Internal Environments
(Chapter 2)
Comp. Physiol.
Last revision: 8/26/98
What is Comparative Physiology? If you ask different people you will
probably get slightly different answers. A common definition would be
something like “Comparison of contrasting physiological processes in
different animal groups as a function of their environments or lifestyles.” Some people refer to the study of physiological adaptations to
different environments as Environmental Physiology. Your textbook
takes a systems approach in comparing physiological adaptations to
the environment. To that, I will add an integrative dimension by
bringing in some more cell physiology and molecular biology.
Overhead: Withers 1-2 (species distribution)
Medical objectives have driven physiology research on mammals and
created a knowledge base that by far supercedes what we know
about the physiology of all other animal groups combined. If we put
this into the perspective of ALL vertebrates representing less than 4%
of all species it is easy to realize how shallow our understanding of
physiology really is. There are scientists that call themselves
comparative physiologists because they look at a specific system in
several species of mammals (e.g., mouse, rat, rabbit). To truly study
the biological diversity of physiological mechanisms you must clearly
work on a wider basis than that.
In this course, we will include more invertebrate physiology than what
you probably were exposed to in BIO350, but because of the
disproportionate knowledge regarding vertebrates you will get a
considerable chunk of vertebrate physiology as well.
Overhead: Claude Bernard
Claude Bernard was a French scientist during the 19th century and he
has often been referred to as the “Father of Physiology.” He was the
first one to realize the importance of a constant internal environment
for life to exist in a varying external environment. He wrote 1959, “La
fixité du miliue interieur, c’est la condition de la vie libre,” which
loosely translated means that a constant internal environment is a
required for a free life.
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Many years later, 1929, an English physiologist, Cannon, suggested
that the maintenance of a constant interior environment should be
called homeostasis. Cannon also concluded that although the interior
environment is kept constant there is a continuous exchange
between the body and the outer world. Such exchanges include
temperature, respiratory gasses, water, and inorganic elements.
Draw: body integument & cell
ECF


Osmotic conc. varies
Ionic conc. varies


Osmotic conc.
varies
Ionic conc.
constant
There are basically 2 levels at which regulation to maintain
homeostasis can occur: 1) at the body integument and 2) at the cell
membrane. The extracellular fluids of most animals vary greatly in ion
concentrations as well as in the total concentration of solutes, the
osmotic concentration.
Overhead: text (definitions)
Ionoconformers: Animals that have the same ion concentrations of
extracellular fluids as the outer environment. A few animal species
ionoconform fully (e.g., the jellyfish, Aurelia)
Ionoregulators: Animals whose extracellular fluids have different ionic
composition from the environment. This category would embrace
most multicellular animals.
Osmoconformers: Allow the total concentration of solutes in body
fluids to be the same as that of the exterior environment. A majority of
marine species osmoconform.
Osmoregulate: Animals that to some extent regulate the osmotic
concentration of their body fluids. That is, the osmolarity of their
extracellular fluids is different from that of their environment.
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In all cases I can think of, ionoconformers are also osmoconformers.
Many marine animals are osmoconformers but do ionoregulate.
Typically, these animals substitute some of the inorganic ions for
organic compounds, such as amino acids or urea.
HOMEOSTASIS
Overhead: cell composition
The ultimate reason to regulate the salt content of the body fluids is to
make it easier for the cells in the body to keep a constant ionic
composition.
The ionic environment of animal cells is always dominated by K+
whereas the concentrations of Na+ and Cl- are typically very low. This
pattern is basically the same in organisms spanning from protozoa to
vertebrates and arthropods. In contrast to the fairly uniform profile of
inorganic ions, the organic content of cells varies enormously
between life forms. Usually there are high concentrations of amino
acids, proteins, and other nitrogenous compounds.
Overhead: BIO550 homeostasis & regulation
Ions and other osmolytes are of course not the only variables that
need to be regulated to maintain internal homeostasis. In more
advanced animals, homeostasis include constancy of blood gases,
blood pressure, and temperature. Typically, in more primitive animals
less physicochemical variables are kept constant.
It is important to distinguish between homeostasis and regulation.
Just because a variable is constant it does not automatically mean
that it is regulated and vice versa.
A very good example is temperature in aquatic animals from the polar
regions where the seasonal variation in water temperature is minimal.
For example, the temperature of the Antarctic waters where the ice
fish lives is –1.9°C with an annual variation of less than 1°C. Thus,
the temperature of ice fish blood is much more constant than human
blood without being regulated.
On the same token, regulation of a variable does not necessarily
imply perfect homeostasis. Rather, regulation brings with it a certain
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degree of constancy. In fact, few systems are so well regulated that
the internal environment is not affected by external conditions.
Overhead: Withers 2-4 (shrimp osmoregulation)
For example, many aquatic invertebrates display a limited degree of
osmoregulation – especially in dilute waters. The mud shrimp is a
osmoconformer in seawater but osmoregulates when the osmolarity
of the water falls below 1000 mOsm/kg. This other shrimp,
Callianassa, osmoconforms throughout its range of ambient osmotic
concentration tolerance.
The terms conformation and regulation do not only apply to osmotic
regulation, but in principle to all physiochemical variables (e.g.,
thermoconformation vs. thermoregulation).
TOLERANCE
At a certain point, the external environment becomes too extreme for
continued survival. If your are a regulator or not, the conditions of the
body fluid become altered beyond tolerable limits for normal cellular
function.
Overhead: Withers 2-5 (temperature tolerance and resistance)
Here is an example of the ability of the chum salmon to survive at
different temperatures. Like most salmonids, the chum salmon
prefers cold water but there is a lower limit of course.
This graph shows the time chum salmon, from this particular study,
could survive at different temperatures. Between approximately 2 and
19°C the fish showed infinite tolerance to the temperature. This range
is called the zone of Tolerance. Below and above the zone of
tolerance the fish are not functioning properly and will die off in a
time-dependent manner. The time it takes for the fish to die depends
on how far off they are from the zone of tolerance. The zones of
resistance are the ranges, at which the animal can survive for some
period of time but will eventually succumb.
Overhead: BIO550 (tolerance/resistance definitions)
Tolerance and resistance to unfavourable environmental conditions
are naturally different between species, but there are also differences
between populations and even between individuals within a
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population. The traditional methods of determining resistance of a
population is to determine the time it takes for 50% of the population
to die. The value is called the LT50 and is given in units of time.
Similarly, the tolerance to a chemical can be quantified by
determining either the dose or concentration of a chemical that kills
50% of the population within a certain time period. LD50 refers to the
dose that kills 50% whereas LC50 refers to the concentration that is
lethal to 50% of the individuals.
Overhead: Withers 2-6 (Dose-mortality curve for NH3)
The relationship between time to death and concentration; or %
mortality and concentration of a chemical is usually described by a
sigmoid curve.
This is the relationship between the water ammonia concentration of
the water and the % mortality of a carp population. The LC50 is
defined by the concentration that kills 50% of the individuals. In this
case, the LC50 was found to be 1.68 mM NH3.
Overhead: BIO550 (acclimatization, acclimation, adaptation)
Individuals and populations are able to alter their ranges of tolerance
and resistance over a period of time in response to an altered
environment. One of the most familiar examples of such changes is
the readjustments that occur in response to travel to high altitudes.
Many individuals, when they first arrive at a high altitude, experience
transient “mountain sickness.” It is characterized by headache,
irritability, insomnia, breathlessness, and nausea and vomiting. This
syndrome develops 2-24 h after arrival at altitude and lasts 4-6 days.
After the first week the symptoms are largely gone and we have
become acclimatized to the new altitude.
Acclimatization is the term used to describe readjustment
phenomenon in the natural environment.
When the same readjustment is observed in the laboratory or
imposed by anthropogenic changes in the environment, it is referred
to as acclimation. Thus, acclimatization and acclimation are usually
accomplished by the same mechanisms.
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Adaptation to the environment should not be confused with
acclimatization and acclimation. Whereas acclimatization and
acclimation are phenotypic changes, adaptation is an alteration at the
genetic level to adjust to a specific environment.
Overhead: Marr et al., 1996, Fig. 2 (acclimation/adaptation)
Marr J.C.A., Lipton J., Cacela D., Hansen J.A., Bergman H.L., Meyer J.S. and Hogstrand C. (1996).
Relationship between rainbow trout growth, tissue copper concentrations, and copper exposure
duration. Aquat. Toxicol. 36, 17-30.
Having said that does not mean that there is no connection between
acclimation/acclimatization and adaptation. In fact, sometimes the
adaptation is to more easily acclimate. For example, animals that
have lived many generations in contaminated areas more easily
acclimate to toxicants than animals that are raised in clean areas.
Clark Fork River,
Montana
Metal (Cu, Zn,
Pb,etc) levels in CF:
“0.2P”  “1P”
CF-BT
H-BT
Hatchery
(clean
water)
clean
water
Clean water
until tolerance
and resistance
of CF-BT =
H-BT
3 – 5 weeks
Resistance
test at 1P
0.2P
The Clark Fork River in Montana is such a place. There has been
mining going on there for a very long time and the fish born in the
area shows a strong ability to acclimate to metals. In this study,
young brown trout were brought from the CF River and kept in clean
water until they showed no difference from hatchery (H) brown trout
in metal tolerance and resistance. The fish were then exposed to the
same metals mixture as that present in the CF River. After 3 and 5
weeks of exposure, the metal concentration was increased above the
tolerance range to quantify resistance.
CF brown trout showed much greater degree of resistance than
hatchery reared browns. This difference was likely due to an
adaptation to the CFR environment.
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Overhead: W2-7 (thermal tolerance polygon)
What you can gain in tolerance at one end of the tolerance range,
during acclimatization, you usually lose in the other end of the
spectrum. For example, during summer fish have higher tolerance to
warm temperatures than in the winter. Conversely, they are more
tolerant to cold in the winter than in the summer.
Overhead: W2-8 (homeostasis models)
Let us now talk a little bit about some possible ways of maintaining
homeostasis.
A. The simplest system is one which is simply based on an
equilibrium between the animal and a constant environment. The
temperature in an icefish, which we were discussing, is such an
example. In an equilibrium system, the value of the internal
variable changes with changes in the external environment.
B. Homeostasis can be kept without specific regulatory mechanisms
even if the variable in the animal is not in equilibrium with the
surroundings. For example, ammonia excretion in fish is a passive
process and the rate of ammonia excretion is directly proportional
to the ammonia level of the blood, which in turn is determined by
the rate of ammonia production and the diffusion constant of
ammonia across the gills. Thus, as long as ammonia production is
constant the ammonia excretion is also constant and the blood
ammonia level is at a steady-state. This steady-state homeostasis
is an open system because there is no direct regulatory
mechanism linking the ammonia concentration of the blood to
either its production or elimination.
C. The 3rd model here is of a closed feedback system. In such a
system there is an active physiological regulation of the constancy
of the variable in the body. That is, there is a set-point for the
variable and its constancy is regulated by a system involving some
kind of sensor mechanism and an effector system. The example
given here is of feedback regulation of the osmotic concentration
of the hemolymph in the brine shrimp, Artemia. Brine shrimp can
live in a broad range of salinities and they maintain a relatively
constant osmotic concentration of their body fluid irrespective of
the osmolarity of the external medium. In this case the osmolarity
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of the body fluid is the regulated variable and Na pumps in the gills
the effector mechanism.
Overhead: W2-9 (negative feedback)
(A) The most common form of feedback regulation is negative
feedback. In its simplest form such a regulatory system operates like
a thermostat that breaks a circuit at a set temperature. A simple onoff switch will not provide a particularly high precision in keeping the
regulated variable constant and for important physiological variables
there are often additional components involved in the regulatory
system. If we take temperature as an example, we have both heat
and cold receptors that activate several opposing effectors (e.g.
vasodilatation – vasoconstriction; sweating – piloerection of body
hair). Furthermore, we have temperature receptors in the skin as well
as in the hypothalamus. Such a multiple control system greatly
increases the precision of regulation and provides greater flexibility.
(B) Another advanced feature present in our thermoregulatory system
is proportional control, which means that the magnitude of the
effector response (e.g. rate of heating) is proportional to the error.
The difference between the actual temperature and the set
temperature is processed through a feedback transfer function. This
transfer function converts the error into a compensatory signal. In the
human thermoregulatory system, the transfer function is multiplication
with the constant value of 32. Other regulatory system will have other
transfer functions.
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