Topic 20 WATER AND ION BALANCE AND NITROGEN EXCRETION lectures 3132 OBJECTIVES 1 Be able to differentiate the terms euryhaline stenohaline osmoconformer and osmoregulator 2 Be able to describe the osmoregulatory problems and physiological solutions found in freshwater vs marine teleost boney fish 3 How do marine mammals reptiles and birds conserve water and get rid of excess salt 4 How do terrestrial insects conserve water 5 Understand the functional anatomy of the mammalian kidney and the processes of filtration and reabsorption 6 What are the major nitrogen excretory products and how is the nature of the N product excreted correlated with water availability in the environment Animals are 65 80 water dissolved in this fluid are inorganic ions small and large molecular weight organic molecules and gases There is a continuous exchange of water and small molecules between the animal and its environment the nature of the exchange is related to the environment Aquatic animals We can identify a variety of kinds of aquatic habitats which differ very substantially in terms of dissolved solutes mostly salts We usually define salt concentration by the term parts per thousand o oo 10 o oo means that for every 990 ml of water there are 10 g of salt 1 2 3 4 freshwater salt 3 o oo brackish salt 3 25 o oo marine salt 25 45 o oo hypersaline salt 45 300 o oo salt lakes like the Dead Sea where salt may approach saturation point and salts actually precipitate and go out of solution Recall our lecture on membrane transport and the concept of osmotic flow of water from areas of high water concentration to areas of low water concentration This very problem impacts aquatic organisms since the water concentration and salt can vary with habitat Aquatic animals can be divided into two categories with respect to their capacity to tolerate changes in the salt in their environments 1 stenohaline animals which are very intolerant of changes in salt most freshwater and marine species are stenohaline however there are many many exceptions like salmon eels etc 2 euryhaline animals which can survive large changes in salt in their habitats virtually all estuarine species are euryhaline 1 Animals can also be characterized with respect to the relationship between the solute concentration of their body fluids and the salt in their habitats Before we talk about this let me define another termOsmolarity an index of the solute concentration in a solution is directly proportional to the solute concentration thus the higher the osmolarity the lower the water content Osm a salt Osm a 1 water 1 osmoconformers animals that have an internal osmolarity that is always equal to the external osmolarity That is when the external salt changes the internal osmolarity passively adjust until it is equal to the external osmolarity animals literally gain or lose water Most marine invertebrates and cartilaginous fish sharks rays etc are osmoconformers 2 Osmoregulators animals that maintain a constant internal osmolarity in spite of the fact that the external osmolarity may be different In effect these animals are in osmotic disequilibrium with their habitats characteristic of all boney fish aquatic amphibians reptiles birds mammals and freshwater hypersaline invertebrates Fig 44 14Marine teleost fish are hypo osmotic with respect to the habitat lose water and gain salts They compensate by drinking seawater and actively transporting excess salts out of the gills also produce minimal urine Freshwater teleost fish are hyper osmotic gain water and lose salts produce copious dilute urine to get rid of excess water and conserve salts actively transport salts from the water into the body through the gills think of the problems that salmon face when they return to their ancestral spawning streams after spending years in the marine environment Marine mammals reptiles and birds Are hypo osmotic with respect to seawater so they face continuous influx of salt and efflux of body water Two generalized adaptations 1 drink large amounts of seawater 2 get rid of excess salts a mammals produce a very concentrated urine low in volume high in salt content b marine reptiles have salt secreting glands which dump very salty secretion into eyes turtles nose marine iguanas and mouth sea snakes c birds have salt secreting glands which empty into the nostril 2 Terrestrial animals Water loss can be real problem in most terrestrial animals it can occur by a variety of ways 1 evaporation across body surface 2 respiratory water loss when air is expired it is water saturated 3 excretory water losses urine feces Except for organisms living in moist terrestrial habitats the conservation of water is an important feature of the physiology of these creatures As the habitat gets dryer water conservation becomes even more critical as shown by the kangaroo rat vs human comparison in fig 44 16 rat does not have access to water Some examples of water conservation mechanisms in terrestrial animals Insects fig 44 20 represent the most successful group of animals with as many as 5 million species many of which occupy very dry habitats These animals have a common digestive and urinary tract consisting of Malphigian tubules which empty into the GI tract fig 44 20 the resulting materials pass into the intestine and then the rectum where as much as 98 of the water will be reabsorbed the feces is essentially dry Mammals have what is known as the glomerular kidney that is a kidney each of which consists of several million filtration the glomerulus and water reabsorption structures called the nephron unit Fig 44 21 functional anatomy of the mammalian kidney What does the kidney do 1 filters the blood makes a cell free protein free filtrate which has the same osmolarity as that of the blood the filtrate urine is then modified by the kidney 2 secretes additional materials into the urine substances like acids bases toxins etc are transferred from the blood into the urine 3 reabsorbs substances from the urine water salts small molecular weight organic materials are transferred back into the blood The final urine is usually very concentrated in terms of solute mostly urea and some salt and has a low volume We ll talk about filtration and reabsorption in some detail Filtration 1 blood enters the glomerulus a meshwork of capillaries by way of the afferent arteriole 3 2 blood pressure in the capillaries forces water and low molecular wgt materials through a filtration device which excludes
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