Editing Gas exchange (section)

===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.