lymphatic vessel
Meaning of lymphatic vessel: Half of the circulation runs one way only: these tubes gather the fluid that leaks out of capillaries and hand it back to the blood.
Definition of lymphatic vessel
A lymphatic vessel is a thin-walled tube that carries lymph, the clear fluid squeezed out of blood capillaries into the tissues, back toward the bloodstream. These vessels begin as blind-ended capillaries between cells, join into larger collecting channels fitted with one-way valves, and pass through lymph nodes along the route. They form a one-directional network rather than a closed loop.
Where does the fluid come from?
Blood capillaries leak, and they are built to. Pressure inside them pushes water and small proteins out into the spaces between cells, and only most of that is drawn back in further along. What remains would swell the tissues within hours if it stayed. Lymphatic capillaries collect it. Their walls are made of overlapping cells tethered to the surrounding tissue by fine filaments, so when fluid builds up the flaps are pulled open and it flows in. Once inside, the same fluid is called lymph. Material too large to re-enter a blood capillary leaves this way instead, including proteins, cell debris, bacteria and fat absorbed from a meal.
How does lymph move without a pump?
There is no heart on this side of the circulation. The larger vessels are divided by valves into short segments, and muscle in the wall of each segment squeezes in sequence, pushing the contents onward. Outside help matters as much: contracting skeletal muscle presses on the vessels, breathing draws fluid up through the chest, and the pulse of a neighboring artery kneads the vessel beside it. Flow is therefore slow and depends on movement. The collected fluid drains into two large ducts at the base of the neck and empties into the veins there.
How do they differ from blood vessels?
| Feature | Lymphatic vessel | Blood vessel |
|---|---|---|
| Direction | One way, toward the neck | A closed circuit |
| Driving force | Muscle, breathing, valves | The heart |
| Entry of fluid | Through openable flaps | Sealed wall |
| Filters on the route | Lymph nodes | None |
The two networks meet only at their ends, which is why a blockage on this side causes swelling rather than a change in blood vessel pressure. The vessels, nodes and lymphoid organs together make up the lymphatic system, and the fluid they carry is lymphatic fluid.
Also known as
Used in a sentence
Tumor cells were seen within a lymphatic vessel at the edge of the specimen.
Good to know
Lymphatic capillaries admit particles and cells too large to re-enter a blood capillary, which is what makes them a ready route for infection and for tumor cells leaving a primary site.
Word origin
Where “lymphatic vessel” comes from
From the Latin lympha, meaning clear water, a word once used for spring water and applied to the pale fluid in these vessels.