Editing Gas exchange (section)

===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>