<Dd> d q d t = J A (\ displaystyle (\ frac (dq) (dt)) = JA) </Dd> <P> Single - celled organisms such as bacteria and amoebae 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, therefore 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) of its length, but its external surface area increases only with the square (L) 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 (dx 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 nm thick; but in larger organisms such as roundworms (Nematoda) the equivalent exchange surface - the cuticle - is substantially thicker at 0.5 μm . </P> <P> 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 systems, which transport the gases evenly to all the body tissues regardless of their distance from the gas exchanger . </P> <P> Some multicellular organisms such as flatworms (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 flagellated 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 . </P>

Which body structure provides actual diffusion of respiratory gases in lungs