Body Fluid
Compartments
1
Lecture outline
I. Water compartments of the body
A.Intracellular
B. Extracellular
i. Interstitial
ii. Plasma
iii. Transcellular
II. Compare/contrast water compartments
A. Size
B. composition
C. Osmolality
III. How do we have different composition/ movement of solutes
A. Different permeability
B. Types of transport across the membrane for solutes--Protein transporters
C. Review of Simple diffusion of solutes
IV. Movement of water
A. Osmosis-movement across cell membranes due to unequal particles
B. Hydrostatic pressure- movement across capillaries
V. Examples of when water vs. solute moves
VI. Definition of osmotic pressure
A. Examples of osmotic pressure differences in body fluid compartments
VII. Tonicity vs osmolality
A. Examples of tonicity and osmolality
2
Body Fluid Compartments
• Why do you need to understand body fluid
compartments and osmolarity calculations?
• Many of you will be applying IV care for patients,
and sometimes doctors make mistakes, so you
need to be able to catch these errors.
• Most medical solutions are calculated in units
that don’t require a periodic table of elements,
but if someone miscalculates a solution, and you
inject it, and the patient crashes, you are just as
liable, and you will be sued.
3
Water
• Water makes up 60% of
our body weight
• Divide this into two
compartments
– Intracellular water
– Extracellular water
4
Compartments
*
Lumen of stomach
• Intracellular Fluid
(30-40% Body Wt)
• Extracellular Fluid
These are
stomach
epithelial cells
*
– Interstitial fluid (the water
immediately outside cells,
between and around cells)
(16%)
– Plasma fluid (the water
inside blood vessels, but not in
(4-5%)
– Transcellular fluid (the
blood cells)
*
*
water enclosed in chambers
lined by epithelial
membranes) (1-3%)
5
Body Fluid Compartments
• If you manipulate one body fluid compartment, it has an
effect on another compartment.
• Body fluid compartments have different sizes and
volumes, and different compositions.
• Although the volume and substances dissolved in the
fluid of one compartment is different than another
compartment, each compartment has the same
number of particles dissolved in the water.
6
Body Fluid Compartments
• Therefore, size and composition (what particles are
dissolved in the water) does not have an effect on the
number of particles dissolved in the water of each
compartment.
• If you could count every particle dissolved in the water of
that compartment, you would see that in all the
compartments there are the same number of particles:
300 million particles per liter, expressed as “300
million osmoles” or “300 mili-osmoles”.
• It could also be described as having “an osmolarity of
300”.
7
Body Fluid Compartments
• That means that there are 300 million particles (or 300
milliosmoles, abbreviated 300 mOsm) dissolved in each
liter of water in each compartment.
• If one compartment has more particles than another one
next to it, and if those particles cannot reach equal
numbers on their own because the cell membrane
blocks their passage, water will try to dilute the
compartment with the higher number of particles until
they are at the same number of particles per liter.
• Water always moves across the compartments
because cell membranes always allow water to pass.
8
Body Fluids compartments
 Different compositions (different amounts of individual
particles). For example, one coffee cup has 1 teaspoon of
instant coffee and 2 teaspoons of sugar, another cup has 3
teaspoons of coffee and 1 teaspoon of sugar.
 Different volumes (one coffee cup holds 8 oz, another holds
16 oz)
 Same osmolalities (total number of particles) When you
evaporate away the liquid in both coffee cups and count each
coffee grain and each sugar grain, there are 300 million total
grains per liter in both cups.
Plasma
Interstitial
Intracellular
9
0.3 Osmolal = 300 mOsmolal (actually closer to 280mOsmolal)
Diffusion
• If the plasma becomes diluted to 260 mOsm, and the cells next to a
blood vessel are still at 300 mOsm, the cells now have more
particles. By a law of physics, all particles want to move from an
area of high concentration to an area of low concentration…That is
why perfume will diffuse out of the bottle and fill up the room if you
leave the top off.
• Therefore, since the particles in the cells are 300 mOsm and the
particles in the blood vessel are 260 mOsm, which way do the
particles want to move?
• The particles in the cells will want to move into the plasma.
• However, the cell membranes will not allow them to pass, so they
are stuck inside the cell.
• The next force of nature that will kick in is water diffusion. Water also
wants to move from its area of high concentration to an area of low
concentration.
10
Diffusion
• If I have 2 identical cups with the same amount of water, and I add
20 teaspoons of lemonade to one cup and 10 teaspoons of
lemonade to the other cup, which cup is more concentrated with
lemonade? The one with the most particles (20 teaspoons). Which
one is more concentrated with water? The one with the fewest
particles (10 teaspoons).
• If these two solutions were in body fluid compartments that were
next to each other, and if the particles cannot move from their area
of high concentration to low concentration, then water will move from
its area of high concentration (which was the more watery cup; the
one that had only 10 teaspoons of lemonade) to its area of low
concentration (the cup with low WATER concentration was the one
with 20 teaspoons of lemonade).
• That is the same thing as saying that particles suck water.
11
Diffusion
• What will move, in order to dilute the cells? Water. Why?
Because particles suck!
• In the case where the plasma has 260 mOsm (260
particles per liter) and the nearby cells have 300 mOsm
(300 particles per liter), will the cells will draw the water
into themselves, or will the plasma draw water into the
blood vessel?
• The compartment with more particles (the cells) will suck
the water in. Therefore, water will move from the plasma
to the adjacent cells.
• What will that do to the cells? They will lyse (rupture).
• When would that ever happen in real life?
12
Diffusion
• When would that ever happen in real life?
 January 12th, 2007, A 28 year-old mother of three from
Sacramento decides to go on a radio program to
compete in a contest called, “Hold your Wee for a Wii.”
They offered a prize to the person who could drink the
most water in two hours without going to the bathroom.
 During this contest, she drank 6 liters in 2 hours (3 liters
an hour). The maximum the kidney can filter is 1 liter an
hour.
 After the contest, she called her co-workers to say she
wasn’t coming to work because her head hurt so badly.
 Later she is found dead.
13
Diffusion
• She was OVER hydrated, so the original 300
particles per liter in her plasma were now at 300
particles per 2 liters, since the excess water
increased her blood volume.
• That means there were only 150 particles per
liter, so overall, there were now fewer particles in
the plasma than in the adjacent cells.
• Therefore, the adjacent cells had more particles,
and they sucked in the water.
• Her brain cells also sucked up the water until
they ruptured and exploded in her skull.
14
Overhydration
 Drinking too much water changes the
extracellular osmolality.
 When the plasma is too dilute (too much water,
too few solutes), water will leave the
bloodstream to enter the tissues, where there
are more solutes (solutes SUCK!).
 Water will enter the tissues (intracellular body
fluid compartment), including the brain.
 The excess water will cause the brain to swell.
15
Overhydration
• Thus, we learn that if a person is overhydrated, the plasma will be diluted below
300 mOsm, but the cells still have 300 mOsm
in particles.
• So, the cells will draw in more water from the
plasma and the cells will enlarge and rupture.
• She should have been given an IV that was
hypertonic (greater than 300 mOsm) to
balance out the number of particles in the
plasma so it matched the number of particles in
the cells.
16
Dehydration
• The opposite is true for someone who is dehydrated.
• Since the original number of particles was 300 million particles per
liter, and then the patient became dehydrated, they would now have
300 million particles per half a liter (since they lost plasma volume
due to dehydration), so their plasma is actually at 600 mOsm per
liter.
• Therefore, if a patient is mildly dehydrated, you will give an IV
that was hypotonic (less than 300 mOsm, be careful of the drip
rate) to balance out the number of particles per liter within the
plasma and within the adjacent cells.
• This has the same effect as having the patient drink some water, but
the iv will work faster.
17
Dehydration
• If a doctor accidentally tells you to give a mildly dehydrated patient
an IV solution that is hypertonic (greater than 300mOsm), the plasma
will have more particles than the cells, and the cells will have the
water sucked out of them, which also causes death.
• Hypertonic solutions are only okay for an overhydrated person, or a
dehydrated person who has lost particles, such as from blood or
electrolyte loss after surgery.
• Understanding body fluid compartments is important!
18
Patient Case 1
• Patient with a history of severe congestive
heart failure, admitted for treatment of
sepsis (infection) and severe dehydration.
Low BP=70/40, High Pulse-110bpm.
• Do you give an iv solution that is hyper,
hypo, or isotonic? Why?
19
Patient Case 1
• Give Isotonic solution (ringers solution).
• Since the person is severely dehydrated,
you do not want to overhydrate the cells
with a hypotonic solution and you don't
want to make the dehydration worse by
giving him a hypertonic solution, thus you
give him an isotonic solution.
20
Patient Case 2
• Patient, age 40, hypertensive, who has
been on diuretic therapy. Skin and mucous
membranes are dry, and he's complaining
of a headache. High BP=200/120.
• Do you give an iv solution that is hyper,
hypo, or isotonic? Why?
21
Patient Case 2
• Give hypotonic solution
• Since the skin and mucous membranes
are dry you want to give a hypotonic
solution so that the cells can become more
hydrated than with a isotonic solution.
22
Patient Case 3
• A 35 year old patient recently underwent
abdominal surgery. Her NG tube is
draining 1200 ml of fluid per shift, she's
losing large amounts of acid and
electrolytes. Arterial blood gas (ABG)
analysis shows a pH of 7.60 (alkalosis).
• Do you give an iv solution that is hyper,
hypo, or isotonic? Why?
23
Patient Case 3
• Give hypertonic solution.
• Since she lost acid and electrolytes, you
want to increase the osmolality by giving
the hypertonic solution (such as hypertonic
normal saline) to replace the electrolyte
loss.
24
Body Weight in Water
• There are 100 trillion cells in your body, 25% of them are red blood
cells (RBC’s). Dead RBCs are the reason why your pee is yellow
and your poop is brown! You will understand why, later in the
semester.
• About 60% of your body weight is from water. How can you
calculate your water weight? For every 2.2 pounds, you are 1
kg in weight. Then multiply that number by 0.6 to see how
much water is in your body.
• Water makes up 60% of our body weight
– 70 kg man X 0.6 = 42 kg = 42 L of water is in his body
– How much of your own weight is water? If you weigh 150 lbs:
150 lbs/ 2.2 = 68 kg
68 x 0.6 = 40.8 Liters
25
Body Fluid Compartments
• The total amount of water in your body is divided into two
compartments
– Intracellular water is inside of your cells. Most of your water is
here.
– Extracellular water is outside of your cells. There are three
types.
• Interstitial fluid (the water immediately outside cells, between
and around cells) (16%)
• Plasma fluid (the water inside blood vessels, but not in blood
cells)
(4-5%)
• Transcellular fluid (the water enclosed in chambers lined by
epithelial membranes, including the GI tract and synovial
joints) (1-3%)
26
Composition of Compartments
• All compartments are not the same size. Which is the
biggest? Intracellular
• What’s the smallest? Trancellular
• The inside of each cell is low sodium and calcium, and
high in potassium and proteins (there are four times as
many proteins in cells than there are in plasma).
• Outside of cells (in the plasma) are high in sodium and
calcium, low in potassium and proteins.
• Sodium has the highest extracellular fluid to intracellular
fluid concentration ratio for most mammalian cells.
27
Different compositions across the
membrane: How can this be?
28
Homeostasis of Osmolarity
• As stated, if you could count all the solutes
(particles) inside and outside of the cell, they are
the same number (300 mOsm). Why does it
need to be that way?
• All particles pull water to them, whether the
particle is glucose, calcium, a protein, salt, etc.
We don’t want a net gain or loss of fluid across
the cell membrane or the cell will shrink or burst.
Not all compartments have the same volume
liquid, but they all have the same number of
particles per liter.
29
Cell membranes are
semi-permeable
• If the numbers of particles are always the same,
how can we have higher numbers of potassium
ions inside of the cell compared to the outside of
the cell?
• Won’t the potassium ions want to move down
their concentration gradient towards equilibrium?
• Yes, they will want to, but the cell membranes
are designed to be semi-permeable (they will
only let certain substances come into and go out
of the cell). The cell membranes prevent
potassium (and other particles) from crossing. 30
Diffusion
• If you have a cell immersed in pure water, will it shrink or burst?
• The particle concentration is higher in the cell than in pure water, so
the particles want to leave the cell and enter the pure water.
• However, they cannot do that because they are blocked in by the
cell membrane.
• That means there are more particles on the inside of the cell than in
the pure water. Particles suck, so pure water will get sucked into the
cell until the cell bursts.
• In theory, if the substance we are talking about was a particle that
can cross the cell membrane whenever it wants to, it would simply
diffuse across the cell membrane until it reached equilibrium, so the
cell would not burst.
31
• What particles can cross the
cell membrane?
• Gases (O2, CO2)
• Lipids and lipid-loving
(hydrophobic or lipophilic)
substances, such as alcohol
32
Functions of Membrane- Selective
Permeability and Transport
• Selectively permeable- allows some
substances to pass between
intracellular and extracellular fluids
• Only small uncharged molecules or fat
soluble molecules can pass through
membrane without help. They get
through by one of two types of ways:
Passive transport means that energy
(ATP) is not needed to get a particular
substance across the cell membrane.
Active transport means that ATP is
used.
33
Membrane Function
There are three types of
Passive Transport:
1) Simple Diffusion
2) Facilitated Diffusion
3) Osmosis
All of these involve
particles crossing from
high to low concentration.
Osmosis is diffusion of
WATER across a CELL
MEMBRANE.
Osmosis only happens if the solute is
permeable across the cell membrane!
http://www.northland.cc.mn.us/biology/BIOLOGY1111/animations/passive1.swf
34
Permeability
• A water-hating (hydrophobic) substance can cross a
cell membrane.
• A very small water-loving (hydrophilic) substance
can also cross, such as gasses or a water molecule
itself.
• However, a larger water-loving molecule needs a
special protein channel in the cell membrane to help it to
cross.
– If it does not require energy (ATP), it is called
facilitated diffusion (facilitated means “helping”).
– If it requires ATP, it is crossing by an active transport
mechanism.
35
Facilitated diffusion
• Facilitated diffusion is when an ion
wants to travel down its concentration
gradient, but there is a channel in the cell
membrane that opens and closes by a
protein which enlarges or shrinks to open
or block the channel (remember, this is still
passive transport, so it does not need
ATP).
36
Three types of
Passive Transport:
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
37
Active Transport
• Active Transport is when a substance needs to
move against its concentration gradient (it is
moved from an area of low concentration on one
side of the cell membrane to an area of high
concentration on the other side of the cell
membrane).
• It accomplishes this because a protein
embedded in the cell membrane grabs onto the
substance and drags it across the cell
membrane (this requires ATP).
38
Active Transport
• There are three main types of active
transport:
 Ion Pumps
 Cotransport
 Endocytosis
• Because these are active transport
mechanisms, ATP is used.
http://www.northland.cc.mn.us/biology/BIOLOGY1111/animations/active1.swf
39
Transport of Water
• There are two ways that water can move:
 Osmosis
 Hydrostatic pressure
40
Movement of water
• Always passive
• Pores (called aquaporins) in the cell
membrane serve as conduits (conducting
channel)
• Osmosis
– Water wants to move from its area of high
concentration (less particles in the water) to its
area of low concentration (more particles in
the water). When it crosses a cell membrane
to do this, it is called osmosis.
– In a solution, water is called the solvent and
the particles are called the solutes.
• Hydrostatic pressure
– This is the pressure of the fluid exerted on the
vessels, or container.
If you squeezed on this
bottle to get the water to
shoot out, what kind of
pressure would this
simulate?
41
Hydrostatic Pressure
• Squeeze a water bottle to shoot the water out, this is
hydrostatic pressure. Hydrostatic pressure is the
pressure of the water exerting on the blood vessel
wall. If you push harder, the water will shoot farther.
• The hydrostatic pressure of water being filled in a
balloon will exceed the capacity of the balloon and pop.
• That is how water moves between cells and into cells so
that plasma becomes interstitial fluid. The plasma leaks
out between the cells that make up the capillaries.
• If you have a swollen ankle and apply an ace wrap, you
are applying hydrostatic pressure to force interstitial fluid
back into the plasma. Hydrostatic pressure is not the
movement across a cell membrane. That is osmosis. 42
Osmosis
• Osmosis is movement of water across the
cell membrane. Osmosis will occur when there
is a difference of particle concentration on each
side of a cell membrane.
• Osmotic pressure can be measured. If there is
more water on one side of a membrane than the
other side of the membrane, the water will move
down its concentration gradient. This occurs
when there are more particles on one side of the
membrane than the other, but the particles are
not free to diffuse across until they reach
equilibrium.
43
Dialysis Tubing Demonstration
• Dialysis tubing is a flexible tube that looks like plastic sausage
tubes, but they are semipermeable like a real cell membrane.
Dialysis tubing is used in laboratory demonstrations about osmosis
because it is not permeable to glucose but water can cross it.
• It helps you learn about the body because glucose also cannot get
across the body’s cell membranes, but water can. In a laboratory
demonstration, water will move into dialysis tubing until it
reaches a certain column height. It does not continue to climb
higher and higher in the column indefinitely, because gravity
will be exerting forces on it too (hydrostatic pressure).
Eventually, the water will reach a certain height and then stop.
• The point at which is does this is when the hydrostatic pressure
caused by the gravity is equal to the osmotic pressure of the water
trying to get into the tube. If we did this experiment in outer space,
all of the water would cross over the membrane and the column
would rise until all the water is gone.
44
Movement of Water in the GI Tract
• Imagine that you take some aspirin and wash it down
with water. If the aspirin particles can get across the cell
membrane of your intestinal tract, there will be no net
gain or loss of water in the compartment.
• But if you eat a bunch of cellulose (fiber), it cannot cross
a cell membrane. There will be more particles in the GI
tract lumen, so the GI lumen will suck water from the
nearby cells into the intestinal lumen.
• That is how laxatives work! A laxative will cause the
osmolality of the GI tube to be increased, so water will
move from the surrounding cells into the GI lumen. If
they extract fluid from the body and in excess, they may
cause dehydration.
45
Hydrostatic Pressure
• Osmotic pressure is the amount of
hydrostatic pressure required to stop
osmosis from moving water from low to
high concentration across a cell
membrane.
• Osmotic pressure is attributed to the
osmolarity of a solution.
• The solution with the highest number of
particles will have the highest
hydrostatic pressure.
46
Membrane
Function
• Osmosis is movement of
water to an area with more
solutes
– This happens when the cell
membrane is permeable to water
but not solute
– direction of osmosis is
determined by differences in
total solute concentrations
– Hypo-osmotic (few particles)
– Hyper-osmotic (many particles)
– Water always moves! Watch
your body fluid compartments
In what direction will osmosis go?
What side of the tube is hypo-osmotic?
Which side is hyper-osmotic?
When would osmosis stop?
47
What if we were in space?
Review: Implications of Concentration and
Osmolality Differences Across Membranes
• First, let’s focus
on a solute that
can move across
the membrane.
• Let’s say these
green particles
are aspirin
molecules in the
stomach.
• How would
these molecules
move across the
body fluid
compartments?
• Diffusionrandom
movement of
particles from
“high to low”
Plasma
GI tract
http://www.indiana.edu/~phys215/lecture/lecnotes/diff.html
48
Review: Implications of Concentration and
Osmolality Differences Across Membranes
•
•
•
•
•
Now, let’s say
you’ve eaten fiber
(cellulose)
You can’t absorb
it!
There are more
particles in one
body fluid
compartment
What will happen?
Water movement
– Osmosis
– Hydrostatic
pressure
This is the basis for
how diuretics and
laxatives work!
Plasma
GI tract
49
Osmotic Pressure
• Osmosis occurs when water moves from a
solution w/ fewer particles to one with more
particles because the particles can’t move
across the membrane!
– Remember “particles suck (....the water).”
• Osmotic pressure is the amount of pressure
required to stop osmosis from happening
(hydrostatic pressure).
• Water is likely to enter a solution that
contains lots of particles that are
impermeable across a membrane– thus the
solution is said to have a high osmotic
pressure!
50
Osmotic Pressure:
the amount of hydrostatic pressure (force of fluid exerted on
the vessel wall) required to counter osmosis
Osmotic pressure
is attributed to the
osmolarity
of a solution
Isosmotic - has same
osmolarity as body fluids
(same number of particles, or
300 mOsm)
Hyperosmotic - higher
osmolarity than body fluids
Hyposmotic- lower
osmolarity than body fluids
Figure 4-10;51
Guyton & Hall
Osmotic Pressure
• In a U-shaped tube separated by a membrane,
water moves to the side with more particles
(Particles suck). If a compartment has a high
number of particles that cannot cross the cell
membrane, water will move from the area
with fewer particles to the area with more
particles.
• Osmotic pressure is the amount of hydrostatic
pressure you need to apply to stop the water
from moving (from the top of the tube where the
particles are highest).
52
• What will happen to a cell placed in the
following solutions?
• Isosmotic (300 mOsm): no net gain or
loss of water.
• Hyperosmotic (600 mOsm): particles
suck, so solution will suck the water
from the cell, which will shrink.
• Hyposmotic (100 mOsm): particles
suck, so cell will suck water from the
solution and burst.
53
Diffusion
• The above example assumes that the particles in the cell
cannot diffuse out, which is usually the case.
• However, there are particles (such as urea) that can
cross a membrane. Urea will diffuse out of one
compartment and into another, down its concentration
gradient. As it does so, water will also be diffusing back
and forth down its own concentration gradient, but
overall, there is no net gain or loss of water in a
compartment when particles are able to diffuse
across the membrane.
54
Example with Diabetes
• Increased sugar in the blood
• Relatively less water due to increased
solute concentration
• Describe the movement of water when the
mOsmolal changes! (It flows into the
plasma)
300
300
Water flow
S
S
S
S
S
304mOsm
Interstitial space
Intercellular
fluid
Plasma
55
Example with Diabetes
• When you eat, sugars are absorbed into plasma.
Normally, insulin transports these sugars into the cells,
but in a diabetic with no insulin, the sugars stay in high
concentration in the plasma. That raises the plasma
above 300 mOsm, while the interstitial fluid is still at 300.
Where will water go? Water will move from the interstitial
fluid into the plasma. Now, the interstitial space has less
water, but the same number of particles, so it may be at
300 particles per HALF a liter, instead of 300 particles
per liter. To calculate the number of particles per liter,
multiply by 2 and you will see that the interstitial space
has actually become 600 mOsm.
56
Example with Diabetes
• Water flows into the blood vessel until the osmolarity in the blood
vessel decreases to 300 mOsm (normal). However, the interstitial
space now has less water, but the same number of particles, so it
may be at 300 particles per HALF a liter, instead of 300 particles per
liter, which is the same as saying it has become 600 mOsm. Where
will water go now?
• Water will then go from the cells into the interstitial space
and the person gets dehydrated.
S
S
S
300
Water flow
600
Water flow
S
300mOsm
Interstitial space
Intercellular
fluid
Plasma
57
Polydipsea
• The condition where a person drinks a
lot of water because they are thirsty is
called polydipsea, and is characteristic of
a person with diabetes.
58
Test yourself- by picking which one
of these is correct
•
What is the proper sequence of events in
diabetes mellitus?
a) Cells lose water to ECF via osmosis; ECF osmotic
pressure rises; Solute concentrations increase in ECF.
b) Solute concentrations increase in ECF; Cells lose water to
ECF via osmosis; ECF osmotic pressure rises
c) Solute concentrations increase in ECF; ECF osmotic
pressure rises; cells lose water to ECF via osmosis
d) Water is lost from the ECF; Solute concentrations increase
in ECF; ECF osmotic pressure rises; Cells lose water to
ECF via osmosis.
What if the question described an athlete who was dehydrated?
59
Same thing occurs with a
dehydrated athlete
•
There is less water in the plasma, so the mOsm there is higher than normal
(304 mOsm). Water flows into the blood vessel until the osmolarity in the
blood vessel decreases to 300 mOsm (normal). However, the interstitial
space now has less water, but the same number of particles, so it may be at
300 particles per HALF a liter, instead of 300 particles per liter, which is the
same as saying it has become 600 mOsm. Where will water go now?
• Water will again go from the cells into the interstitial space. The
blood volume becomes normal but the person is thirsty. If the
condition lasts for too long, blood volume (and blood pressure) will
go down.
S
S
S
300
Water flow
300
Water flow
S
304mOsm
Interstitial space
Intercellular
fluid
Plasma
60
Note this poor child’s swollen distended
belly. This condition is called ascites.
It occurs from lack of dietary protein.
Alcoholics and elderly people also may
lack dietary protein.
Kwashiorkor- “disease of deposed
child” (no longer suckled).
•Occurs in economically
disadvantaged countries that use
cornmeal as their primary food source
•Corn has no protein
•Edema (interstitial accumulation of
fluid) is caused by low levels of
plasma proteins (particles in the
blood!).
• In Kwashiorkor, the plasma osmotic
pressure is low compared to the
transcellular osmotic pressure
Used with permission given by A. Imholtz
http://academic.pg.cc.md.us/~aimholtz/
That means the plasma has more
water, less particles than normal. The
fluid moves from the plasma into the
interstitial space and then into an area
of low resistance, which is the
61
abdominal cavity.
Example with Malnourishment
• We need all of our essential amino acids, and problems
can occur if you are deficient in only one amino acid.
• All compartments, including the plasma, should be 300
mOsm.
• There are many plasma proteins; the most abundant is
albumin, which is made in the liver.
• If your diet is deficient in an amino acid, the liver cannot
make enough plasma proteins.
• If albumin numbers decline, particles in the plasma
decrease.
62
Example with Malnourishment
• Plasma decreases to 200 mOsm.
• Other compartments will suck the water from the plasma
and edema will result.
• Therefore, lack of dietary protein causes a low mOsm in
the blood plasma.
• Since there are more particles in the interstitial space,
water will move from the plasma to the interstitial
compartment.
63
Ascites
• The peritoneal cavity is the space outside of the digestive organs
and under the skin and fat.
• It is an area of low resistance; there is not much there to hold back
something that is pressing from the inside. The fluid goes from the
plasma into the peritoneal compartment (outside of the GI organs,
deep to the skin), and the belly becomes distended with fluid (the
condition is called ascites).
• It is not to be confused with a full stomach; ascites is characteristic
of malnutrition and other diseases.
• Edema (excess interstitial fluid) also occurs in legs, since there is
no pressure there to keep it in. Ascites is not just a problem of poor
countries. Other people who often get ascites are alcoholics and the
elderly who don’t eat their protein.
• When malnourished, the body will break down the proteins in the
muscles to get what is needed elsewhere. Alcoholics drink their diet;
64
the liver becomes so scarred that it can’t make proteins.
Acites
• Bile is made in the liver and excreted into the digestive
tract by the bile duct.
• Ascites puts pressure on the bile duct, causing “portal
hypertension”. This blockage prevents the release of
bilirubin, so this yellow pigment enters the tissues, turning
the skin yellow, especially the white parts of the eyes. This
yellow appearance of the skin is called jaundice.
• When excess bilirubin enters the brain, it causes nerve cell
death. This is why alcoholics still seem drunk when they
are sober.
• Bilirubin is the result of RBC death. Parts of the RBC can
be recycled (such as the iron), but the bilirubin (part of
hemoglobin) needs to be eliminated. Bilirubin is what
causes the color of urine and feces.
65
Our book refers to tonicity more than
osmolality....are they interchangeable?
• NO!!!!!!!
• Tonicity refers to the number of
particles per kg of water in each
of two compartments, where the
membrane that separates them
is impermeable to the particles.
Differences in tonicity between
the two compartments always
leads to water moving from one
compartment to the other.
Osmolality refers to the number
of particles- regardless of
permeability
• Some solutes (primarily urea)
are freely permeable to cell
membranes (exhibit passive
transport). A hyperosmotic
solution may cause only
transient shifts in the body under
steady-state conditions.
Figure 25-5; Guyton and Hall
66
Urea is Permeable
• Why is it important to understand this
about urea?
• Because urea is reabsorbed in the early
stage of kidney filtration, and then
excreted in the later stage. During the
early stage, it does not cause water to be
reabsorbed with it, so the kidneys are free
to excrete the excess water.
67
Tonicity vs. Osmolality
• Tonicity and osmolality are not the same word.
Tonicity refers to the number of particles in 1kg
of water. Will there be a shift of water across the
cell membrane? Most of the time, particles don’t
move so the water does.
• If this was always the case, tonicity and
osmolality are the same number. But
substances, such as urea, can move across a
cell membrane, so there is only a transient shift
in water. In these cases, the resulting osmolality
and tonicity are different.
68
For example:
• 300mmol/L sucrose
• isosmotic to our bodily fluids!
300mmol/Kg sucrose
300mmol/kg sucrose
is also isotonic- no
change in cell volume
It’s osmolality is also
its effective osmolality
(tonicity).
69
• Place a cell in isotonic solution: There are
equal particles and no net gain or loss of
water.
• This solution is considered to be Isotonic
and isosmotic.
70
What about 600mOsmolal Sucrose?
•Hyper-osmotic
•Hypertonic
• Place a cell in
hypertonic solution:
The cell will shrink.
• This solution is
hypertonic and
hyperosmotic.
Water will
move out of
the cell.
71
What about 200mOsmolal Sucrose?
•Hypo-osmotic
•Hypotonic
• Place a cell in
hypotonic solution:
The cell will swell.
• This solution is
hypotonic and hypoosmotic.
Water will
move into the
cell.
72
Let’s see what happens with urea
400mmol/Kg urea
Hyper-osmotic
Isotonic (long-term)
73
Difference between tonicity and
osmolarity
• Not all hyperosmotic solutions are hypertonic.
• If you place a cell in a solution with 400 mOsm
of urea, the urea (a particle) will move into the
cell until it equalizes.
• The water will transiently shift from the solution
to the cell until it equalizes, with no net gain or
loss of water from the cell. The cell will not
shrink, nor will it swell.
• This solution is hyperosmotic but isotonic.
• Urea is important for proper kidney function, so it
74
needs to be able to cross a membrane.
Study Tip:
• How to remember what a cell does in hypertonic water?
• Hypertonic has an “e” in it. Make the letter e with your
body. See how you have to curl up and shrink to make
this letter? A cell will curl up and shrink in hypertonic
solution.
• How to remember what a cell does in hypotonic water?
• Hypotonic has an “o” in it. Make the letter o with your
arms over your head. See how you have made yourself
bigger? A cell will swell up in hypotonic solution.
75