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-[[Image:3DScience respiratory labeled.jpg|thumb|&nbsp;|300px|The Respiratory System]]
-Among [[quadruped]]s, the '''respiratory system''' generally includes tubes, such as the [[bronchi]], used to carry [[air]] to the [[lungs]], where gas exchange takes place. A [[diaphragm (anatomy)|diaphragm]] pulls air in and pushes it out. Respiratory systems of various types are found in a wide variety of organisms. Even trees have respiratory systems.
-In [[human]]s and other [[mammal]]s, the respiratory system consists of the airways, the lungs, and the respiratory muscles that mediate the movement of air into and out of the body.  Within the [[alveolar]] system of the lungs, molecules of [[oxygen]] and [[carbon dioxide]] are passively exchanged, by [[diffusion]], between the gaseous environment and the blood.  Thus, the respiratory system facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation.
-The system also helps to maintain the acid-base balance of the body through the efficient removal of carbon dioxide from the blood.
-==Anatomy==
-In humans and other animals, the respiratory system can be conveniently subdivided into an [[upper respiratory tract]] (or ''conducting zone'') and [[lower respiratory tract]] (''respiratory zone''), trachea and lungs.
-Air moves through the body in the following order:
-*[[Nostrils]]
-*[[Nasal cavity]]
-*[[Pharynx]] (naso-, oro-, laryngo-)
-*[[Larynx]] (voice box)
-*[[Vertebrate trachea|Trachea]] (wind pipe)
-*[[Thoracic]] cavity (chest)
-*[[Bronchi]] (right and left)
-*[[Alveoli]] (site of gas exchange)
-===Upper respiratory tract/conducting zone===
-The conducting zone begins with the [[nares]] (nostrils) of the [[nose]], which open into the [[nasopharynx]] (nasal cavity). The primary functions of the nasal passages are to: 1) filter, 2) warm, 3) moisten, and 4) provide resonance in speech. The nasopharynx opens into the [[oropharynx]] (behind the oral cavity). The oropharynx leads to the [[laryngopharynx]], and empties into the [[larynx]] (voicebox), which contains the [[vocal cords]], passing through the [[glottis]], connecting to the [[Vertebrate trachea|trachea]] (wind pipe).
-===Lower respiratory tract/respiratory zone===
-The trachea leads down to the [[thorax|thoracic cavity]] (chest) where it divides into the right and left "main stem" [[bronchus|bronchi]]. The subdivision of the bronchus are: primary, secondary, and tertiary divisions (first, second and third levels). In all, they divide 16 more times into even smaller [[bronchiole]]s.
-The bronchioles lead to the respiratory zone of the lungs which consists of [[respiratory bronchiole]]s, [[alveolar duct]]s and the [[alveoli]], the multi-lobulated sacs in which most of the gas exchange occurs.
-==Ventilation==
-[[Ventilation (physiology)|Ventilation]] of the lungs is carried out by the muscles of respiration.
-===Control===
-Ventilation occurs under the control of the autonomic nervous system from the part of the [[brain stem]], the [[medulla oblongata]] and the [[pons]]. This area of the brain forms the respiration regulatory center, a series of interconnected neurons within the lower and middle brain stem which coordinate respiratory movements. The sections are the [[pneumotaxic center]], the [[apneustic center]], and the dorsal and ventral respiratory groups. This section is especially sensitive during infancy, and the neurons can be destroyed if the infant is dropped or shaken violently. The result can be death due to "[[shaken baby syndrome]]."<ref>*[http://www.usask.ca/medicine/prevent/sbs.html Fact sheet on Shaken Baby Syndrome]</ref>
-===Inhalation===
-[[Inhalation]] is initiated by the [[diaphragm (anatomy)|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 [[strap muscles of the neck]].
-Inhalation is driven primarily by the [[diaphragm (anatomy)|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.
-===Exhalation===
-[[Exhalation]] is generally a passive process, however active or ''forced'' exhalation is achieved by the [[abdominal muscles|abdominal]] and the [[internal intercostal muscles]].
-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.<ref>A simple [[:wikibooks:Constructing school science lab equipment/Bell jar model lung|model of how the lungs are inflated]] can be built from a [[bell jar]]</ref>
-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.
-==Circulation==
-The right side of the heart pumps blood from the [[right ventricle]] through the [[pulmonary semilunar valve]] into the [[pulmonary trunk]]. The trunk branches into right and left [[pulmonary arteries]] to the [[pulmonary blood vessel]]s. The vessels generally accompany the [[airway]]s and also undergo numerous branchings. Once the gas exchange process is complete in the pulmonary capillaries, blood is returned to the left side of the heart through four [[pulmonary veins]], two from each side. The [[pulmonary circulation]] has a very low resistance, due to the short distance within the lungs, compared to the [[systemic circulation]], and for this reason, all the pressures within the pulmonary blood vessels are normally low as compared to the pressure of the systemic circulation loop.
-Virtually all the body's blood travels through the lungs every minute. The lungs add and remove many chemical messengers from the blood as it flows through pulmonary capillary bed . The fine capillaries also trap blood clots that have formed in systemic veins.
-==Gas exchange==
-The major function of the respiratory system is [[gas exchange]].  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]].
-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), and are permeable to gases. The alveoli are lined with pulmonary capillaries, the walls of which are also thin enough to permit gas exchange.  All gases diffuse from the alveolar air to the blood in the pulmonary capillaries, as carbon dioxide diffuses in the opposite direction, from capillary blood to alveolar air.  At this point, the pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide.  Exhalation follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration.
-In an average resting adult, the lungs take up about 250ml of oxygen every minute while excreting about 200ml of carbon dioxide. During an average breath, an adult will exchange from 500 ml to 700 ml of air. This average breath capacity is called [[tidal volume]].
-==Development==
-The respiratory system lies dormant in the human fetus during pregnancy. At birth, the respiratory system is drained of fluid and cleaned to assure proper functioning of the system. If an infant is born before forty weeks gestational age, the newborn may experience respiratory failure due to the 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 enhance the development of type II alveolar cells. <ref>[http://ajpregu.physiology.org/cgi/content/full/281/3/R770 Department of Environmental Biology, University of Adelaide, Adelaide, South Australia]</ref>
-==Role in communication==
-The movement of gas through the larynx, pharynx and mouth allows [[human]]s to speak, or ''[[phonation|phonate]]''. Because of this, gas movement is extremely vital for communication purposes.
-==Conditions of the respiratory system==
-[[Respiratory disease|Disorders of the respiratory system]] can be classified into four general areas:
-*Obstructive conditions  (e.g., [[emphysema]], [[bronchitis]], [[Allergic asthma|asthma attack]]s)
-*Restrictive conditions  (e.g., [[fibrosis]], [[sarcoidosis]], alveolar damage, pleural effusion)
-*Vascular diseases  (e.g., [[pulmonary edema]], [[pulmonary embolism]], [[pulmonary hypertension]])
-*Infectious, environmental and other "diseases"  (e.g., [[pneumonia]], [[tuberculosis]], [[asbestosis]], particulate pollutants) [[cough]]ing is of major importance, as it is the body's main method to remove dust, mucus, saliva, and other debris from the lungs. Inability to cough can lead to infection. Deep breathing exercises may help keep finer structures of the lungs clear from particulate matter, etc.
-The respiratory tract is constantly exposed to [[microbe]]s due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent [[pathogen]]s from entering the body.
-Disorders of the respiratory system are usually treated internally by a [[Pulmonology|pulmonologist]] or Respiratory Physician.
-==Gas exchange in plants==
-Plants use [[carbon dioxide]] gas in the process of [[photosynthesis]], and then exhale [[oxygen]] gas, a waste product of photosynthesis. However, plants also sometimes respire as humans do, using [[oxygen]] and producing [[carbon dioxide]].
-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]].{{Fact|date=February 2007}} 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.
-==See also==
-*[[Liquid breathing]]
-*[[Aquatic respiration]]
-*[[Involuntary control of respiration]]
-*[[Gill]]
-*[[Respiratory tract]]
-*[[Major systems of the human body]]
-*[[Muscles of respiration]]
-==References==
-*Perkins, M. 2003.  Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.
-*[http://www.wrongdiagnosis.com/medical/respiration_regulatory_center_printer.htm Medical Dictionary]
-==Notes==
-{{reflist}}
-==External links==
-*[http://www.scienceaid.co.uk/biology/humans/respiratorysystem.html Science aid: Respiratory System] A simple guide for high school students
-*[http://www.leeds.ac.uk/chb/lectures/anatomy7.html Introduction to Respiratory System]
-*[http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookRespsys.html#Diseases%20of%20the%20Respiratory%20Sys A high school level description of the respiratory system]
-*[http://www.bio.umass.edu/biology/bemis/FAOV4/Ch18.doc The Respiratory System] Univerity level
-{{Organ systems}}
-{{Respiratory_system}}
-{{Lung}}
-{{Respiratory physiology}}
-{{Respiratory pathology}}
-[[Category:Exercise physiology]]
-[[Category:Respiratory system]]
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