In this video, I’ll explain how two different
pressures (hydrostatic and osmotic pressure) drive fluid out of and back into your blood
vessels. These pressures might seem a bit unimportant at first but they’ll help us
explain some interesting cases of edema, or tissue swelling. So, enough talking.
:) Let’s do it! Welcome to “Physio Flip”! Here we have a tissue, like muscle or
nervous tissue, which is being supplied with blood by this network of capillaries.
Capillaries, with their thin walls, pores, and gaps are specially designed to
allow for the exchange of nutrients, gases, wastes, and fluid between the blood
and the tissues. They’re the place in the cardiovascular system where we can
get stuff into and out of the blood. For example, as blood flows into the capillary on
the arteriole end, you’ll notice that some fluid is filtered out of the blood vessel. However,
we normally don’t want that fluid to stay in the interstitial space as this could lead to fluid
collecting in the tissues, causing swelling. Many of you have probably experienced this before:
if you have ever stayed standing for too long, you might have noticed that your ankles and
feet swell up or experience “edema”. This is caused by fluid staying outside the blood
vessel, within the interstitial space, causing the tissue to swell. To prevent this swelling from
happening, much of the fluid that was filtered out is normally reabsorbed back into the capillary at
the venous end. So fluid out, and fluid back in. But why was fluid forced out in the first
place, and how does it get reabsorbed? Let’s zoom in first on the fluid being
filtered out to see what’s going on. The pressure that drives fluid movement out
of the blood in the capillaries is called hydrostatic pressure. Instead of some fancy
definition of what hydrostatic pressure is, I’m just going to show you, using an example
that some of you may be able to try out at home! Here I have a straw, like what you might use
to drink. What I’m going to do is put my finger on this end of the straw, preventing fluid
from leaving. Then I’m going to fill up the straw with water. If I were to take this
straw and poke a hole in it over here, what do you think would happen? You might
have guessed it: water will begin to leak out. Why? Well, all the water in the straw is being
pulled down by gravity, which then applies pressure to the water down here by my finger.
This pressure is termed hydrostatic pressure, and generates an outward pressure that
will drive fluid out of the straw. You might be thinking: “Alright dude, we don’t
care about the straw. What’s with the straw?!” Well, what I just showed you is basically the
same thing that happens in our capillaries: our cardiovascular system is basically just a
long straw with “holes” in the capillaries. In our capillaries, due to the amount of fluid in
our cardiovascular system and the hydrostatic pressure in the arteriole end of
the capillaries, fluid is forced out by hydrostatic pressure through microscopic
pores and channels in the capillary walls. But if hydrostatic pressure were all we
had, fluid would collect in the tissues and we’d experience edema, or swelling. Time
for our second key term: osmotic pressure. Osmotic pressure is based on 1 key fact: by
osmosis, water always flows towards regions in the body that have a higher osmolarity, or
a higher concentration of solute particles. For example, let’s say we have some water,
surrounding a vessel shown here in yellow. This vessel has pores or small “holes” which allow
certain particles to cross. Inside this vessel, I’ll put a high concentration of some particle
(shown here in orange). Notice that this particle’s concentration is much higher on the
inside of the vessel, compared to the outside. But if this particle is relatively large, like
a protein molecule, and cannot leave this vessel through the small pores, that means that the
concentration of these large, protein molecules will remain high inside the vessel and
remain low on the outside of the vessel. Since water always flows towards regions
with a higher concentration of particles, what do you think water is going to do? Yup-in
this case, water is going to be reabsorbed back into this vessel, towards the
higher concentration of particles. This situation is almost exactly what
is happening in your capillaries. In your blood you’ll find a higher
concentration of proteins (like albumin) compared to outside the blood vessels in the
interstitial space. Because of this higher concentration of proteins in your blood, fluid
tends to be reabsorbed back into the blood, driven by osmotic pressure. But for now, what we’ve
learned is that hydrostatic pressure tends to drive fluid out of your blood vessels,
osmotic pressure tends to drive it back in. Quick side note: this osmotic pressure, caused by
large molecules like proteins, is called colloid osmotic pressure (or oncotic pressure). So,
as I mention osmotic pressure in this video, I’m really talking specifically about “colloid
osmotic pressure”. Also, we do have other solutes in our blood besides proteins, like Na+ and Cl-.
However, these do not contribute to the osmotic pressure or the reabsorption of fluid. If you’re
curious as to why, checkout the comments below! Zooming back out, let’s put
it all together to see how hydrostatic and osmotic pressure filter
and reabsorb fluid at the capillaries. On the arteriole end of the capillary,
the hydrostatic pressure is higher than the osmotic pressure. This causes fluid to
be forced out into the interstitial space. But as hydrostatic pressure decreases as we
move through the capillary, the osmotic pressure eventually becomes equal to and even greater
than the hydrostatic pressure, causing nearly all of the filtered fluid to be reabsorbed
back into the capillary at the venous end. You might notice that there is still a small
fraction of fluid sitting in the tissues. Not all of it was reabsorbed back by osmotic pressure.
That’s where our final player comes in: the lymphatic system. Vessels of the lymphatic system
will help reabsorb the remaining small fraction of filtered fluid that wasn’t reabsorbed by osmotic
pressure, helping us to prevent swelling or edema. There you have it! In the next videos, we’ll
review a few cases where fluid does collect within the tissue, caused by abnormal changes
in hydrostatic pressure or osmotic pressure. For example, I’ll explain in detail why your ankles
swell up when you sit or stand for too long.