lymphatic fluid
Meaning of lymphatic fluid: Clear in an arm, milky after a fatty meal: how it looks depends entirely on which tissue it has just drained.
Definition of lymphatic fluid
Lymphatic fluid is the pale fluid that drains out of body tissues into lymphatic vessels and travels back to the bloodstream. It begins as plasma pushed out of small blood vessels into the spaces between cells. What is collected carries dissolved protein, cell waste, fat absorbed from the gut and immune cells, and it filters through lymph nodes along the way.
Where does it come from?
From a leak that happens by design. Blood pressure inside the smallest vessels pushes water and dissolved substances out through their walls into the tissue, feeding the cells. Most of that fluid is drawn back in further along, but not all of it, and the remainder would swell the tissue within hours if nothing collected it. Blind-ended lymphatic capillaries take up the surplus through overlapping flaps that open under pressure and close behind it. Once inside a lymphatic vessel the fluid is called lymph. It moves without a pump of its own, squeezed along by contracting muscle, by breathing and by the vessel walls themselves, with one-way valves preventing return. It reenters the bloodstream through large veins at the base of the neck.
Is it always clear?
| Source | Appearance | Why |
|---|---|---|
| Limbs and skin | Pale straw, clear | Water, salts and protein only |
| Gut, after a fatty meal | Milky white | Loaded with absorbed fat droplets |
| An inflamed area | Cloudy | Rich in immune cells and debris |
Fat absorbed from a meal does not enter the bloodstream directly. It is packaged into droplets too large for blood capillaries and is taken up instead by lymphatics in the intestinal lining, turning that lymph milky. The milky version has its own name, chyle, and a leak of it into the chest cavity produces a distinctive milky collection.
What happens when it cannot drain?
Fluid backs up into the tissue and stays there, producing a firm swelling that does not shift with elevation the way ordinary swelling does. Because the fluid is protein-rich, it draws in more water and over time provokes scarring and thickening of the skin. Blocked drainage follows surgery that removes nodes, radiation, injury, or infection with the parasitic worms that cause filariasis. Removal of an axillary lymph node group during breast surgery is a well recognized cause of swelling in the arm, which is why the lymphatic system is mapped carefully before nodes are taken.
Also known as
Used in a sentence
Lymphatic fluid collected in the wound bed after the node dissection.
Good to know
Lymph draining the intestine after a fatty meal is milky rather than clear because it carries absorbed fat, and swelling caused by blocked lymphatic drainage is protein-rich and firm rather than the shifting swelling of excess water alone.
Word origin
Where “lymphatic fluid” comes from
Latin lympha, clear spring water, describes how the fluid looks, and the Greek adjective ending gives lymphatic.