Editing Gas exchange

{{Short description|Process by which gases diffuse through a biological membrane}}
{{Continuum mechanics}}
'''Gas exchange''' is the physical process by which gases move passively by [[Diffusion#Diffusion vs. bulk flow|diffusion]] across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable [[membrane]], or a [[biological membrane]] that forms the boundary between an organism and its extracellular environment.

[[File:Maiah Flores - Wikipedia Digital Diagram.pdf|thumb|Gas exchange]]

Gases are constantly consumed and produced by [[Metabolism|cellular and metabolic reactions]] in most living things, so an efficient system for gas exchange between, ultimately, the interior of the cell(s) and the external environment is required. Small, particularly unicellular organisms, such as [[bacterium|bacteria]] and [[protozoa]], have a high [[Surface-area-to-volume ratio|surface-area to volume ratio]]. In these creatures the gas exchange membrane is typically the [[cell membrane]]. Some small multicellular organisms, such as [[flatworm]]s, are also able to perform sufficient gas exchange across the [[skin]] or [[cuticle]] that surrounds their bodies. However, in most larger organisms, which have small surface-area to volume ratios, specialised structures with convoluted surfaces such as [[gill]]s, [[Pulmonary alveolus|pulmonary alveoli]] and [[Leaf#Mesophyll|spongy mesophyll]]s provide the large area needed for effective gas exchange. These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the [[mammal]]ian [[lung]], the spongy mesophyll, which is found inside the leaves of [[Euphyllophyte|some kinds of plant]], or the gills of those [[mollusc]]s that have them, which are found in the [[Mantle (mollusc)|mantle]] cavity.

In [[aerobic organism]]s, gas exchange is particularly important for [[Respiration (physiology)|respiration]], which involves the uptake of [[oxygen]] ({{Chem|O|2}}) and release of [[carbon dioxide]] ({{Chem|CO|2}}). Conversely, in [[photosynthesis|oxygenic photosynthetic organisms]] such as most [[Embryophyte|land plants]], uptake of carbon dioxide and release of both oxygen and water vapour are the main gas-exchange processes occurring during the day. Other gas-exchange processes are important in less familiar organisms: ''e.g.'' carbon dioxide, [[methane]] and [[hydrogen]] are exchanged across the cell membrane of [[methanogen|methanogenic archaea]]. In [[nitrogen fixation]] by [[diazotroph]]ic bacteria, and [[denitrification]] by [[heterotrophic]] [[bacteria]] (such as ''[[Paracoccus denitrificans]]'' and various [[pseudomonadaceae|pseudomonads]]),<ref>{{cite journal | last1 = Carlson | first1 = C. A. | last2 = Ingraham | first2 = J. L. | year = 1983 | title = Comparison of denitrification by ''Pseudomonas stutzeri'', ''Pseudomonas aeruginosa'', and ''Paracoccus denitrificans'' | journal = Appl. Environ. Microbiol. | volume = 45 | issue = 4| pages = 1247–1253 | doi = 10.1128/AEM.45.4.1247-1253.1983 | pmid = 6407395 | pmc = 242446 | bibcode = 1983ApEnM..45.1247C | doi-access = free }}</ref> nitrogen gas is exchanged with the environment, being taken up by the former and released into it by the latter, while [[giant tube worms]] rely on bacteria to oxidize [[hydrogen sulfide]] extracted from their deep sea environment,<ref>C.Michael Hogan. 2011. [http://www.eoearth.org/article/Sulfur?topic=49557 ''Sulfur''. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC] {{webarchive |url=https://web.archive.org/web/20121028080550/http://www.eoearth.org/article/Sulfur?topic=49557 |date=October 28, 2012 }}</ref> using dissolved oxygen in the water as an electron acceptor.

[[Diffusion]] only takes place with a [[concentration gradient]]. [[Gases]] will flow from a high [[concentration]] to a low concentration.
A high [[oxygen]] concentration in the [[Pulmonary alveolus|alveoli]] and low oxygen concentration in the [[capillaries]] causes oxygen to move into the capillaries.
A high [[carbon dioxide]] concentration in the capillaries and low [[carbon]] [[dioxide]] concentration in the alveoli causes carbon dioxide to move into the alveoli.

==Physical principles of gas-exchange==

===Diffusion and surface area===

The exchange of gases occurs as a result of [[Molecular diffusion|diffusion]] down a concentration gradient. Gas molecules move from a region in which they are at high concentration to one in which they are at low concentration. Diffusion is a [[Laws of thermodynamics|passive process]], meaning that no energy is required to power the transport, and it follows [[Fick's laws of diffusion|Fick's law]]: {{citation needed|date=April 2017}}

:<math>J = -D \frac{d \varphi}{d x} </math>

In relation to a typical biological system, where two compartments ('inside' and 'outside'), are separated by a membrane barrier, and where a gas is allowed to spontaneously diffuse down its concentration gradient:{{citation needed|date=April 2017}}
* ''J'' is the flux, the [[amount of substance|amount of gas]] diffusing per unit area of membrane per unit time. Note that this is already scaled for the area of the membrane.
* ''D'' is the [[mass diffusivity|diffusion coefficient]], which will differ from gas to gas, and from membrane to membrane, according to the size of the gas molecule in question, and the nature of the membrane itself (particularly its [[viscosity]], [[temperature]] and [[hydrophobicity]]).
* ''φ'' is the [[Thermodynamic activity|concentration]] of the gas.
* ''x'' is the position across the thickness of the membrane.
* d''φ''/d''x'' is therefore the concentration gradient across the membrane. If the two compartments are individually well-mixed, then this is simplifies to the difference in concentration of the gas between the inside and outside compartments divided by the thickness of the membrane.
* The negative sign indicates that the diffusion is always in the direction that - over time - will destroy the concentration gradient, ''i.e.'' the gas moves from high concentration to low concentration until eventually the inside and outside compartments reach [[List of types of equilibrium|equilibrium]].

[[File:Fick's Law for gas-exchange surface.png|center|'''Fig. 1.''' Fick's law for gas-exchange surface]]

Gases must first dissolve in a liquid in order to diffuse across a [[membrane]], so all biological gas exchange systems require a moist environment.<ref name="Piiper1971">{{Cite journal|vauthors=Piiper J, Dejours P, Haab P, Rahn H | title= Concepts and basic quantities in gas exchange physiology| journal =[[Respiration Physiology]]| volume=13| issue= 3| year=1971| pages=292–304| doi=10.1016/0034-5687(71)90034-x| pmid= 5158848}}</ref> In general, the higher the concentration gradient across the gas-exchanging surface, the faster the rate of diffusion across it. Conversely, the thinner the gas-exchanging surface (for the same concentration difference), the faster the gases will diffuse across it.<ref name="Kety1951">{{Cite journal| author=Kety SS| title= The theory and applications of the exchange of inert gas at the lungs and tissues| journal =[[Pharmacological Reviews]]| volume=3|year=1951| issue= 1| pages=1–41| pmid= 14833874}}</ref>

In the equation above, ''J'' is the [[flux]] expressed per unit area, so increasing the area will make no difference to its value. However, an increase in the available surface area, will increase the ''amount'' of gas that can diffuse in a given time.<ref name="Kety1951"/> This is because the amount of gas diffusing per unit time (d''q''/d''t'') is the product of ''J'' and the area of the gas-exchanging surface, ''A'':

:<math>\frac{d q}{d t} = J A</math>

[[Unicellular organism|Single-celled organisms]] such as [[bacteria]] and [[amoeba]]e do not have specialised gas exchange surfaces, because they can take advantage of the high surface area they have relative to their volume. The amount of gas an organism produces (or requires) in a given time will be in rough proportion to the volume of its [[cytoplasm]]. The volume of a unicellular organism is very small; thus, it produces (and requires) a relatively small amount of gas in a given time. In comparison to this small volume, the surface area of its [[cell membrane]] is very large, and adequate for its gas-exchange needs without further modification. However, as an organism increases in size, its surface area and volume do not scale in the same way. Consider an imaginary organism that is a cube of side-length, ''L''. Its volume increases with the cube (''L''<sup>3</sup>) of its length, but its external surface area increases only with the square (''L''<sup>2</sup>) of its length. This means the external surface rapidly becomes inadequate for the rapidly increasing gas-exchange needs of a larger volume of cytoplasm. Additionally, the thickness of the surface that gases must cross (d''x'' in Fick's law) can also be larger in larger organisms: in the case of a single-celled organism, a typical cell membrane is only 10&nbsp;nm thick;<ref name="Scneiter1999">{{cite journal|last1=Schneiter|first1=R|last2=Brügger|first2=B|last3=Sandhoff|first3=R|last4=Zellnig|first4=G|last5=Leber|first5=A|last6=Lampl|first6=M|last7=Athenstaedt|first7=K|last8=Hrastnik|first8=C|last9=Eder|first9=S|last10=Daum|first10=G|last11=Paltauf|first11=F|last12=Wieland|first12=FT|last13=Kohlwein|first13=SD|title=Electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of the lipid molecular species composition of yeast subcellular membranes reveals acyl chain-based sorting/remodeling of distinct molecular species en route to the plasma membrane.|journal=The Journal of Cell Biology|date=1999|volume=146|issue=4|pages=741–54|pmid=10459010|doi=10.1083/jcb.146.4.741|pmc=2156145}}</ref> but in larger organisms such as [[Nematode|roundworms]] (Nematoda) the equivalent exchange surface - the cuticle - is substantially thicker at 0.5 μm.<ref name="Cox1981">{{cite journal|last1=Cox|first1=G. N.|title=Cuticle of ''Caenorhabditis elegans'': its isolation and partial characterization|journal=The Journal of Cell Biology|date=1 July 1981|volume=90|issue=1|pages=7–17|doi=10.1083/jcb.90.1.7|pmid=7251677|pmc=2111847}}</ref>

===Interaction with circulatory systems===

[[File:Comparison of con- and counter-current flow exchange.jpg|300px|thumb|right|'''Fig. 2.'''  A comparison between the operations and effects of a '''cocurrent and a countercurrent flow exchange system''' is depicted by the upper and lower diagrams respectively. In both it is assumed (and indicated) that red has a higher value (e.g. of temperature or the partial pressure of a gas) than blue and that the property being transported in the channels therefore flows from red to blue. Note that channels are contiguous if  effective exchange is to occur (i.e. there can be no gap between the channels).]]

In [[multicellular]] organisms therefore, specialised respiratory organs such as gills or lungs are often used to provide the additional surface area for the required rate of gas exchange with the external environment. However the distances between the gas exchanger and the deeper tissues are often too great for diffusion to meet gaseous requirements of these tissues. The gas exchangers are therefore frequently coupled to gas-distributing [[circulatory system]]s, which transport the gases evenly to all the body tissues regardless of their distance from the gas exchanger.<ref>{{cite web |url=http://www.frozenevolution.com/xii5-multicellular-organisms-can-overcome-certain-evolutionary-constraints-imposed-unicellular-organ |title=Frozen Evolution |last=Flegr |first=Jaroslav |website=Frozen Evolution |access-date=21 March 2017}}</ref>

Some multicellular organisms such as [[flatworm]]s (Platyhelminthes) are relatively large but very thin, allowing their outer body surface to act as a gas exchange surface without the need for a specialised gas exchange organ. Flatworms therefore lack gills or lungs, and also lack a circulatory system. Other multicellular organisms such as [[sponges]] (Porifera) have an inherently high surface area, because they are very porous and/or branched. Sponges do not require a circulatory system or specialised gas exchange organs, because their feeding strategy involves one-way pumping of water through their porous bodies using [[flagellum|flagellated]] [[choanocyte|collar cells]]. Each cell of the sponge's body is therefore exposed to a constant flow of fresh oxygenated water. They can therefore rely on diffusion across their cell membranes to carry out the gas exchange needed for respiration.<ref>{{cite web |url=https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-respiratory-system-39/systems-of-gas-exchange-219/the-respiratory-system-and-direct-diffusion-830-12073/ |title=The respiratory system and direct diffusion |website=Boundless |access-date=19 March 2017}}</ref>

In organisms that have circulatory systems associated with their specialized gas-exchange surfaces, a great variety of systems are used for the interaction between the two.

In a [[countercurrent flow]] system, air (or, more usually, the water containing dissolved air) is drawn in the ''opposite'' direction to the flow of blood in the gas exchanger. A countercurrent system such as this maintains a steep concentration gradient along the length of the gas-exchange surface (see lower diagram in Fig. 2). This is the situation seen in the [[Fish gill|gills]] of fish and [[Gill|many other aquatic creatures]].<ref name=campbell /> The gas-containing environmental water is drawn unidirectionally across the gas-exchange surface, with the blood-flow in the gill capillaries beneath flowing in the opposite direction.<ref name=campbell>{{cite book|last1=Campbell|first1=Neil A.|title= Biology|edition= Second|publisher= Benjamin/Cummings Publishing Company, Inc|location= Redwood City, California|date= 1990|pages=836–838|isbn=978-0-8053-1800-5}}</ref><ref name="Hughes1972">{{Cite journal| author=Hughes GM| title=Morphometrics of fish gills| journal=[[Respiration Physiology]]| volume=14| issue=1–2| year=1972| pages=1–25| doi=10.1016/0034-5687(72)90014-x| pmid=5042155}}</ref><ref name=storer>{{cite book|last1=Storer|first1=Tracy I.|last2=Usinger|first2=R. L.|last3=Stebbins|first3=Robert C.|last4=Nybakken|first4=James W.|title=General Zoology|edition=sixth|publisher=McGraw-Hill|location=New York|date=1997|pages=[https://archive.org/details/generalzoolog00stor/page/668 668–670]|isbn=978-0-07-061780-3|url-access=registration|url=https://archive.org/details/generalzoolog00stor/page/668}}</ref> Although this theoretically allows almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood.<ref name=campbell />

Alternative arrangements are [[Bird anatomy#Respiratory system|cross current systems]] found in birds.<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=15|pages=2455–2462|date=2011|doi=10.1242/jeb.052548|pmid=21753038|doi-access=free}}</ref><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> and dead-end air-filled sac systems found in the [[lung]]s of mammals.<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><ref name=tortora1>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofan1987tort |url-access= registration |pages=[https://archive.org/details/principlesofan1987tort/page/570 570–580]|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 978-0-06-350729-6 }}</ref> In a [[Countercurrent exchange#Cocurrent flow—half transfer|cocurrent flow]] system, the blood and gas (or the fluid containing the gas) move in the same direction through the gas exchanger. This means the magnitude of the gradient is variable along the length of the gas-exchange surface, and the exchange will eventually stop when an equilibrium has been reached (see upper diagram in Fig. 2).<ref name=campbell />
Cocurrent flow gas exchange systems are not known to be used in nature.

==Mammals==
The gas exchanger in mammals is internalized to form lungs, as it is in most of the larger land animals.{{citation needed|date=July 2017}} Gas exchange occurs in microscopic dead-end  air-filled sacs called [[pulmonary alveolus|alveoli]], where a very thin membrane (called the [[blood-air barrier]]) separates the blood in the alveolar capillaries (in the walls of the alveoli) from the alveolar air in the sacs.
[[File:An alveolus, is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the.png|thumb|300 px|'''Fig. 3.'''  An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has the form of a hollow cavity. They occur in the mammalian lung. They are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.]][[File:Alveolar Wall.svg|thumb|300 px|left|'''Fig. 4.'''  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&nbsp;I&nbsp;[[epithelial]] cells (or type&nbsp;1&nbsp;[[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the alveolar capillary blood.]]

== Exchange membrane ==

The membrane across which gas exchange takes place in the alveoli (i.e. the blood-air barrier) is extremely thin (in humans, on average, 2.2&nbsp;μm thick).<ref name="grays" /> It consists of the [[Pneumocytes|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium|endothelial cells]] of the pulmonary capillaries (Fig. 4).<ref name="grays" /><ref name="s-cool">{{Cite web| title= Gas Exchange in humans| url=http://www.s-cool.co.uk/a-level/biology/gas-exchange/revise-it/gas-exchange-in-humans| access-date= 19 March 2013}}</ref> The large surface area of the membrane comes from the folding of the membrane into about 300&nbsp;million alveoli, with diameters of approximately 75–300&nbsp;μm each. This provides an extremely large surface area (approximately 145&nbsp;m<sup>2</sup>) across which gas exchange can occur.<ref name="grays" />

===Alveolar air===
[[File:Alveolar air.png|thumb|right|300 px|'''Fig. 5.''' The changes in the composition of the alveolar air during a normal breathing cycle at rest. The scale on the left, and the blue line, indicate the partial pressures of carbon dioxide in kPa, while that on the right and the red line, indicate the partial pressures of oxygen, also in kPa (to convert kPa into mm Hg, multiply by 7.5).]]

[[File:Alveolus.jpg|thumb|300 px|left|'''Fig. 6.''' A diagrammatic histological cross-section through a portion of lung tissue showing a normally inflated [[Pulmonary alveolus|alveolus]] (at the end of a normal exhalation), and its walls containing the [[Pulmonary circulation|alveolar capillaries]] (shown in cross-section). This illustrates how the alveolar capillary blood is completely surrounded by alveolar air. In a normal human lung all the alveoli together contain about 3 liters of alveolar air. All the alveolar capillaries contain about 100 ml blood.]]

[[Atmosphere of Earth|Air]] is brought to the alveoli in small doses (called the [[tidal volume]]), by [[breathing]] in ([[inhalation]]) and out ([[exhalation]]) through the [[Respiratory tract|respiratory airways]], a set of relatively narrow and moderately long tubes which start at the nose or mouth and end in the alveoli of the lungs in the chest. Air moves in and out through the same set of tubes, in which the flow is in one direction during inhalation, and in the opposite direction during exhalation.

During each inhalation, at rest, approximately 500&nbsp;ml of fresh air flows in through the nose. It is warmed and moistened as it flows through the nose and [[pharynx]]. By the time it reaches the trachea the inhaled air's temperature is 37&nbsp;°C and it is saturated with water vapor. On arrival in the alveoli it is diluted and thoroughly mixed with the approximately  2.5–3.0&nbsp;liters of air that remained in the alveoli after the last exhalation. This relatively large volume of air that is semi-permanently present in the alveoli throughout the breathing cycle is known as the [[functional residual capacity]] (FRC).<ref name=tortora1 />

At the beginning of inhalation the airways are filled with unchanged alveolar air, left over from the last exhalation. This is the [[Dead space (physiology)|dead space]] volume, which is usually about 150&nbsp;ml.<ref>{{cite web|title=Dead space volume - Oxford Reference|url=http://www.oxfordreference.com/view/10.1093/oi/authority.20110803095704195}}</ref> It is the first air to re-enter the alveoli during inhalation. Only after the dead space air has returned to the alveoli does the remainder of the tidal volume (500&nbsp;ml - 150&nbsp;ml = 350&nbsp;ml) enter the alveoli.<ref name=tortora1 /> The entry of such a small volume of fresh air with each inhalation, ensures that the composition of the FRC hardly changes during the breathing cycle (Fig. 5).<ref name=tortora1 /> The alveolar [[Pulmonary gas pressures|partial pressure of oxygen]] remains very close to 13–14&nbsp;[[Pascal (unit)|kPa]] (100&nbsp;mmHg), and the [[Pulmonary gas pressures#Partial pressure of carbon dioxide|partial pressure of carbon dioxide]] varies minimally around 5.3&nbsp;kPa (40&nbsp;mmHg) throughout the breathing cycle (of inhalation and exhalation).<ref name=tortora1 /> The corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21&nbsp;kPa (160&nbsp;mmHg) and 0.04&nbsp;kPa (0.3&nbsp;mmHg) respectively.<ref name=tortora1 />
[[File:Gas exchange.jpg|thumb|right|300 px|'''Fig. 7.''' A highly diagrammatic illustration of the process of gas exchange in the mammalian lungs, emphasizing the differences between the gas compositions of the ambient air, the alveolar air (light blue) with which the alveolar capillary blood equilibrates, and the blood gas tensions in the pulmonary arterial (blue blood entering the lung on the left) and venous blood (red blood leaving the lung on the right). All the gas tensions are in kPa. To convert to mm Hg, multiply by 7.5.]]

This alveolar air, which constitutes the FRC, completely surrounds the blood in the alveolar capillaries (Fig. 6). Gas exchange in mammals occurs between this alveolar air (which differs significantly from fresh air) and the blood in the alveolar capillaries. The gases on either side of the gas exchange membrane equilibrate by simple diffusion. This ensures that the partial pressures of oxygen and carbon dioxide in the blood leaving the alveolar capillaries, and ultimately circulates throughout the body, are the same as those in the FRC.<ref name=tortora1 />

The marked difference between the composition of the alveolar air and that of the ambient air can be maintained because the [[functional residual capacity]] is contained in dead-end sacs connected to the outside air by long, narrow, tubes (the airways: [[nose]], [[pharynx]], [[larynx]], [[trachea]], [[bronchi]] and their branches and sub-branches down to the [[bronchioles]]). This anatomy, and the fact that the lungs are not emptied and re-inflated with each breath, provides mammals with a "portable atmosphere", whose composition differs significantly from the [[Great Oxygenation Event|present-day ambient air]].<ref>{{cite book |last1=Lovelock |first1=James | title=Healing Gaia: Practical medicine for the Planet|url=https://archive.org/details/healinggaiaprac00love |url-access=registration |pages=[https://archive.org/details/healinggaiaprac00love/page/21 21]–34, 73–88|location=New York |publisher=Harmony Books |date=1991|isbn= 978-0-517-57848-3}}</ref>

The composition of the air in the FRC is carefully monitored, by measuring the partial pressures of oxygen and carbon dioxide in the arterial blood. If either gas pressure deviates from normal, reflexes are elicited that change the rate and depth of breathing in such a way that normality is restored within seconds or minutes.<ref name=tortora1 />

===Pulmonary circulation===
{{Main |Pulmonary circulation}}
All the blood returning from the body tissues to the right side of the [[heart]] flows through the [[Pulmonary circulation|alveolar capillaries]] before being pumped around the body again. On its passage through the lungs the blood comes into close contact with the alveolar air, separated from it by a very thin diffusion membrane which is only, on average, about 2&nbsp;μm thick.<ref name=grays /> The gas pressures in the blood will therefore rapidly equilibrate with those in the [[Pulmonary alveolus|alveoli]], ensuring that the arterial blood that circulates to all the tissues throughout the body has an [[Blood gas tension|oxygen tension]] of 13−14&nbsp;kPa (100&nbsp;mmHg), and a [[Blood gas tension|carbon dioxide tension]] of 5.3&nbsp;kPa (40&nbsp;mmHg). These arterial partial pressures of oxygen and carbon dioxide are [[Homeostasis#Levels of blood gases|homeostatically controlled]]. A rise in the arterial <math>P_{{\mathrm{CO}}_2}</math>, and, to a lesser extent, a fall in the arterial <math>P_{{\mathrm{O}}_2}</math>, will reflexly cause deeper and faster breathing until the blood gas tensions return to normal. The converse happens when the carbon dioxide tension falls, or, again to a lesser extent, the oxygen tension rises: the rate and depth of breathing are reduced until blood gas normality is restored.

Since the blood arriving in the alveolar capillaries has a <math>P_{{\mathrm{O}}_2}</math> of, on average, 6&nbsp;kPa (45&nbsp;mmHg), while the pressure in the alveolar air is 13&nbsp;kPa (100&nbsp;mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3&nbsp;liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has a <math>P_{{\mathrm{CO}}_2}</math> of also about 6&nbsp;kPa (45&nbsp;mmHg), whereas that of the alveolar air is 5.3&nbsp;kPa (40&nbsp;mmHg), there is a net movement of carbon dioxide out of the capillaries into the alveoli. The changes brought about by these net flows of individual gases into and out of the functional residual capacity necessitate the replacement of about 15% of the alveolar air with ambient air every  5 seconds or so. This is very tightly controlled by the continuous monitoring of the arterial blood gas tensions (which accurately reflect partial pressures  of the respiratory gases in the alveolar air) by the [[Aortic body|aortic bodies]], the [[Carotid body|carotid bodies]], and the [[Central chemoreceptors|blood gas and pH sensor]] on the anterior surface of the [[medulla oblongata]] in the brain. There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the [[bronchioles]] and [[Pulmonary circulation|pulmonary capillaries]], and are therefore responsible for directing the flow of air and blood to different parts of the lungs.

It is only as a result of accurately maintaining the composition of the 3 liters alveolar air that with each breath some carbon dioxide is discharged into the atmosphere and some oxygen is taken up from the outside air. If more carbon dioxide than usual has been lost by a short period of [[hyperventilation]], respiration will be slowed down or halted until the alveolar <math>P_{{\mathrm{CO}}_2}</math> has returned to 5.3&nbsp;kPa (40&nbsp;mmHg). It is therefore strictly speaking untrue that the primary function of the respiratory system is to rid the body of carbon dioxide "waste". In fact the total concentration of carbon dioxide in arterial blood  is about 26&nbsp;mM (or 58&nbsp;ml per 100 ml),<ref name=ciba>{{cite book |last1=Diem |first1=K. | last2=Lentner |first2=C. | chapter= Blood – Inorganic substances| title= in: Scientific Tables | edition= Seventh |location=Basle, Switzerland |publisher=CIBA-GEIGY Ltd. |date=1970 |page=571}}</ref> compared to the concentration of oxygen in saturated arterial blood of about 9&nbsp;mM (or 20&nbsp;ml per 100 ml blood).<ref name=tortora1 /> This large concentration of carbon dioxide plays a pivotal role in the [[Acid-base homeostasis|determination and maintenance of the pH of the extracellular fluids]]. The carbon dioxide that is breathed out with each breath could probably be more correctly be seen as a byproduct of the body's extracellular fluid [[Homeostasis#Levels of blood gases|carbon dioxide]] and  [[Homeostasis#Blood pH|pH homeostats]]

If these homeostats are compromised, then a [[respiratory acidosis]], or a [[respiratory alkalosis]] will occur. In the long run these can be compensated by renal adjustments to the H<sup>+</sup> and HCO<sub>3</sub><sup>−</sup> concentrations in the plasma; but since this takes time, the [[hyperventilation syndrome]] can, for instance, occur when agitation or anxiety cause a person to breathe fast and deeply<ref>{{cite journal|last=Shu|first=BC |author2=Chang, YY |author3=Lee, FY |author4=Tzeng, DS |author5=Lin, HY |author6=Lung, FW|title=Parental attachment, premorbid personality, and mental health in young males with hyperventilation syndrome.|journal=Psychiatry Research|date=2007-10-31|volume=153|issue=2|pages=163–70|pmid=17659783|doi=10.1016/j.psychres.2006.05.006|s2cid=3931401 }}</ref> thus blowing off too much CO<sub>2</sub> from the blood into the outside air, precipitating a set of distressing symptoms which result from an excessively high pH of the extracellular fluids.<ref name="Edward Newton">{{cite web |url=http://www.emedicine.com/emerg/topic270.htm |title=eMedicine - Hyperventilation Syndrome: Article by Edward Newton, MD |access-date=2007-12-20 }}</ref>

Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with [[hemoglobin]]. The oxygen is held on the hemoglobin by four [[Iron(II) oxide|ferrous iron]]-containing [[heme]] groups per hemoglobin molecule. When all the heme groups carry one O<sub>2</sub> molecule each the blood is said to be "saturated" with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase the oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as HCO<sub>3</sub><sup>−</sup> ions in the plasma. However the conversion of dissolved CO<sub>2</sub> into HCO<sub>3</sub><sup>−</sup> (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and alveolar capillaries on the other. The reaction is therefore catalyzed by [[carbonic anhydrase]], an [[enzyme]] inside the [[red blood cell]]s.<ref name="Raymond&Swenson2000">{{Cite journal|vauthors=Raymond H, Swenson E | title=The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs| journal=[[Respiration Physiology]]| volume=121| issue=1| year=2000| pages=1–12| doi=10.1016/s0034-5687(00)00110-9| pmid=10854618}}</ref> The reaction can go in either direction depending on the prevailing partial pressure of carbon dioxide. A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as [[carbamino]] groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO<sub>2</sub>, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26&nbsp;mM (or 58&nbsp;ml/100 ml),<ref name=ciba /> compared to the concentration of oxygen in saturated arterial blood of about 9&nbsp;mM (or 20&nbsp;ml/100 ml blood).<ref name=tortora1 />

==Other vertebrates==
===Fish===
[[File:Tuna Gills in Situ 01.jpg|300 px|thumb|left|'''Fig. 8.''' Gills of tuna showing filaments and lamellae]]

The dissolved oxygen content in [[fresh water]] is approximately 8–10&nbsp;milliliters per liter compared to that of air which is 210&nbsp;milliliters per liter.<ref name="Advanced Biology">{{cite book|title=Advanced Biology|author1=M. b. v. Roberts |author2=Michael Reiss |author3=Grace Monger |pages=164–165|publisher=Nelson|year=2000|location=London, UK}}</ref> Water is 800 times more dense than air<ref name=tyson>{{cite book|last1=Tyson|first1=P. D.|last2=Preston-White|first2=R.A.|title=The Weather and Climate of Southern Africa|edition=Second|date=2013|publisher=Oxford University Press| location=Cape Town, South Africa|page=14|isbn=9780195718065}}</ref> and 100 times more viscous.<ref name="Advanced Biology"/> Therefore, oxygen has a diffusion rate in air 10,000 times greater than in water.<ref name="Advanced Biology"/> The use of sac-like lungs to remove oxygen from water would therefore not be efficient enough to sustain life.<ref name="Advanced Biology"/> Rather than using lungs, gaseous exchange takes place across the surface of highly vascularized [[Fish gill|gill]]s. Gills are specialised organs containing [[Gill filament|filaments]], which further divide into [[lamella (anatomy)|lamellae]]. The lamellae contain [[capillaries]] that provide a large surface area and short diffusion distances, as their walls are extremely thin.<ref name="Newstead1967">{{Cite journal| author=Newstead James D | title=Fine structure of the respiratory lamellae of teleostean gills| journal=[[Cell and Tissue Research]]| volume=79| issue=3| year=1967| pages=396–428| doi=10.1007/bf00335484| pmid=5598734| s2cid=20771899}}</ref> Gill rakers are found within the exchange system in order to filter out food, and keep the gills clean.

Gills use a [[countercurrent flow]] system that increases the efficiency of oxygen-uptake (and waste gas loss).<ref name=campbell /><ref name="Hughes1972" /><ref name=storer/> Oxygenated water is drawn in through the mouth and passes over the gills in one direction while blood flows through the lamellae in the opposite direction. This [[countercurrent exchange|countercurrent]] maintains steep concentration gradients along the entire length of each capillary (see the diagram in the [[#Interaction with circulatory systems|"Interaction with circulatory systems"]] section above). Oxygen is able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water.<ref name="Hughes1972"/> The deoxygenated water will eventually pass out through the [[Operculum (fish)|operculum]] (gill cover). Although countercurrent exchange systems theoretically allow an almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood.<ref name=campbell />

===Amphibians===
Amphibians have three main organs involved in gas exchange: the lungs, the skin, and the gills, which can be used singly or in a variety of different combinations. The relative importance of these structures differs according to the age, the environment and species of the amphibian. The skin of amphibians and their larvae are highly vascularised, leading to relatively efficient gas exchange when the skin is moist. The larvae of amphibians, such as the pre-metamorphosis [[tadpole]] stage of [[frog]]s, also have external [[gills]]. The gills are absorbed into the body during [[metamorphosis]], after which the lungs will then take over. The lungs are usually simpler than in the [[amniote|other land vertebrates]], with few internal septa and larger alveoli; however, toads, which spend more time on land, have a larger alveolar surface with more developed lungs. To increase the rate of gas exchange by diffusion, amphibians maintain the concentration gradient across the respiratory surface using a process called [[buccal pumping]].<ref name="Brainerd">{{cite journal |last=Brainerd |first=E. L.  |date=1999 |title=New perspectives on the evolution of lung ventilation mechanisms in invertebrates|journal=Experimental Biology Online |volume=4 |issue= 2|pages=1–28 |doi=10.1007/s00898-999-0002-1 |bibcode=1999EvBO....4b...1B |s2cid=35368264 }}</ref> The lower floor of the mouth is moved in a "pumping" manner, which can be observed by the naked eye.

===Reptiles===
All [[reptile]]s breathe using lungs. In [[squamate]]s (the [[lizards]] and [[snakes]]) ventilation is driven by the [[core (anatomy)|axial musculature]], but this musculature is also used during movement, so some squamates rely on [[buccal pumping]] to maintain gas exchange efficiency.<ref name ="reptiles">{{cite journal |last1=Taylor |first1=E. W. |last2=Campbell |first2= H. A.|last3=Leite|first3=C|last4=Abe|first4=A. S.|last5=Wang|first5=T|title= Respiration in reptiles |journal= Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=148  |pages=S110–S111 |doi=10.1016/j.cbpa.2007.06.431 |year=2007}}</ref>

Due to the rigidity of [[turtle]] and [[tortoise]] shells, significant expansion and contraction of the chest is difficult. Turtles and tortoises depend on muscle layers attached to their shells, which wrap around their lungs to fill and empty them.<ref name="Klein2003">{{cite journal | last=Klein | first=Wilfied |author2=Abe, Augusto |author3=Andrade, Denis |author4= Perry, Steven | title=Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, ''Tupinambis merianae'' (Teidae: Reptilia) | journal=Journal of Morphology | volume=258 | issue=2 | year=2003 | pages=151–157 | doi=10.1002/jmor.10136 | pmid=14518009| s2cid=9901649 }}</ref> Some aquatic turtles can also pump water into a highly vascularised mouth or [[cloaca]] to achieve gas-exchange.<ref name="Orenstein 2001">{{cite book | last=Orenstein | first=Ronald | title=Turtles, Tortoises & Terrapins: Survivors in Armor | publisher=Firefly Books | year=2001 | isbn=978-1-55209-605-5 | url-access=registration | url=https://archive.org/details/turtlestortoises0000oren }}</ref><ref name="Feder1985">{{cite journal|last=Feder|first=Martin E.|author2=Burggren, Warren W. |title=Cutaneous gas exchange in vertebrates: design, patterns, control and implications|journal=Biological Reviews|date=1985|volume=60|issue=1|pages=1–45|doi=10.1111/j.1469-185X.1985.tb00416.x|pmid=3919777|s2cid=40158158|url=http://www.biol.unt.edu/~burggren/pdfs/1985/%2849%29Feder,Burggren1985BR.pdf}}</ref>

[[Crocodile]]s have a structure similar to the mammalian diaphragm - the diaphragmaticus - but this muscle helps create a unidirectional flow of air through the lungs rather than a tidal flow: this is more similar to the air-flow seen in [[birds]] than that seen in mammals.<ref>{{cite journal|last=Farmer|first=CG|author2=Sanders, K |title=Unidirectional airflow in the lungs of alligators|journal=Science|year=2010|volume=327|issue=5963|pages=338–340|doi=10.1126/science.1180219|pmid=20075253|bibcode=2010Sci...327..338F|s2cid=206522844}}</ref> During inhalation, the diaphragmaticus pulls the liver back, inflating the lungs into the space this creates.<ref name="Farmer2012">{{cite journal |author1=Farmer, C. G. |author2=Carrier D. R. | year=2000 | title=Pelvic aspiration in the American alligator (''Alligator mississippiensis'') | journal=Journal of Experimental Biology | volume=203 |issue=11 | pages=1679–1687 |doi=10.1242/jeb.203.11.1679 |pmid=10804158}}</ref><ref>{{cite journal|author1=Munns, S. L. |author2=Owerkowicz, T. |author3=Andrewartha, S. J. |author4=Frappell, P. B. | year=2012| title=The accessory role of the diaphragmaticus muscle in lung ventilation in the estuarine crocodile ''Crocodylus porosus''| journal=Journal of Experimental Biology| volume=215| pages=845–852|doi=10.1242/jeb.061952| issue=5 | pmid=22323207| doi-access=free}}</ref> Air flows into the lungs from the bronchus during inhalation, but during exhalation, air flows out of the lungs into the bronchus by a different route: this one-way movement of gas is achieved by aerodynamic valves in the airways.<ref>{{Cite journal | author1=Farmer, C. G. | author2=Sanders, K. | year=2010 | title=Unidirectional airflow in the lungs of alligators | journal=Science | volume=327 | issue=5963 | pages=338–340 | doi=10.1126/science.1180219 | pmid=20075253 | url=http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./Unidirectional%20Airflow%20in%20the%20Lungs%20of%20Alligators.pdf | bibcode=2010Sci...327..338F | s2cid=206522844 | access-date=2017-04-20 | archive-url=https://web.archive.org/web/20160624213901/http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./Unidirectional%20Airflow%20in%20the%20Lungs%20of%20Alligators.pdf | archive-date=2016-06-24 | url-status=dead }}</ref><ref>{{cite journal |author1=Schachner, E. R. |author2=Hutchinson, J. R. |author3=Farmer, C. | year=2013 | title=Pulmonary anatomy in the Nile crocodile and the evolution of unidirectional airflow in Archosauria | journal=PeerJ |volume=1 |page=e60 |doi=10.7717/peerj.60 |pmid=23638399 |pmc=3628916 |doi-access=free }}</ref>

===Birds===
{{Main|Bird anatomy#Respiratory system}}
[[File:BirdRespiration.svg|thumb|right|'''Fig. 10.''' Inhalation-exhalation cycle in birds.]]
[[File:Cross-current exchanger.jpg|thumb|300 px|left|'''Fig. 9.'''  A diagrammatic representation of the cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs 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= graham /> 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.]]
Birds have [[Bird anatomy#Respiratory system|lungs but no diaphragm]]. They rely mostly on [[air sacs]] for [[Ventilation (physiology)|ventilation]]. These air sacs do not play a direct role in gas exchange, but help to move air unidirectionally across the gas exchange surfaces in the lungs. During inhalation, fresh air is taken from the trachea down into the posterior air sacs and into the [[parabronchi]] which lead from the posterior air sacs into the lung. The air that enters the lungs joins the  air which is already in the lungs, and is drawn forward across the gas exchanger into anterior air sacs. During exhalation, the posterior air sacs force air into the same [[parabronchi]] of the lungs, flowing in the same direction as during inhalation, allowing continuous gas exchange irrespective of the breathing cycle. Air exiting the lungs during exhalation joins the air being expelled from the anterior air sacs (both consisting of "spent air" that has passed through the gas exchanger) entering the trachea to be  exhaled  (Fig. 10).<ref name=AvResp /> Selective [[bronchoconstriction]] at the various bronchial branch points ensures that the air does not ebb and flow through the bronchi during inhalation and exhalation, as it does in mammals, but follows the paths described above.

The unidirectional airflow through the parabronchi exchanges respiratory gases with a ''crosscurrent'' blood flow (Fig. 9).<ref name="graham"/><ref name="AvResp"/> The partial pressure of O<sub>2</sub> (<math>P_{{\mathrm{O}}_2}</math>) in the parabronchioles declines along their length as O<sub>2</sub> diffuses into the blood. The capillaries leaving the exchanger near the entrance of airflow take up more O<sub>2</sub> than capillaries leaving near the exit end of the parabronchi.  When the contents of all capillaries mix, the final <math>P_{{\mathrm{O}}_2}</math> of the mixed pulmonary venous blood is higher than that of the exhaled air, but lower than that of the inhaled air.<ref name= graham /><ref name=AvResp />

==Plants==
Gas exchange in plants is dominated by the roles of carbon dioxide, oxygen and [[water vapor]]. {{Chem|CO|2}} is the only carbon source for [[autotroph]]ic growth by [[photosynthesis]], and when a plant is actively photosynthesising in the light, it will be taking up carbon dioxide, and [[transpiration|losing water vapor]] and oxygen. At night, plants [[cellular respiration|respire]], and gas exchange partly reverses: water vapor is still lost (but to a smaller extent), but oxygen is now taken up and carbon dioxide released.<ref name = "Whitmarsh_1999">{{cite book | veditors = Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee | title = Concepts in Photobiology: Photosynthesis and Photomorphogenesis | chapter = Chapter 2: The Basic Photosynthetic Process | vauthors = Whitmarsh J, Govindjee | year = 1999 | publisher = Kluwer Academic Publishers | location = Boston | isbn = 978-0-7923-5519-9 | chapter-url = https://books.google.com/books?id=dqSuoOtDM1cC&q=photosynthesis+reaction+equation+generalized&pg=PA13 | page = 13 }}</ref>

[[File:Leaf anatomy.svg|thumb|400 px|'''Fig. 11.''' A stylised cross-section of a [[euphyllophyte]] plant leaf, showing the key plant organs involved in gas exchange]]
Plant gas exchange occurs mostly through the leaves. Gas exchange between a leaf and the atmosphere occurs simultaneously through two pathways: 1) epidermal cells and cuticular waxes (usually referred as '[[Plant cuticle|cuticle]]') which are always present at each leaf surface, and 2) [[stoma]]ta, which typically control the majority of the exchange.<ref>{{Cite journal|last1=Márquez|first1=Diego A.|last2=Stuart-Williams|first2=Hilary|last3=Farquhar|first3=Graham D.|date=2021-03-01|title=An improved theory for calculating leaf gas exchange more precisely accounting for small fluxes|url=https://www.nature.com/articles/s41477-021-00861-w|journal=Nature Plants|volume=7|issue=3|language=en|pages=317–326|doi=10.1038/s41477-021-00861-w|pmid=33649595 |bibcode=2021NatPl...7..317M |s2cid=232090898 |issn=2055-0278|hdl=1885/238421|hdl-access=free}}</ref> Gases enter into the photosynthetic tissue of the leaf through dissolution onto the moist surface of the palisade and spongy [[mesophyll]] cells. The spongy mesophyll cells are loosely packed, allowing for an increased surface area, and consequently an increased rate of gas-exchange. Uptake of carbon dioxide necessarily results in some loss of water vapor,<ref name=" K. Raschke, 1976">{{Cite journal| author=K. Raschke | title= How Stomata Resolve the Dilemma of Opposing Priorities | journal =[[Phil. Trans. R. Soc. Lond. B]]| volume=273 | issue= 927 |year=1976  | pages=551–560 | doi=10.1098/rstb.1976.0031| bibcode=1976RSPTB.273..551R| doi-access=free }}</ref> because both molecules enter and leave by the same stomata, so plants experience a gas exchange dilemma: gaining enough {{Chem|CO|2}} without losing too much water. Therefore, water loss from other parts of the leaf is minimised by the waxy cuticle on the leaf's [[epidermis]]. The size of a stoma is regulated by the opening and closing of its two [[guard cell]]s: the turgidity of these cells determines the state of the stomatal opening, and this itself is regulated by water stress. Plants showing [[crassulacean acid metabolism]] are drought-tolerant [[xerophyte]]s and perform almost all their gas-exchange at night, because it is only during the night that these plants open their stomata. By opening the stomata only at night, the water vapor loss associated with carbon dioxide uptake is minimised. However, this comes at the cost of slow growth: the plant has to store the carbon dioxide in the form of [[malic acid]] for use during the day, and it cannot store unlimited amounts.<ref name="Ting2985">{{cite journal|doi=10.1146/annurev.pp.36.060185.003115|title=Crassulacean Acid Metabolism|year=1985|last1=Ting|first1=I P|journal=Annual Review of Plant Physiology|volume=36|issue=1|pages=595–622|hdl=10150/552219|hdl-access=free}}</ref>

[[File:portable gas exchange measurements.jpg|thumb|right|alt=see adjacent text| '''Fig. 12.'''  High precision gas exchange measurements reveal important information on [[plant physiology]]]]

Gas exchange measurements are important tools in plant science: this typically involves sealing the plant (or part of a plant) in a chamber and measuring changes in the concentration of carbon dioxide and water vapour with an [[infrared gas analyzer]]. If the environmental conditions ([[humidity]], {{Chem|CO|2}} concentration, light and [[temperature]]) are fully controlled, the measurements of {{Chem|CO|2}} uptake and water release reveal important information about the {{Chem|CO|2}} [[Assimilation (biology)|assimilation]] and [[transpiration]] rates. The intercellular {{Chem|CO|2}} concentration reveals important information about the photosynthetic condition of the plants.<ref name="Von Caemmerer and Farquhar, 1981">{{Cite journal|author1=S Von Caemmerer |author2=GD Farquhar | title= Some relationships between the biochemistry of photosynthesis and gas exchange of leaves |journal =[[Planta (journal)|Planta]]| volume=153 |issue=4 |year=1981 | pages=376–387 | doi=10.1007/bf00384257|pmid=24276943 |title-link=photosynthesis |bibcode=1981Plant.153..376V |s2cid=22760377 }}</ref><ref>{{citation |title=Portable Gas Exchange Fluorescence System GFS-3000. Handbook of Operation |date=March 20, 2013 |url=http://www.walz.com/downloads/manuals/gfs-3000/gfs-3000_Manual_8a.pdf |access-date=October 21, 2014 |archive-date=December 15, 2017 |archive-url=https://web.archive.org/web/20171215121153/http://www.walz.com/downloads/manuals/gfs-3000/gfs-3000_Manual_8a.pdf |url-status=dead }}</ref> Simpler methods can be used in specific circumstances: [[hydrogencarbonate indicator]] can be used to monitor the consumption of {{Chem|CO|2}} in a solution containing a single plant leaf at different levels of light intensity,<ref name="BBC">[https://www.bbc.co.uk/education/guides/zxtcwmn/revision/3 BBC Bitesize - GCSE Biology - Gas exchange in plants]</ref> and oxygen generation by the pondweed ''[[Elodea]]'' can be measured by simply collecting the gas in a submerged test-tube containing a small piece of the plant.

==Invertebrates==

The mechanism of gas exchange in invertebrates depends their size, feeding strategy, and habitat (aquatic or terrestrial).

[[File:Porifera body structures 01.png|thumb|'''Fig. 13.'''  Diagram representing the body structure of Porifera. The diagram shows the mechanism of water uptake for sponges. Yellow: [[pinacocytes]], red: choanocytes, grey: [[mesohyl]], pale blue: water flow]]

The [[sponge]]s (Porifera) are sessile creatures, meaning they are unable to move on their own and normally remain attached to their [[Substrate (marine biology)|substrate]]. They obtain nutrients through the flow of water across their cells, and they exchange gases by simple diffusion across their cell membranes. Pores called [[Ostium (sponges)|ostia]] draw water into the sponge and the water is subsequently circulated through the sponge by cells called [[choanocyte]]s which have [[flagellum|hair-like structures]] that move the water through the sponge.<ref>Anderson, D. (2001) ''Invertebrate Zoology'' Oxford University Press</ref>

[[File:Coral reef at palmyra.jpg|thumb|'''Fig. 14.'''  Cnidarians are always found in aquatic environments, meaning that their gas exchange involves absorbing oxygen from water.]]

The [[cnidarian]]s include [[corals]], [[sea anemones]], [[jellyfish]] and [[hydras]]. These animals are always found in aquatic environments, ranging from fresh water to salt water. They do not have any dedicated [[respiratory organs]]; instead, every cell in their body can absorb oxygen from the surrounding water, and release waste gases to it. One key disadvantage of this feature is that cnidarians can die in environments where water is [[water stagnation|stagnant]], as they deplete the water of its [[oxygen]] supply.<ref name="cnidarians">{{cite web |url=http://study.com/academy/lesson/cnidaria-respiratory-system.html |title= Cnidarian Respiratory System|author=<!--Not stated--> |website=study.com |access-date=20 March 2017}}</ref> Corals often form symbiosis with other organisms, particularly photosynthetic [[dinoflagellate]]s. In this [[symbiosis]], the [[coral]] provides shelter and the other organism provides nutrients to the coral, including oxygen.{{citation needed|date=November 2023}}

[[File:Giant roundworm (265 11) Cross-section.jpg|thumb|180 px|left|'''Fig. 15.''' Cross section of a nematode.]]

The [[nematode|roundworms]] (Nematoda), [[flatworm]]s (Platyhelminthes), and many other small invertebrate animals living in aquatic or otherwise wet habitats do not have a dedicated gas-exchange surface or circulatory system. They instead rely on [[diffusion]] of {{Chem|CO|2}} and {{Chem|O|2}} directly across their cuticle.<ref>{{cite web |url=http://study.com/academy/lesson/nematode-respiratory-system.html |title= Nematode Respiratory System|author=<!--Not stated--> |website=study.com |access-date=20 March 2017}}</ref><ref>{{cite web |url=http://rspp.weebly.com/platyhelminthes.html |title= Platyhelminthes Respiratory System|author=<!--Not stated--> |website=rspp.weebly.com|access-date=20 March 2017}}</ref> The cuticle is the [[semi-permeable]] outermost layer of their bodies.{{citation needed|date=November 2023}}

Other aquatic invertebrates such as most [[mollusc]]s (Mollusca) and larger [[crustacean]]s (Crustacea) such as [[lobster]]s, have gills analogous to those of fish, which operate in a similar way.

[[File:Actias selene 5th instar spiracles sjh.jpg|thumb|left|200 px|'''Fig. 16.''' Photographic representation of spiracles.]]
Unlike the invertebrates groups mentioned so far, [[insect]]s are usually terrestrial, and exchange gases across a moist surface in direct contact with the atmosphere, rather than in contact with surrounding water. The insect's [[exoskeleton]] is impermeable to gases, including water vapor, so they have a more specialised gas exchange system, requiring gases to be directly transported to the tissues via a complex network of tubes. This respiratory system is separated from their circulatory system. Gases enter and leave the body through openings called [[Spiracle (arthropods)|spiracle]]s, located laterally along the [[thorax]] and [[abdomen]]. Similar to plants, insects are able to control the opening and closing of these spiracles, but instead of relying on [[turgor pressure]], they rely on [[muscle contraction]]s.<ref>{{cite journal |last1=Lane |first1=N. J. |last2=Harrison |first2= J. B. |date=1986 |title= Junctions and the cytoskeleton in insect tissues |journal=Journal of Cell Biology |volume=103 |issue=5 |pages= A69 }}</ref> These [[muscle contractions|contractions]] result in an insect's abdomen being pumped in and out. The spiracles are connected to tubes called [[tracheae]], which branch repeatedly and ramify into the insect's body. These branches terminate in specialised [[tracheole|tracheole cells]] which provides a thin, moist surface for efficient gas exchange, directly with cells.<ref>Klowden, M. J. 2007. Physiological systems in insects. Elsevier/Academic Press. pp. 440–442</ref>

The other main group of terrestrial [[arthropod]], the [[arachnid]]s ([[spider]]s, [[scorpion]], [[mite]]s, and their relatives) typically perform gas exchange with a [[book lung]].<ref name="Garwood">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y  |name-list-style=amp|doi-access=free }}</ref>

==Summary of main gas exchange systems==

{| class="wikitable"
|-
!  !! Surface area !! Diffusion distance !! Maintaining concentration gradient !! Respiratory organs
|-
| '''Human''' || Total alveoli<ref name="Basset1987">{{Cite journal|vauthors=Basset J, Crone C, Saumon G | title=Significance of active ion transport in transalveolar water absorption: a study on isolated rat lung| journal=[[The Journal of Physiology]]| volume=384| year=1987| pages=311–324| doi=10.1113/jphysiol.1987.sp016456| pmid=3656149| pmc=1192264}}</ref> = 70–100&nbsp;m<sup>2</sup>|| Alveolus and capillary (two cells)|| Constant blood flow in capillaries; breathing || Lungs
|-
| '''Fish''' || Many lamellae and filaments per gill || Usually one cell || Countercurrent flow || Gills
|-
| '''Insects''' || Specialised tracheole cell || One cell || Buccal pumping || Spiracles
|-
| '''Sponges''' || Ostia pores || One cell || Water movement || None
|-
| '''Flatworms''' || Flat body shape || Usually one cell || Countercurrent flow || None
|-
| '''Cnidarians''' || Oral arms || Usually one cell || Water movement || None
|-
| '''Reptiles''' || Many lamellae and filaments per gill{{clarify|reason=what gills?|date=April 2020}} || Alveolus and capillary (two cells) || Countercurrent flow || Lungs
|-
| '''Amphibians''' || Many lamellae and filaments per gill || Alveolus and capillary (two cells) or one cell || Countercurrent flow || Lungs, skin and gills
|-
| '''Plants''' || High density of stomata; air spaces within leaf || One cell || Constant air flow || Stomata
|}

==See also==
* {{annotated link|Respiratory system}}

==References==
{{Reflist}}

{{authority control}}

[[Category:Biological processes]]
[[Category:Gases|*]]