5 March, 2012
Capillaries are so good at allowing the change for materials the reason is that is the
morphology. The fact that they have very thin walls and packed close to the cells, they are trying
to get the nutrients and gasses to. They are in the right location and they don’t offer a large
barrier for diffusion.
This graph shows what makes capillaries even more ideal for allowing exachange.
It has to do with what happens to blood velovity aas the blood transit through the capailliares.
The blood flow is not the same blood velocity. These are two different parameters.
Blood flow= is volume per min or time (V/min)
Velocity = is distance per time (distance/ time )—gives no concept of vol , only distance
Since there are so many millions of capillaries, they represent a VAST cross-sectional area in
the circulatory system. The pic shows large aorta arteries to capillaries to the veins .
Because we have millions of capillaries, altogether they represent the LARGEST CROSS-
SECTIONALAREA component of the circulation. (Just because they are so many millions of
-The vol of blood that’s exiting the heart (100ml/ min) we want to have that (100ml/min) going all
the way through the circulation , (closed loop). Ideally in any location it’s going to be flowing at
100ml/min. That is going to represent blood flow, that should be pretty close the same thing at
different locations throughout the circulation.
Blood velocity is dependent on cross-sectional area.If we have certain blood that is packed in
aorta its, going to be travelling along the aorta at a certain velocity (mm/sec). But as we get that
vol of blood (100ml) and it reaches the capillary and its distributed to million diff capilliaries. The
100ml of blood is travelling a total of distance in length very slowly. Since its distributed to so
Velocity drops off drastically a lot in the capillaries compared to the large blood vessels. The
velocity picks up again as the blood gets back channelled into fewer and fewer blood vessels.
This is similar to water flowing through a river, and then into a lake and then into a river again.
Blue dye (blood 100ml used for tracking) , it travels forward is much slower.
Each individual component of this fluid at a slower velocity because its now spread out over a
long distance. As it channels back into the narrow vein again, its going to speed up again. This
is conceptually what happens to our blood as it goes through capillaries. It allows for more time
for diffusion to take place (key advantage for slowing the velocity).. It allows O2 to be off of hamemoglobin into the plasma, diffusing out of the circulation into the surrounding tissue. And
for nutrients to go that path and metabolites to go another direction.
If blood travels too quickly , it limits the amount of exchange that can occur and don’t improve
the delivery of substances.
Movement of substances across the capillary
-Our capillary is nice thin wall structure –(endothelial cells , they have fairly tight connections to
each other but they are not completely impermiant , they have small pores or clefts that
separate the individual cells.And so there is going to opportunity for substances to move across
the wall of endothelial cells. And also for substances to move out through the pores. What path
they take depends on the type of molecule
Lipid soluble substances (like gasses) don’t have to find any particular location, they can just
start diffusing right across the plasma membrane right into the tissue and back again. All they
need is a diffusion gradient to start. This process is very efficient since it has the entire capillary
surface as the exchange location (no limiting compartment).
Other things also need to move from plasma to tissue (amino acids, glucose, ions, metabolites),
these substances are not lipid soluble but water soluble. That means they can’t FREELY move
across the plasma membrane, they need to have water filled location to pass through. So
predominatnly, these substances are moving through the water filled clefts/pores which are
formed where endothelial cells meet with each other (makes a loose junction, with small pockets
so that water & small substances can pass through). Very much limited pathway, because there
is going to be fewer locations of these pores compared to the ENTIRE SURFACE OF THE
CAPILLARY. So this process is limited by how many pores there are and how large in size these
They will moving down concentration gradients by diffusion. The pores are small,which means
they set an upper limit to what can get through. These pores between the cells, are actually
creating a permeability barrier. Based on the size, what can squeeze through and what
cannot. So if you are small, and water soluble, its easier to get through,. If red blood cell THERE
IS NO WAY YOU WILL FIT THROUGH HERE. NOT A LARGE PROTEIN will fit here.
Lots of diff proteins , that are part of plasma, (albumin, immunoglobulins, big proteins that are
part of cloating cascades) these proteins are all quite large, and they are not capable of fitting
through these pores. So we got trapped plasma proteins that will not be able to leake out under
normal situations. Very important feature, that proteins are stuck in the plasma. There is limited amount of protein gets taken across the capillary wall, by a process of vesicle-
mediated endocytosis (proteins taken in on vesicle, vesicle moves across the cell, and then
exocytocsed on the outer side) So this protein now has been able to get across the capillary
wall and through these vesicles. Not very efficient process, and only a few proteins move this
NOT EVERY CAPILLARY IS MADE EQUALLY for instance, the capaillieies that are in the
brain , have a much tighter permeability barrier, which means the pores between the cells are
non-existent, (the cells are so tightly jammed up against each other , that there is no access
through water-filled channels to get fluids and dissolve substances out). Everything has to be
taken across via specific transport mechanism. This is what protects the brain, called blood
brain barrier (helps to protect it from nasty things that could get into the blood stream)
On the extreme opposite we have capillaries within the liver, these capillaries , these
endothelial cells have very loose junctions, ( they don’t come close to touching each other, they
have large gaps in between them which allows for a lot of substances to travel through) very
loose permeability barrier and allows a lot of substances. This allows liver to do its function
controlling for toxic substances that are in the blood.
For most part we have limited , but somewhat present permeability.
How is capillary exchange regulated ? (POSTED ON MOODLE)
Sometimes we need more exchange than others, we have more active tissue, metabolism goes
up , when muscles contract, more O2 , more nutrients for the cells in the tissue to work. So in
that rate, how can we control this exchange process? One of the ways our body manipulate
show this exchange happens is by controlling how much blood flow enters the capillaries. So
how is capillary blood flow regulated? It’s established by what happens what happens further
upstream. Capillary blood flow is determined what happens in immediate upstream network
of blood vessels.
Feed Artery branches into smaller arteriole arteries.
If we have constricted blood vessels, very little blood enters the capillaries, if we have
vasodilated blood vessles, then there is a lot more blood flwo that will enter the capillaries. By
increasing blood flow, the primary way we increase the amount of exchange that can happen.
Similar to hyperaemia , if a muscle Is active and producing metabolites, its going to cause
vasodilation of these blood vessels. So we have more blood entering into the capillary networks,
so we have more units per time passing through and also we are able to increase the
concentration gradients because we are supplying much more of a substance within the same amount of time. So , for instance, if we got a capillary , that has two red blood cells in it, those
red blood cells are bringing a certain amount of oxygen within them that will move into the
tissue. We have a concentration gradient established allowing for the diffusion of oxygen from
the plasma into the tissue. When we vasodilate those arterioles that are upstream of the
capillary, we end up packing the capillary with a lot of blood cells.
When we have vasodilation of the arterioles we end up packing our capillaries with red blood
cells. So there is no plasma in between only plasma in between (just cell and cell). Oxygen is
trapped within this portion of this capillary, we are enhancing the gradeitn to allow more O2 to
diffuse, since we packed our capillary with so much to begin with. This will alow for enhanced
delivery of substances and enchanced removal of substances.
How capillaries regulate fluid balance in the circulatory system? This has nothing to do with the
exchange of gasses or nutrients; this is just bulk FULID movement.
Why do we have this fluid exchange? Whats causing it to occur? A couple of different
forces that can cause fluid to move either out of the capillary or into the capillary, we will talk
about what these forces and what takes place. Hydrostatic pressure is our major forces .
Osmotic pressure is the other major force but there is hydrostatic pressures on both
compartments. We got our capillary compartment and our interstitial compartment and osmotic
forces on either sides. Hydrostatic pressure inside the capillary is the blood pressure , that’s
the force of fluid pushing out against the wall of the capillary and if the capillaries are
leaky or got pores in it, which will allow fluid to move .
As the fluid pressure pushes out the fluid will come squirting out of the holes that are in the
capillary wall (THAT’S OUTWARDS force, CAPILLARY HYDROSTATIC PRESSURE (P ). C
There is fluid in the extracellular space, that fluid is also going to exert a certain pressure ( its
very low pressure) since the total volume of interstitial space is VAST compared to the small
fixed volume of the capillary.
So the size of these arrow is not equivalent. Much greater hydrostatic pressure inside the
capillary compared to the very weak hydrostatic fluid inside the interstitial. The interstitial
pressure would push fluid out of the interstitial and back towards the capillary(direction of
--Osmotic pressures are determined by dissolved proteins, since ions balance between places.
Proteins are stuck in one compartment or the other. Proteins are stuck in one compartment or
the other, since this is a barrier to their movement.
-Lot of plasma protein inside, these plasma protein exert osmotic pressure, they draw fluid
towards them, water is going to be drawn from high concentration to low concentration. Presence of all the proteins jammed into the plasma, means water is in low concentration so
water wants to move towards the proteins to balance that pressure.
--Osmotic forces always draw water towards them, if there was proteins dissolved in the
interstitial fluid then that interstitial fluid proteins will draw fluid towards them. But in most cases,