SLIDE 2 Body fluid compartments: closed circulatory system
Osmoregulation is about regulating various aspects of the body fluids of animals
The animal is separated from its external environment. The external environment refers to
either freshwater, marine, or terrestrial. These are the 3 major types of environment animal
physiologists usually talk about
The animal is separated from the external environment by the integument. The integument is
the boundary between the environment and the animal, so for animals this would be the skin.
Inside the animal are several body fluids. The most important body fluid is probably the
intracellular fluid, the fluid that’s found within the cells.
The ICF is where all metabolic reactions in the take place in the intracellular fluids so the amount
and composition of the fluid is important because if it changes too much it might have
disastrous effects on the enzymes that are carrying out the metabolic reactions.
Everything is geared towards assuring that the ICF is well regulated to make metabolism
What could affect the composition of ICF?
o One thing is water moving either into the cell from ECF, or water from the ICF moving
out to the ECF. If you have net movement of water into or out of the cell, it might
concentrate or dilute the contents of the fluids in the cell, therefore effect ions and
solute, therefore have a negative effect on the proteins in that cell.
The ICF is separated from two kinds of extracellular fluid. One is the blood plasma, the other is
the interstitial fluid, and this fluid is directly surrounding the cell.
The blood plasma is separated from the cell by 2 boundaries, one is the plasma membrane
which closes the ICF, and the other is the endothelium.
The endothelium is a layer of cells, and a single layer of cells at some points that separates the
blood plasma from the interstitial fluid.
In animals with closed circulatory systems, another major fluid is the lymph. The lymph is as
close to the interstitial fluid you can get.
You have fluid coming from the blood plasma, crossing the endothelium to get into the
interstitial fluid space, the lymph is simply the interstitial fluid as it’s returning to the blood.
When you think of the lymphatic system and you think of lymph, the fluid that’s come out of the
blood plasma to surround the cell which is now called the interstitial fluid, the lymph the
interstitial fluid that’s getting into the lymphatic system and going back to the blood plasma.
There isn’t a complete separation between interstitial fluid and blood plasma because that
interstitial fluid is taken back to the blood plasma and comes back so a fluid circulation is going
SLIDE 3 Body fluid compartments: open circulatory system
If you don’t have a closed circulatory system, you don’t have blood plasma because you don’t
have a distinction between the blood plasma and other extracellular fluids, you only have one
kind of extracellular fluid which is named haemolymph. The haemolymph is akin to the blood and the lymph and the interstitial fluid in an animal with
an open circulatory system.
Regardless of the circulatory system, in osmoregulation you want to regulate various aspects of
these body fluids. The 3 aspects we try to regulate are osmotic pressure, ionic composition, and
When you talk about osmoregulation, you talk about how do animals regulate these 3
components of their body fluid.
You talk about how they regulate osmotic pressure which is the total concentration of solutes
dissolved and fluid. Ionic composition can be how animals regulate sodium, potassium, calcium
ions and how they regulate volume example keeping the volume of the cell constant and
causing it to not burst.
SLIDE 4 Osmotic Pressure
Osmotic pressure regulation refers to how animals regulate how much concentration of solute
in total is present in their body fluids
You can demonstrate osmotic pressure by taking a U shaped tube and divide it in the middle by
a semipermeable membrane which is permeable to water but not the solutes dissolved in
The left side has just water; the right side is a solution which could be salt water. You fill them
up in equal amounts and let it sit for a while.
Water will move from the left side to the right side to try to balance the concentration of
sodium chloride on both sides.
The osmotic pressure was higher on the right side because of the presence of solutes. Water
moves from low osmotic pressure to high osmotic pressure.
You can see that you still have water on the left side and solutes on the right side of the tube
and there still isn’t equal concentration on both sides. This is because another kind of pressure
opposes osmotic pressure which is called hydrostatic pressure. As water starts to build up on
the right side, the weight of the water puts pressure back down and that limits how high the
water can move up on the right column.
Osmotic pressure drives the water in the direction of solutes, and hydrostatic drives it in the
Whatever the osmotic pressure of the extracellular fluid is, the cell usually has to have the same
osmotic pressure. You’ll always find the osmotic pressure of ICF is the same as ECF so there’s no
osmotic gradient for the net movement of water. So we’re interested in how animals regulate
the total amount of solute dissolved in their extracellular fluids.
The problem with animals is that you have ICF of a given osmotic pressure; you regulate the ECF
to have the same osmotic pressure as the ICF so there’s no net movement in and out of the cell,
but the ECF osmotic pressure may or may not be the same as the external environment. So
there may or may not be gradients for water to come into or leave the animal if there’s
differences in osmolarity between the ECF of animals and the environments. SLIDE 5 Ionic composition of body fluids
In the graph, the concentration in the extracellular fluids on the left and the concentration in
intracellular fluids on the right of various kinds of ions are shown.
The extracellular fluids are highlighted in red for the 6 animals. You’ll notice that in each case,
there are two ions that dominate extracellular fluids of these animals, which are sodium (blue)
and chloride (orange). In all animals, their extracellular fluid (blood plasma, interstitial fluid,
haemolymph) are dominated by sodium and chloride.
Why might this be the case?
o If you take a look at the ionic composition of seawater, the two ions that dominate sea
water are sodium and chloride.
o It’s thought that animal life likely evolved in the oceans; therefore the animals that
evolved in the oceans simply evolved to have extracellular fluids mimicked the
compositions of seawater in which they evolved.
The concentration is given in a unit called milliosmolar. An osmolar only accounts for things that
are only soluble in water. Anything that’s not soluble in water can’t contribute to osmolarity of
the solution. So milliosmolar simply means the total concentration of all solvents, disregarding
anything that isn’t soluble in water.
On the left, the animal’s osmolarity is about a thousand mOsM. On the right the animals have
lower osmolarity, meaning they have a smaller total solute content in their extracellular fluid.
If you look at each of the animal’s intracellular fluid composition, you’ll notice they’re poor in
sodium and chloride. Instead, ICF is dominated by potassium in most animals.
SLIDE 6 Osmoregulation: Marine
We take different animals in different environment and ask how they regulate how much total
stuff is dissolved in their ECF.
In the marine environment, sea water, the osmolarity is about 1000 mOsM, meaning there’s
roughly 1000 millimoles of solutes worth dissolved in every litre of seawater
Looking at the pictures we put these animals into 3 groups. The squid and the crab have ECF
osmolarity that’s roughly the same as the seawater. The shark has slightly higher osmolarity
than where it lives. Then we have these fish, the mammal, and the reptile, all of which have
much lower osmolarity than the environment in which they live.
So if you look at the fish, it has a lower osmotic pressure than the seawater, so that’s going to
draw water out of the fish. Water will always be trying to go outside the fish so it has to figure
out how to get it back.
SLIDE 7 Osmoregulation: Marine: Isosmotic marine invertebrates and hagfish
The hagfish is a vertebrate; it’s a jawless fish. All marine invertebrates as well as the hagfish are
isosmotic. Their ECF osmotic pressure is always the same as the seawater in which they live.
o ECF = Seawater The squid and the crab (marine invertebrates) have ECF osmolarity that’s similar to seawater. In
these animals, there’s no osmotic gradient between their ECF and the water in which they live.
Therefore, there’s no drive towards the water into or out of the animal, so there’s no net
movement of water in or out.
At the same time, their ECF is largely made of inorganic ions like sodium and chloride that are
present at similar levels in seawater, so there’s no net movement of ions into or out of the
These animals are also called osmoconformers meaning they’ll take on the osmolarity of their
environment. The seawater has high osmotic pressure but it’s also a stable environment so it
doesn’t change much in terms of salinity, so the animals can rely on this environment for no net
movement of water to occur.
In the starfish example, the arrow is pointing towards its madreporite which is the structure that
allows for a direct connection between the haemolymph of the animal and the seawater. So
when you talk about the animals’ ECF being similar to the seawater in which they evolved, the
starfish may have had the seawater as their extracellular fluid because the madreporite is the
opening to the extracellular fluid environment which allows the seawater to come into the
animal and be its extracellular fluid.
SLIDE 8 Osmoregulation: Marine- Hyposmotic marine teleosts
Marine teleost means marine fish which are a class of bony fish. These are hyposmotic fish
meaning their ECF osmotic pressure is lower than seawater. Water is always trying to get out of
the fish and into the seawater
Even though these fish live in seawater they run the risk of dehydration.
o Considerable osmotic gradient between external environment and ECF
Net movement of water out of the animal: dehydration is a problem!
At the same time, their blood ions are also present at much lower concentrations than the
seawater, so ions like sodium and chloride are constantly trying to get into the animal and they
have to actively pump it out into the water.
o ECF contains predominantly inorganic ions at levels much lower than seawater
Net movement of ions into the animal: need to be excreted!
SLIDE 9 Osmoregulation: Marine- Hyposmotic marine telehosts
The solution for dehydration is to just drink the water. So marine fish drink water, but the water
it’s drinking is the water that surrounds it, which contains lots of sodium and chloride ions.
When they drink the seawater, the salt water enters the gut; the body fluids surrounding the gut
are much less concentrated with these ions, so this makes the problem worse.
The fish actively pump a large proportion of the sodium and chloride ions into their body fluids.
This creates very salty body fluids that surround the gut, and leaves behind fresh water. This
sets up an osmotic gradient which favors water from the gut into the animal’s body fluids. This solves the problem of dehydration, but at the same time the fish has too much ions in its
body fluids. To deal with this, the marine fish actively secrete sodium and chloride ions across
Na+/K+ ATPase and Sodium/Potassium/2 Chloride transporter is involved in this.
The gills of all fish are made up of 3 types of cells. The majority of the cells are pavement cells
which align the surface of the gills to allow gas exchange. Another cell found is known as the
chloride cell or “mitochondria-rich cell”. The accessory cell is always adjacent to a chloride cell.
These fish are trying to get rid of sodium and chloride out of the blood and into the seawater.
It begins at the sodium/potassium pump, pumping sodium from the chloride cell into the blood.
Potassium comes back into the chloride cell in exchange.
This builds a gradient for sodium to want to come back into the chloride cell, and it does that
through the sodium/potassium/2 chloride transporter which allows the sodium to come back
into the cell coupled with potassium and 2 chlorides. The potassium is pumped back out.
The cell gets enriched with chloride, they want to get out of the cell and the only way they can
do that is through the apical channel which is the chloride channel which faces the water side.
So chloride is moved from the blood into the cell, and into the seawater.
How do we get rid of sodium? Sodium leaks out of the fish from the blood through the space
between the accessory cell and the adjacent chloride cell.
What drives this movement of sodium? The loss of chloride which leads to an electrostatic
interaction. As the chloride enters the seawater, its negative charge attracts the positive charge
of the sodium.
SLIDE 10 Osmoregulation: Marine- Slightly hyperosmotic marine elasmobranchs and coelacanths
Marine elasmobranchs include sharks, skates and rays.
Marine elasmobranchs and coelacanths are hyperosmotic, so they have osmotic pressure
greater than the surrounding seawater. This means water is constantly coming in through
osmosis from the environment into the animal’s body fluids. Thus, these animals don’t need to
o Modest osmotic gradient between external environment and ECF
Slight movement of water into the animal
No need to drink seawater!
Due to this, they don’t have to worry about overloading ions like the marine teleost and don’t
have to spend as much energy to pump ions out.
Thus, sharks, skates, and rays don’t drink seawater unlike the marine teleost.
The ECF of these a