mammals


Horses are obligate nasal breathers. That is, they are different from many other mammals in that they do not have the option of breathing through their mouths and must take in air through their nose.

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.



The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as air sacs. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli. See Avian respiratory system for a detailed description of these and other features.



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.

Anatomy of Respiratory System in Invertebrates

Sponges and Jellyfish

These animals lack specialized organs for gas exchange, instead taking in gases directly from the surrounding water.

Flatworms and Annelids


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) .

Physiology of Respiratory System in Mammals

For more detailed descriptions see also Respiratory physiology or Respiration.















Gas exchange

The major function of the respiratory system is gas exchange between the external environment and an organism's circulatory system. In humans and mammals, this exchange facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation. As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained, two opposing conditions could occur: 1) respiratory acidosis, a life threatening condition, and 2) respiratory alkalosis.


Non-respiratory Functions


The movement of gas through the larynx, pharynx and mouth allows humans to speak, or phonate. Vocalization, or singing, in birds occurs via the syrinx, an organ located at the base of the trachea. The vibration of air flowing across the larynx (vocal chords), in humans, and the syrinx, in birds, results in sound. Because of this, gas movement is extremely vital for communication purposes.

Temperature control

Panting in dogs

Coughing and sneezing

The phlegm is removed from the body by coughing, and sneezing.

Development of Respiratory System in Animals

Humans and Mammals

The respiratory system lies dormant in the human fetus during pregnancy. At birth, the respiratory system has under-developed lungs. This is due to the incomplete development of the alveoli type II cells in the lungs, necessary for the production of surfactant. The infant lungs do not function due to collapse of alveoli caused by surface tension of water remaining in the lungs, which in normal cases would be prohibited by the presence of surfactant. This condition may be avoided by giving the mother a series of steroid shots in the final week prior to delivery, which will have weard the development of type II alveolar cells.

Disease and the Respiratory System



Disorders of the respiratory system are usually treated internally by a pulmonologist or respiratory physician.

Respiratory System in Plants

Gas exchange in plants



See also


  • Medical Dictionary


External links

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