00:00
Finally, let’s talk about which physical
principles influence the blood circulation.
00:08
We talked a little bit about this before.
Remember that the blood velocity – the speed
with which the blood is travelling in the
blood vessels – is greatest just after it
leaves the left ventricle, just above the
aortic valve in the aorta. And the speed decreases
progressively with decreasing vessel size
or diameter. There’s a significant drop
off in speed in the arterioles – remember
the resistance vessels. And then there’s
very slow flow through the capillaries. That’s
important because that’s when all the work
is being done of exchanging oxygen, nutrients
and waste products. So that you can see that
the speed in the aorta is 30 cm/s for the
blood whereas in the capillaries it’s only
0.026 cm/s. But that’s very important because
we want plenty of time for the red blood cells
to give up their oxygen and get in the carbon
dioxide from the cells and give off nutrients
to the cells.
01:19
This is one of the physical principles that’s
most important in the cardiovascular system
is called the law of continuity. That is the
volume of flow per second has to be constant
throughout the entire system. And, of course
we know that because there’s so much greater
surface area in the capillaries so that, even
though the flow is slower, it ends up being
constant and the flow continues in a continuous
circle.
01:48
You can see the little diagram below showing
you the total surface area being much larger
in the capillaries where the velocity is much
slower. And also the blood pressure is much
higher where the cross-sectional area is smaller.
And the pressure is much lower in the capillaries
where the blood is doing its work of exchanging
oxygen and nutrients for waste products. So
here we see a very important law of the capillary.
02:20
The physical principles governing capillary
function are extremely important and were
discovered by an English physiologist named
Starling in the early part of the twentieth
century. He is also the Starling who came
up with the rubber-band law of the heart in
which the heart contracts more when you stretch
it more. In any case, the capillary law of
Starling talks about fluid passage in and
out of the capillary. There are two forces
here: the blood pressure pushes fluid out
of the capillary and the oncotic pressure
made by dissolved proteins in the blood tends
to pull fluid by diffusion back into the capillary.
03:04
You can see on the left near the arteriole
end of the capillary that the blood pressure
is 32 mm Hg and the osmotic or oncotic pressure
is only 22 mm Hg, making for a net outward
pressure of 10 mm Hg at the arteriole end.
03:24
When you get down to the venous end, the blood
pressure has fallen. So now the blood pressure’s
only 15 mm Hg but the oncotic pressure is
still 22 mm Hg. So that there is a net negative
pressure at the venous end of the capillary
of 7 mm Hg. Now those of you who’ve been
following this will know immediately, “Oh
but still there’s a gain of 3 mm Hg along
the length of the capillary in the favour
of outward pressure, which means fluid moving
out of the capillary and into the interstitial
space – that is the space between the cells.”
Now, if that fluid were not removed, it would
result in swelling and oedema. And that’s
where the lymphatic system comes in.
04:14
The lymphatic system drains that interstitial
fluid so that the tissues do not become swollen
with fluid.
04:23
Well, in summary then, in this lecture we’ve
covered the main classes of blood vessels: