12 1 Terrestrial Osmoregulation All animal cells require a balance of ions and other solutes and produce nitrogenous waste that must be cleared from the cell As animal bodies have become larger and more complex they have evolved mechanisms to reduce the workload on their cells by regulating their immediate environments the content of the extracellular fluid Aquatic animals face osmotic and ionic challenges that vary depending on whether they are marine or freshwater and they have adaptations that prevent or facilitate water loss respectively Due to the nature of osmosis this control is effected by regulating solute concentration Aquatic animals have a diversity of glands gills renal glands rectal glands that control what water and solutes enter the body stay in the body and exit the body The clearance of nitrogenous waste is fairly straightforward as it does not need to be processed to reduce its toxicity Terrestrial animal cells have the same basic needs but the osmotic and ionic challenges of living on land are different While aquatic animals are surrounded by water to fulfill their osmotic needs terrestrial animals must acquire it more actively Some animals are able to extract water from the air but this constrains the environments in which they can live Most terrestrial animals are dependent on ingesting water by drinking or eating The water and salts in their food is transported transcellularly and paracellularly across the epithelial layer of their digestive tract to the circulatory system Some water is also produced as an end product of oxidative phosphorylation when O2 is used as the final electron acceptor This is called metabolic water Because the osmotic pressure outside their bodies is much lower than inside terrestrial animals must have adaptations to prevent water loss via evaporation The integument of terrestrial animals can be extensive and variable depending on how arid vs moist their habitat is For example the epithelial cells that compose the skin are connected via tight junctions that prevent paracellular H2O loss via evaporation Instead water is secreted using sweat glands that are under hormonal and neural control Terrestrial animals must also process their nitrogenous waste When proteins are metabolized there are many reactions that result in the production of ammonia NH3 which can be protonated to form ammonium NH4 both of which are toxic Ammonia can diffuse across the cell membrane while ammonium requires a transporter such as the NH4 H exchanger These mechanisms are sufficient in aquatic animals However when nitrogenous waste leaves the body of a terrestrial animal it does not get immediately diluted and diffuse away from the body as it does in aquatic animals To achieve non toxic concentrations of ammonia would require a huge amount of water loss Further terrestrial animals store their urine in their bodies temporarily alleviating the need to urinate constantly Thus the toxic ammonia that is produced as a byproduct of protein metabolism must be converted into something less toxic While most aquatic animals are ammoniateles terrestrial animals are either uricoteles or ureoteles and convert ammonia into uric acid or urea respectively The groupings below are general and there are many exceptions to them For example many animals change their mode of nitrogen excretion in response to water availability Further almost every species has the enzymes to produce any of these molecules but excrete most of their nitrogenous waste in only one form at any given time Recall that the earliest wave of land invaders were invertebrates myriapods followed by their arthropod predators approximately 420 mya In some insects and myriapods the hemolymph that fills their body cavity has long thin tubes that project into it called Malpighian tubules These tubules are only one cell layer thick and the apical sides of the membranes are convoluted and ciliated to increase surface area and draw hemolymph into the tubules respectively These tubules are the site of urine formation via secretion instead of filtering the hemolymph the cells of the tubules use ATP to create electrochemical gradients that drive particular solutes into the lumen for excretion which water will follow The walls of these tubules comprise stellate cells which allow Cl and H2O excretion and principal cells which use an apical H ATPase to control cation secretion The urine travels through the tubule where some selective reabsorption occurs before it is excreted into the hindgut to be expelled with feces via the anus The cells of the Malpighian tubules are not innervated secretion and reabsorption are controlled by hormones that circulate through the hemolymph These hormones affect secretion of water ions and other solutes by binding GPCRs that activate or inhibit membrane transport proteins via a second messenger cascade A few species of land animals mostly reptiles and birds drink seawater which is a good source of both solutes and water but presents a challenge in that it is hyperosmotic to their ECF and cells This allows them to survive without access to freshwater instead relying solely on hyperosmotic sources like seawater or even dietary water Their bodies have adapted to absorb the water without absorbing all of the solutes using salt glands Like the rectal gland in marine fish salt glands excrete hyperosmotic NaCl solutions concentrating the water inside the animal In reptiles these are near the eye and drain near the nostril In birds these are at the base of the beak and drain at the nostrils This is achieved using a countercurrent system between the blood and the lumen of the gland tubules By arranging these two thin walled tubes parallel to each other but flowing in opposite directions this improves the efficiency of solute exchange The combination of channels expressed by the salt gland cells are unclear but they appear to include channels you are familiar with for generating electrochemical gradients and osmoregulation Na K ATPase Na K Cl cotransporter symporter NKCC K channels and Cl channels Recall that the renal glands of freshwater fish are more extensive than marine fish The chloride cells in the gills of both types of fish are important in regulating water and solute balance The renal glands are also important in osmo and iono regulating the ECF by filtering it reabsorbing some substances and excreting the rest The renal glands in freshwater fish are more extensive than that of marine fish
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