Blind ended, highly permeable, with valves and no basement membrane
Expansion
Lymphatic capillaries differ from blood capillaries in four ways
- Blind ended, beginning in the tissues rather than forming a loop
- No continuous basement membrane and large interendothelial gaps, so highly permeable
- Overlapping endothelial flaps acting as one way valves, allowing large proteins, lipids and cells to enter but not leave
- Anchoring filaments tethering them to surrounding collagen, so that tissue oedema pulls them open rather than compressing them
“The wetter the tissue, the wider the lymphatic opens.” That is an elegant piece of design: interstitial fluid accumulation itself increases lymphatic drainage.
Larger vessels acquire smooth muscle and valves, and flow depends on intrinsic contraction, skeletal muscle pumping, respiration and arterial pulsation.
Functions
- Return of interstitial fluid and protein, about 3 litres daily
- Fat absorption through the lacteals of the small intestine, as chylomicrons
- Immune surveillance, delivering antigen and cells to lymph nodes
Absent from: the central nervous system (which uses the glymphatic system instead), cartilage, bone marrow, epidermis and cornea.
Clinical consequences: lymphoedema after node clearance or filariasis is non-pitting and protein rich, unlike venous oedema; and the same permeability that admits protein admits tumour cells, which is why lymphatic spread is a principal route of metastasis.