Respiratory system - Revision history

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2024-09-28T08:45:13Z

Reverted 1 edit by Wondaful2 (talk) to last revision by Hopefulgratitude

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	The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] ~~which consists~~ of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].		The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] consisting of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].

	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

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2024-09-18T06:47:11Z

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	The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] ~~consisting~~ of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].		The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] which consists of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].

	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

Hopefulgratitude: /* Ventilatory volumes */c/e

2024-07-29T14:27:27Z

Ventilatory volumes: c/e

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	The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances (the resting [[tidal volume]] of about 500 ml), and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by [[spirometry]].<ref name=tortora8 /> A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below (Fig. 3):		The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances (the resting [[tidal volume]] of about 500 ml), and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by [[spirometry]].<ref name=tortora8 /> A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below (Fig. 3):

	Not all the air in the lungs can be expelled during maximally forced exhalation ([[Expiratory reserve volume\|ERV]]). This is the [[Lung volumes\|residual volume]](volume of air remaining even after a forced exhalation) of about 1.0–1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume (i.e. [[functional residual capacity]] of about 2.5–3.0 liters, and [[total lung capacity]] of about 6 liters) can therefore also not be measured by spirometry. Their measurement requires special techniques.<ref name=tortora8>{{cite book \|last1= Tortora \|first1= Gerard J. \|last2=Anagnostakos\|first2=Nicholas P.\| title=Principles of anatomy and physiology \|url= https://archive.org/details/principlesofanat05tort \|url-access= registration \|pages=[https://archive.org/details/principlesofanat05tort/page/570 570–572]\|edition= Fifth \|location= New York \|publisher= Harper & Row, Publishers\|date= 1987 \|isbn= 0-06-350729-3 }}</ref>		Not all the air in the lungs can be expelled during maximally forced exhalation ([[Expiratory reserve volume\|ERV]]). This is the [[Lung volumes\|residual volume]] (volume of air remaining even after a forced exhalation) of about 1.0–1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume (i.e. [[functional residual capacity]] of about 2.5–3.0 liters, and [[total lung capacity]] of about 6 liters) can therefore also not be measured by spirometry. Their measurement requires special techniques.<ref name=tortora8>{{cite book \|last1= Tortora \|first1= Gerard J. \|last2=Anagnostakos\|first2=Nicholas P.\| title=Principles of anatomy and physiology \|url= https://archive.org/details/principlesofanat05tort \|url-access= registration \|pages=[https://archive.org/details/principlesofanat05tort/page/570 570–572]\|edition= Fifth \|location= New York \|publisher= Harper & Row, Publishers\|date= 1987 \|isbn= 0-06-350729-3 }}</ref>

	The rates at which air is breathed in or out, either through the mouth or nose or into or out of the [[alveoli]] are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the [[respiratory rate]]. An average healthy human breathes 12–16 times a minute.		The rates at which air is breathed in or out, either through the mouth or nose or into or out of the [[alveoli]] are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the [[respiratory rate]]. An average healthy human breathes 12–16 times a minute.

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	[[File:Alveolar Wall.svg\|thumb\|300 px\|left\|'''Fig. 10''' A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type I [[epithelial]] cells (or type 1 [[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the pulmonary capillary blood.]]		[[File:Alveolar Wall.svg\|thumb\|300 px\|left\|'''Fig. 10''' A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type I [[epithelial]] cells (or type 1 [[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the pulmonary capillary blood.]]

	The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple [[Diffusion#Diffusion vs. bulk flow\|diffusion]],<ref>{{cite book\|last1=Maton\|first1=Anthea\|first2=Jean Susan\|last2= Hopkins\|first3=Charles William\|last3=Johnson\|first4=Maryanna Quon\|last4= McLaughlin\|first5=David\|last5=Warner\|first6=Jill\|last6= LaHart Wright\|title=Human Biology and Health\|publisher=Prentice Hall\|year=2010 \|location=Englewood Cliffs\|pages= 108–118\|isbn=978-0134234359}}</ref> across a very thin membrane (known as the [[blood–air barrier]]), which forms the walls of the [[pulmonary alveoli]] (Fig. 10). It consists of the [[Pneumocytes\|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium\|endothelial cells]] of the alveolar capillaries (Fig. 10).<ref name=grays>{{cite book \|last1=Williams \|first1=Peter L. \|last2=Warwick \|first2=Roger \|last3=Dyson\|first3=Mary \|last4=Bannister \|first4=Lawrence H. \|title=Gray's Anatomy\| pages=1278–1282 \|location=Edinburgh\|publisher=Churchill Livingstone \| edition=Thirty-seventh \|date=1989\|isbn= 0443-041776 }}</ref> This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called [[Pulmonary alveolus\|alveoli]]<ref name=grays /> (each between 75 and 300 µm in diameter) branching off from the respiratory [[bronchiole]]s in the [[lung]]s, thus providing an extremely large surface area (approximately 145 m<sup>2</sup>) for gas exchange to occur.<ref name=grays />		The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple [[Diffusion#Diffusion vs. bulk flow\|diffusion]],<ref>{{cite book\|last1=Maton\|first1=Anthea\|first2=Jean Susan\|last2= Hopkins\|first3=Charles William\|last3=Johnson\|first4=Maryanna Quon\|last4= McLaughlin\|first5=David\|last5=Warner\|first6=Jill\|last6= LaHart Wright\|title=Human Biology and Health\|publisher=Prentice Hall\|year=2010 \|location=Englewood Cliffs\|pages= 108–118\|isbn=978-0134234359}}</ref> across a very thin membrane (known as the [[blood–air barrier]]), which forms the walls of the [[pulmonary alveoli]] (Fig. 10). It consists of the [[Pneumocytes\|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium\|endothelial cells]] of the alveolar capillaries (Fig. 10).<ref name=grays>{{cite book \|last1=Williams \|first1=Peter L. \|last2=Warwick \|first2=Roger \|last3=Dyson\|first3=Mary \|last4=Bannister \|first4=Lawrence H. \|title=Gray's Anatomy\| pages=1278–1282 \|location=Edinburgh\|publisher=Churchill Livingstone \| edition=Thirty-seventh \|date=1989\|isbn= 0443-041776 }}</ref> This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called [[Pulmonary alveolus\|alveoli]]<ref name=grays /> (each between 75 and 300 μm in diameter) branching off from the respiratory [[bronchiole]]s in the [[lung]]s, thus providing an extremely large surface area (approximately 145 m<sup>2</sup>) for gas exchange to occur.<ref name=grays />

	The air contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a [[partial pressure]] of oxygen of 13-14 kPa (100 mmHg), and a [[PCO2\|partial pressure of carbon dioxide]] of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).<ref name=tortora1 /> As mentioned in [[#Mechanics of breathing\|the section above]], the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively.<ref name=tortora1 />		The air contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a [[partial pressure]] of oxygen of 13-14 kPa (100 mmHg), and a [[PCO2\|partial pressure of carbon dioxide]] of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).<ref name=tortora1 /> As mentioned in [[#Mechanics of breathing\|the section above]], the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively.<ref name=tortora1 />

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2024-02-22T17:59:30Z

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← Previous revision		Revision as of 17:59, 22 February 2024
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	The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] consisting of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].		The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] consisting of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].

	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|~~lammelae~~]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

	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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>		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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>

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	During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs [[Anatomical terms of location#Main terminologies\|anteriorly]], and to the pelvis (pubis and ilium in Fig. 17) [[Anatomical terms of location#Main terminologies\|posteriorly]] (forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.<ref name=AvResp />		During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs [[Anatomical terms of location#Main terminologies\|anteriorly]], and to the pelvis (pubis and ilium in Fig. 17) [[Anatomical terms of location#Main terminologies\|posteriorly]] (forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.<ref name=AvResp />

	[[File:Cross-current exchanger.jpg\|thumb\|right\|250 px\|'''Fig. 19''' The [[Countercurrent exchange\|cross-current]] respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs [[Unidirectional respiratory system\|unidirectionally]] (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web\| url = http://www.people.eku.edu/ritchisong/birdrespiration.html \| title = BIO 554/754 – Ornithology: Avian respiration \| access-date = 2009-04-23 \| last = Ritchson \| first = G \| publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal\|last=Scott\|first=Graham R.\|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes\|journal=Journal of Experimental Biology\|volume= 214\|issue=Pt 15\|pages=2455–2462\|date=2011\|doi=10.1242/jeb.052548\|pmid=21753038\|doi-access=}}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]		[[File:Cross-current exchanger.jpg\|thumb\|right\|250 px\|'''Fig. 19''' The [[Countercurrent exchange\|cross-current]] respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs [[Unidirectional respiratory system\|unidirectionally]] (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web\| url = http://www.people.eku.edu/ritchisong/birdrespiration.html \| title = BIO 554/754 – Ornithology: Avian respiration \| access-date = 2009-04-23 \| last = Ritchson \| first = G \| publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal\|last=Scott\|first=Graham R.\|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes\|journal=Journal of Experimental Biology\|volume= 214\|issue=Pt 15\|pages=2455–2462\|date=2011\|doi=10.1242/jeb.052548\|pmid=21753038\|doi-access=\|s2cid=27550864 }}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]

	During inhalation air enters the [[Vertebrate trachea\|trachea]] via the nostrils and mouth, and continues to just beyond the [[syrinx (bird anatomy)\|syrinx]] at which point the trachea branches into two [[Bronchus\|primary bronchi]], going to the two lungs (Fig. 16). The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig. 16).<ref name=AvResp /> The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the [[Anatomical terms of location#Caudal\|caudal]] end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or [[parabronchi]]) where [[gas exchange]] occurs (Fig. 16).<ref name=AvResp /> As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig. 18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.<ref name="Maina2005">{{cite book\|last1=Maina\|first1=John N.\|title=The lung air sac system of birds development, structure, and function; with 6 tables\|date=2005\|publisher=Springer\|location=Berlin\|isbn=978-3-540-25595-6\|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82\|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book\|last=Krautwald-Junghanns\|first=Maria-Elisabeth\|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles\|year=2010\|publisher=Manson Publishing\|location=Germany\|isbn=978-3-89993-049-8\|display-authors=etal}}</ref><ref name=sturkie>{{cite book\|last=Sturkie \|first= P.D. \|editor1-first= P. D \|editor1-last= Sturkie \|title=Avian Physiology \| publisher=Springer Verlag \|location= New York \|date= 1976 \|page = 201 \|isbn= 978-1-4612-9335-4 \|doi= 10.1007/978-1-4612-4862-0\|s2cid= 36415426 }}</ref> From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,<ref name=AvResp /> the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.		During inhalation air enters the [[Vertebrate trachea\|trachea]] via the nostrils and mouth, and continues to just beyond the [[syrinx (bird anatomy)\|syrinx]] at which point the trachea branches into two [[Bronchus\|primary bronchi]], going to the two lungs (Fig. 16). The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig. 16).<ref name=AvResp /> The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the [[Anatomical terms of location#Caudal\|caudal]] end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or [[parabronchi]]) where [[gas exchange]] occurs (Fig. 16).<ref name=AvResp /> As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig. 18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.<ref name="Maina2005">{{cite book\|last1=Maina\|first1=John N.\|title=The lung air sac system of birds development, structure, and function; with 6 tables\|date=2005\|publisher=Springer\|location=Berlin\|isbn=978-3-540-25595-6\|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82\|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book\|last=Krautwald-Junghanns\|first=Maria-Elisabeth\|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles\|year=2010\|publisher=Manson Publishing\|location=Germany\|isbn=978-3-89993-049-8\|display-authors=etal}}</ref><ref name=sturkie>{{cite book\|last=Sturkie \|first= P.D. \|editor1-first= P. D \|editor1-last= Sturkie \|title=Avian Physiology \| publisher=Springer Verlag \|location= New York \|date= 1976 \|page = 201 \|isbn= 978-1-4612-9335-4 \|doi= 10.1007/978-1-4612-4862-0\|s2cid= 36415426 }}</ref> From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,<ref name=AvResp /> the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.

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	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

	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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>		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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>

	==Mammals==		==Mammals==

Autisticeditor 20: Undid revision 1198684962 by Autisticeditor 20 (talk) reverted

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← Previous revision		Revision as of 20:14, 24 January 2024
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	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

	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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>		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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>

	==Mammals==		==Mammals==

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	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammelae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

	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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>		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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>

	==Mammals==		==Mammals==

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	[[File:Alveolar Wall.svg\|thumb\|300 px\|left\|'''Fig. 10''' A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type I [[epithelial]] cells (or type 1 [[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the pulmonary capillary blood.]]		[[File:Alveolar Wall.svg\|thumb\|300 px\|left\|'''Fig. 10''' A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type I [[epithelial]] cells (or type 1 [[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the pulmonary capillary blood.]]

	The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple [[Diffusion#Diffusion vs. bulk flow\|diffusion]],<ref>{{cite book\|last1=Maton\|first1=Anthea\|first2=Jean Susan\|last2= Hopkins\|first3=Charles William\|last3=Johnson\|first4=Maryanna Quon\|last4= McLaughlin\|first5=David\|last5=Warner\|first6=Jill\|last6= LaHart Wright\|title=Human Biology and Health\|publisher=Prentice Hall\|year=2010 \|location=Englewood Cliffs\|pages= 108–118\|isbn=978-0134234359}}</ref> across a very thin membrane (known as the [[blood–air barrier]]), which forms the walls of the [[pulmonary alveoli]] (Fig. 10). It consists of the [[Pneumocytes\|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium\|endothelial cells]] of the alveolar capillaries (Fig. 10).<ref name=grays>{{cite book \|last1=Williams \|first1=Peter L. \|last2=Warwick \|first2=Roger \|last3=Dyson\|first3=Mary \|last4=Bannister \|first4=Lawrence H. \|title=Gray's Anatomy\| pages=1278–1282 \|location=Edinburgh\|publisher=Churchill Livingstone \| edition=Thirty-seventh \|date=1989\|isbn= 0443-041776 }}</ref> This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called [[Pulmonary alveolus\|alveoli]]<ref name=grays /> (each between 75 and 300 µm in diameter) branching off from the respiratory [[bronchiole]]s in the [[lung]]s, thus providing an extremely large surface area (approximately 145 m<sup>2</sup>) for gas exchange to occur.<ref name=grays />		The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple [[Diffusion#Diffusion vs. bulk flow\|diffusion]],<ref>{{cite book\|last1=Maton\|first1=Anthea\|first2=Jean Susan\|last2= Hopkins\|first3=Charles William\|last3=Johnson\|first4=Maryanna Quon\|last4= McLaughlin\|first5=David\|last5=Warner\|first6=Jill\|last6= LaHart Wright\|title=Human Biology and Health\|publisher=Prentice Hall\|year=2010 \|location=Englewood Cliffs\|pages= 108–118\|isbn=978-0134234359}}</ref> across a very thin membrane (known as the [[blood–air barrier]]), which forms the walls of the [[pulmonary alveoli]] (Fig. 10). It consists of the [[Pneumocytes\|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium\|endothelial cells]] of the alveolar capillaries (Fig. 10).<ref name=grays>{{cite book \|last1=Williams \|first1=Peter L. \|last2=Warwick \|first2=Roger \|last3=Dyson\|first3=Mary \|last4=Bannister \|first4=Lawrence H. \|title=Gray's Anatomy\| pages=1278–1282 \|location=Edinburgh\|publisher=Churchill Livingstone \| edition=Thirty-seventh \|date=1989\|isbn= 0443-041776 }}</ref> This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called [[Pulmonary alveolus\|alveoli]]<ref name=grays /> (each between 75 and 300 μm in diameter) branching off from the respiratory [[bronchiole]]s in the [[lung]]s, thus providing an extremely large surface area (approximately 145 m<sup>2</sup>) for gas exchange to occur.<ref name=grays />

	The air contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a [[partial pressure]] of oxygen of 13-14 kPa (100 mmHg), and a [[PCO2\|partial pressure of carbon dioxide]] of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).<ref name=tortora1 /> As mentioned in [[#Mechanics of breathing\|the section above]], the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively.<ref name=tortora1 />		The air contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a [[partial pressure]] of oxygen of 13-14 kPa (100 mmHg), and a [[PCO2\|partial pressure of carbon dioxide]] of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).<ref name=tortora1 /> As mentioned in [[#Mechanics of breathing\|the section above]], the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively.<ref name=tortora1 />

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	The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] consisting of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].		The '''respiratory system''' (also '''respiratory apparatus''', '''ventilatory system''') is a [[biological system]] consisting of specific [[organs]] and structures used for [[gas exchange]] in [[animal]]s and [[plant]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal\|land animals]], the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book \|last1=Campbell \|first1=Neil A. \|title=Biology \|date=1990 \|publisher=Benjamin/Cummings Pub. Co. \|location=Redwood City, Calif. \|isbn=0-8053-1800-3 \|pages=834–835 \|edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called [[pulmonary alveolus\|alveoli]], and in birds, they are known as [[Bird anatomy#Respiratory system\|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal \|last1=Hsia \|first1=CC \|last2=Hyde \|first2=DM \|last3=Weibel \|first3=ER \|author3-link=ER Weibel \|title=Lung Structure and the Intrinsic Challenges of Gas Exchange. \|journal=Comprehensive Physiology \|date=15 March 2016 \|volume=6 \|issue=2 \|pages=827–95 \|doi=10.1002/cphy.c150028 \|pmid=27065169\|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus\|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s, the bronchioles are termed [[Bird anatomy#Respiratory system\|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus\|alveoli]] in mammals and [[Bird anatomy#Respiratory system\|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]] which involves the [[muscles of respiration]].

	In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|~~lammelae~~]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.		In most [[fish]], and a number of other [[aquatic animal]]s (both [[vertebrate]]s and [[invertebrate]]s), the respiratory system consists of [[gill]]s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat [[Gill#Vertebrate gills\|filaments]] and [[Gill#Vertebrate gills\|lammellae]] which expose a very large surface area of highly [[blood vessel\|vascularized]] tissue to the water.

	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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>		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 includes anatomical features such as [[stoma]]ta, that are found in various parts of the plant.<ref>{{cite book\|last=West\|first=John B.\|title=Respiratory physiology-- the essentials\|publisher=Williams & Wilkins\|location=Baltimore\|pages=[https://archive.org/details/respiratoryphysi00west/page/1 1–10]\|isbn=0-683-08937-4\|url=https://archive.org/details/respiratoryphysi00west/page/1\|year=1995}}</ref>

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	During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs [[Anatomical terms of location#Main terminologies\|anteriorly]], and to the pelvis (pubis and ilium in Fig. 17) [[Anatomical terms of location#Main terminologies\|posteriorly]] (forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.<ref name=AvResp />		During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs [[Anatomical terms of location#Main terminologies\|anteriorly]], and to the pelvis (pubis and ilium in Fig. 17) [[Anatomical terms of location#Main terminologies\|posteriorly]] (forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.<ref name=AvResp />

	[[File:Cross-current exchanger.jpg\|thumb\|right\|250 px\|'''Fig. 19''' The [[Countercurrent exchange\|cross-current]] respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs [[Unidirectional respiratory system\|unidirectionally]] (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web\| url = http://www.people.eku.edu/ritchisong/birdrespiration.html \| title = BIO 554/754 – Ornithology: Avian respiration \| access-date = 2009-04-23 \| last = Ritchson \| first = G \| publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal\|last=Scott\|first=Graham R.\|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes\|journal=Journal of Experimental Biology\|volume= 214\|issue=Pt 15\|pages=2455–2462\|date=2011\|doi=10.1242/jeb.052548\|pmid=21753038\|doi-access=}}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]		[[File:Cross-current exchanger.jpg\|thumb\|right\|250 px\|'''Fig. 19''' The [[Countercurrent exchange\|cross-current]] respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs [[Unidirectional respiratory system\|unidirectionally]] (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web\| url = http://www.people.eku.edu/ritchisong/birdrespiration.html \| title = BIO 554/754 – Ornithology: Avian respiration \| access-date = 2009-04-23 \| last = Ritchson \| first = G \| publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal\|last=Scott\|first=Graham R.\|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes\|journal=Journal of Experimental Biology\|volume= 214\|issue=Pt 15\|pages=2455–2462\|date=2011\|doi=10.1242/jeb.052548\|pmid=21753038\|doi-access=\|s2cid=27550864 }}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]

	During inhalation air enters the [[Vertebrate trachea\|trachea]] via the nostrils and mouth, and continues to just beyond the [[syrinx (bird anatomy)\|syrinx]] at which point the trachea branches into two [[Bronchus\|primary bronchi]], going to the two lungs (Fig. 16). The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig. 16).<ref name=AvResp /> The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the [[Anatomical terms of location#Caudal\|caudal]] end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or [[parabronchi]]) where [[gas exchange]] occurs (Fig. 16).<ref name=AvResp /> As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig. 18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.<ref name="Maina2005">{{cite book\|last1=Maina\|first1=John N.\|title=The lung air sac system of birds development, structure, and function; with 6 tables\|date=2005\|publisher=Springer\|location=Berlin\|isbn=978-3-540-25595-6\|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82\|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book\|last=Krautwald-Junghanns\|first=Maria-Elisabeth\|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles\|year=2010\|publisher=Manson Publishing\|location=Germany\|isbn=978-3-89993-049-8\|display-authors=etal}}</ref><ref name=sturkie>{{cite book\|last=Sturkie \|first= P.D. \|editor1-first= P. D \|editor1-last= Sturkie \|title=Avian Physiology \| publisher=Springer Verlag \|location= New York \|date= 1976 \|page = 201 \|isbn= 978-1-4612-9335-4 \|doi= 10.1007/978-1-4612-4862-0\|s2cid= 36415426 }}</ref> From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,<ref name=AvResp /> the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.		During inhalation air enters the [[Vertebrate trachea\|trachea]] via the nostrils and mouth, and continues to just beyond the [[syrinx (bird anatomy)\|syrinx]] at which point the trachea branches into two [[Bronchus\|primary bronchi]], going to the two lungs (Fig. 16). The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig. 16).<ref name=AvResp /> The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the [[Anatomical terms of location#Caudal\|caudal]] end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or [[parabronchi]]) where [[gas exchange]] occurs (Fig. 16).<ref name=AvResp /> As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig. 18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.<ref name="Maina2005">{{cite book\|last1=Maina\|first1=John N.\|title=The lung air sac system of birds development, structure, and function; with 6 tables\|date=2005\|publisher=Springer\|location=Berlin\|isbn=978-3-540-25595-6\|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82\|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book\|last=Krautwald-Junghanns\|first=Maria-Elisabeth\|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles\|year=2010\|publisher=Manson Publishing\|location=Germany\|isbn=978-3-89993-049-8\|display-authors=etal}}</ref><ref name=sturkie>{{cite book\|last=Sturkie \|first= P.D. \|editor1-first= P. D \|editor1-last= Sturkie \|title=Avian Physiology \| publisher=Springer Verlag \|location= New York \|date= 1976 \|page = 201 \|isbn= 978-1-4612-9335-4 \|doi= 10.1007/978-1-4612-4862-0\|s2cid= 36415426 }}</ref> From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,<ref name=AvResp /> the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.