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Respiratory system

A complete, schematic view of the human respiratory system.

In humans and other mammals, 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.[1] The system also helps to maintain the acid-base balance of the body through the efficient removal of carbon dioxide from the blood.



In humans and other animals, the respiratory system can be conveniently subdivided into an upper respiratory tract and lower respiratory tract, trachea and lungs, or into the conducting zone (for gas transport, anywhere from atmosphere to alveoli) and the respiratory zone (the alveolated region where gas exchange occurs). The respiratory zone also contains the transitional zone.

Air moves through the body in the following order

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 trachea (wind pipe).


Ventilation of the lungs is carried out by the muscles of respiration.


Ventilation occurs under the control of the autonomic nervous system from parts of the brain stem, the medulla oblongata and the pons. This area of the brain forms the respiration regulatory center, a series of interconnected brain cells 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 and/or shaken violently. The result can be death due to "shaken baby syndrome."[2]

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 strap 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.


Exhalation is generally a passive process; however, active or forced exhalation is achieved by the abdominal and the internal intercostal muscles. During this process air is forced or exhaled out.

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.[3]

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.


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 vessels. The vessels generally accompany the airways 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.


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.[4]

Role in communication

The movement of gas through the larynx, pharynx and mouth allows humans to speak, or phonate. Because of this, gas movement is extremely vital for communication purposes.

Conditions of the respiratory system

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 mechani 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.

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.[citation needed] 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


  • Perkins, M. 2003. Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.
  • Medical Dictionary


  1. ^ Maton, Anthea; Jean, Hopkins Susan, Johnson Charles William, McLaughlin Maryanna Quon Warner David, LaHart Wright, Jill D. (1995). Human Biology and Health. Englewood Cliffs, New Jersey: Prentice Hall, 108-118. ISBN 0-12-981176-1
  2. ^ *Fact sheet on Shaken Baby Syndrome
  3. ^ A simple model of how the lungs are inflated can be built from a bell jar
  4. ^ Department of Environmental Biology, University of Adelaide, Adelaide, South Australia

External links

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trachea (tracheal rings, carina) • bronchi • main bronchus (right, left) • lobar/secondary bronchi (eparterial bronchus) • segmental/tertiary bronchi (bronchopulmonary segment) • bronchiole • terminal bronchiole

respiratory zone

respiratory bronchiole • alveolar duct • alveolus • alveolar-capillary barrier


parietal pleura (cervical, costal, mediastinal, diaphragmatic) • visceral pleura • pulmonary ligament • recesses (costomediastinal, costodiaphragmatic) • pleural cavity

v • d • eRespiratory system, physiology: respiratory physiologyVolumes lung volumes- vital capacity- functional residual capacity- respiratory minute volume- closing capacity- dead space- spirometry- body plethysmography- peak flow meter- thoracic independent volume- bronchial challenge testAirways ventilation (V)(positive pressure) - breath(inhalation, exhalation) -respiratory rate- respirometer- pulmonary surfactant- compliance- hysteresivity- airway resistanceBlood pulmonary circulation- perfusion (Q)- hypoxic pulmonary vasoconstriction- pulmonary shuntInteractions ventilation/perfusion ratio(V/Q) and scan- zones of the lung- gas exchange- pulmonary gas pressures- alveolar gas equation- hemoglobin- oxygen-haemoglobin dissociation curve(2,3-DPG, Bohr effect, Haldane effect) - carbonic anhydrase(chloride shift) - oxyhemoglobin- respiratory quotient- arterial blood gas- diffusion capacity- DlcoControl of respirationpons(pneumotaxic center, apneustic center) - medulla(dorsal respiratory group, ventral respiratory group) - chemoreceptors(central, peripheral) - pulmonary stretch receptors(Hering-Breuer reflex) Insufficiency high altitude- oxygen toxicity- hypoxia v • d • ePathologyof respiratory system (J, 460-519), respiratory diseasesUpper RT
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Mendelson's syndrome- Atelectasis Other/general Respiratory failure - Influenza - SARS see also congenital (Q30-Q34, 748) v • d • ePrenatal development/Mammaliandevelopmentof respiratory system (overview) Upper Nasal placodeLower Laryngotracheal groove- Lung buds Categories: Respiratory therapy | Respiratory system | Pulmonology | Exercise physiologyHidden categories: Semi-protected against vandalism | All articles with unsourced statements | Articles with unsourced statements since February 2007

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