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
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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  [salt]; Osm  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
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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
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2. blood pressure in the capillaries forces water and low molecular wgt materials
through a filtration device which excludes proteins and cells
3. materials that are filtered-water, salts, urea, small molecular wgt organics like
glucose and amino acids
Reabsorption.
Fig. 44.221. the nephron unit is divided into distinct regions, each having somewhat specialized
functions- proximal convoluted tubule, descending limb of the loop of Henle, thin
segment of the ascending limb, thick segment of the ascending limb, distal
convoluted tubule and collecting duct
2. proximal convoluted tubule- water and salt reabsorption
3. descending limb of the loop of Henle- water reabsorption
4. thin segment of the ascending limb and thick segment of the ascending limb- NaCl
reabsorption
5. collecting duct- urea and water reabsorption
The amount of water that is reabsorbed by the collecting duct, and hence the
concentration of the urine, depends on how permeable the collecting duct membrane is
to water. This permeability is under hormonal control.
If the blood osmolarity rises, the brain triggers thirst and also releases a hormone
called antidiuretic hormone which causes the walls of the collecting duct to become
more permeable to water. The opposite takes place when blood osmolarity is too low.
Nitrogen excretion and the environment.
Ammonia is the ultimate end product of the catabolism of nitrogen-containing
molecules. Ammonia cannot accumulate to very high concentrations because it is toxic.
It is either directly excreted or converted into relatively non-toxic compounds.
Fig. 44.131. most aquatic animals excrete ammonia directly into the environment
2. mammals and terrestrial amphibians produce urea which can be excreted at
relatively high concentrations in urine
3. birds, insects, reptiles like lizards and terrestrial snail convert ammonia into uric acid;
uric acid can be excreted with minimal water loss. In fact, uric acid forms crystals
which can be excreted or actually accumulated in the body as takes place in some
pulmonate (lunged) snails.
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