10 2 Water Breathers As animals get larger their surface area SA and volume V increase but volume increases a factor faster For example for a sphere V 4 3 r3 and SA 4 r2 As radius increases both volume and surface area will increase but volume will increase faster because r is raised to the third power instead of the second as in surface area Thus the area inside the animal that needs oxygen and is producing waste will increase faster than the surface over which diffusion of these substances occurs If we use Fick s Law to predict how much oxygen can diffuse across a given surface we would predict that a unicellular animal can only be about a millimeter in diameter In unicellular animals we see that metabolic rate volume tissue in grams decreases with size to compensate for a slow increase in SA Multicellular animals alter the surface area of their respiratory surface skin gills lungs to accommodate larger bodies For example soil nematodes roundworms are usually less than 7 mm long and other worms can be up to 1 m long However none exceed a few mm thick Any thicker animals have one of three major strategies for gas transport 1 circulate the external medium to complement diffusion across the skin 2 circulatory transport to assist gas diffusion across respiratory surface cutaneous respiration or 3 a specialized respiratory surface circulatory transport As you have seen before only thin simple animals are able to survive without circulatory systems sponges cnidarians insects Aquatic animals that lack circulatory systems often have cavities lined with ciliated or flagellated cells like choanocytes in sponges that facilitate bulk flow of the surrounding water Cnidarians can use muscle contractions to move the water through the mouth into the internal cavity then back out of the mouth The figure below shows how extensive cutaneous respiration is across vertebrates Although gas exchange across the skin occurs in a variety of animals it is most prominent in amphibians Cutaneous respiration requires a capillary bed that underlies the skin and no specialized respiratory organs Although this is a simple system it has several limitations The skin must be very thin for diffusion to occur at a useful rate see Fick s Law which leaves the animal vulnerable to injury and predation Recall that gases must dissolve before they can diffuse into terrestrial animals so the skin must be kept moist which limits the habitat of the animal Because of these two factors the size of the skin over which cutaneous respiration occurs is usually limited Finally an animal can combine a specialized respiratory surface that is optimized for diffusion of gases such as the gills or lungs with a circulatory system to transport those gases throughout the body Gills originate as evaginations of the body surface during embryonic development and can either remain external or be internalized by covering them with a flap Gills are most common in aquatic animals Lungs originate as invaginations of the body surface during embryonic development forming an internal body cavity that contains the external medium Lungs are most common in terrestrial animals although the lungfish is a notable exception Recall that specialized respiratory surfaces are often ventilated where the animal expends energy to increase the bulk flow of the external medium over the respiratory surface The type of ventilation employed is determined by the anatomy of the respiratory organ surface Ventilation can be nondirectional which occurs when animals with external gills wave them through the water tidal in which the medium moves in and back out along a single passageway as is often the case with internalized gills or lungs or unidirectional where the medium enters at one point and exits another traveling constantly along a linear path Unidirectional ventilation is common in aquatic animals most of which have internal gills Although animals cannot dynamically adjust their pathway of air flow they can adjust their pattern of ventilation such as the rate and depth of inhalation and exhalation Gas exchange is also improved by the anatomy of the circulation that underlies the respiratory surface Perfusion of blood close to the respiratory surface is necessary for gases to diffuse to and from the blood and respiratory medium Recall that diffusion of O2 into the blood and CO2 out is dependent on maintaining partial pressure gradients between the external medium and blood so the amount of dissolved O2 and CO2 in the blood is tightly regulated When animals efficiently mix the medium around them via nondirectional ventilation they continually refresh the supply of oxygenated water that comes into contact with their respiratory surface external gill By keeping this gradient high O2 can diffuse from the medium into the blood at maximum rates The oxygenated blood that leaves the gill will have a PO2 that is comparable to the surrounding water having been maximally oxygenated If nondirectional ventilation is poor a boundary layer of stagnant medium will develop around the respiratory surface and gas exchange will be poor due to a lack of partial pressure gradient immediately surrounding the blood supply In the case of tidal breathers they usually cannot expel all of the deoxygenated medium Thus along the single passageway incoming medium will mix with some residual outgoing deoxygenated medium and what is actually in the respiratory cavity will have lower PO2 than the environment When the blood leaves this area it will have PO2 levels comparable to the mixed level in the respiratory cavity In unidirectional ventilation the respiratory medium flows in only one direction across the respiratory surface It enters the animal s respiratory system at one point and exits at another The efficiency of gas exchange between these respiratory systems and the blood that perfuses them depends on their relative directions of flow In concurrent flow the medium and blood flow in the same direction This allows the PO2 to equilibrate between the medium and blood so the final oxygenated state of the blood is not particularly concentrated Countercurrent flow where the medium and blood are flowing in opposite directions is more efficient This provides any given point in the blood a constantly refreshed supply of oxygenated water or air so there is a constant partial pressure gradient for diffusion of gases to occur By the time the blood gets to the end of the respiratory surface it will be
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