Diagram showing the formation of lymph from interstitial fluid (labeled here as "Tissue fluid"). Note: how the tissue fluid is entering the blind ends of lymph capillaries (indicated by deep green arrows)
Lymph capillaries or lymphatic capillaries are tiny, thin-walled vessels, the smallest lymphatic vessels of the lymphatic system, that are closed at one end and are located in the spaces between cells throughout the body, except in the central nervous system, and in non-vascular tissues. The main purpose of these vessels is to drain excess tissue fluids from around the cell ready to be filtered and returned to the venous circulation. This tissue fluid upon entering the lumen (elongated cavity of a tubular structure) is known as the lymph.
Lymphatic capillaries are slightly larger in diameter than blood capillaries and have a unique structure that permits interstitial fluid to flow into them but not out. The ends of endothelial cells that make up the wall of a lymphatic capillary overlap. When pressure is greater in the interstitial fluid than in lymph, the cells separate slightly, like the opening of a one-way swinging door, and interstitial fluid enters the lymphatic capillary. When pressure is greater inside the lymphatic capillary, the cells adhere more closely, and lymph cannot escape back into interstitial fluid. Attached to the lymphatic capillaries are anchoring filaments, which contain elastic fibers. They extend out from the lymphatic capillary, attaching lymphatic endothelial cells to surrounding tissues. Lymph capillaries have a greater oncotic pressure, which is due to the greater concentration of plasma proteins in the lymph.
When excess interstitial fluid accumulates and causes tissue swelling, the anchoring filaments are pulled, making the openings between cells even larger so that more fluid can flow into the lymphatic capillary.
The lymphatic capillary becomes the afferent lymphatic vessel and carries the lymph into a lymph node.