Editing Gas exchange (section)

==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.