You may remember when we talked about capillary
function, fluid is released from the capillaries
at the arteriolar side because the pressure
in the capillary – the hydrostatic pressure
– is higher than the oncotic pressure from
the proteins in the blood.
At the other end, at the venule end, the oncotic
pressure is a little higher than the hydrostatic
pressure. But you’ll remember when we reviewed
this that actually there is still a little
bit of advantage for hydrostatic pressure
so a tiny amount of fluid continues to leak
out of the capillaries.
And remember this was Starling’s law of
the capillary. And so you do need the lymphatics
because you want something to drain that little
bit of tissue fluid, also called interstitial
fluid, in between the cells. And that leads
then to a balance in the cardiovascular system.
Remember: hydrostatic pressure a little higher
at the arteriolar end, oncotic a little higher
at the venous end but overall a tiny advantage
to hydrostatic pressure so that a small amount
of fluid escapes from the capillary into the
tissue space. And the lymphatics take this
fluid away from the tissues and bring it back
to the bloodstream.
So how do we get the fluid to flow through
the lymphatic system?
Well it’s a system very similar to the veins.
First of all, the small lymph vessels have
valves, just like veins. Their muscle
pumping squeezes the lymphatics, pushing the
fluid up in the right direction. When you
take a deep breath, you have negative pressure
in your chest and that tends to suck the fluid
up in the lymph vessels. And many of the larger
lymph vessels have smooth muscle so, just
like the veins, they contract a little bit.
All of these processes move the lymph from
the tissues up into the large veins in the
chest where they’re returned to the cardiovascular
system – to the blood.
So, again, we start with lymph capillaries
draining lymph. They collect in larger lymph
vessels known as lymphatics. These larger
ones have a little bit of smooth muscle. And
then eventually they feed into very large
lymph vessels in the chest that feed into
Large lymph-collecting vessels connect to
the venous system in the chest: on the right
side to the subclavian vein, on the left side
they return into the superior vena cava.
As the lymph capillaries proceed towards the
chest, they’re joined by other capillaries
growing larger and larger. They develop smooth
muscle, they develop adventitia. In other
words, they look a lot like veins but they’re