How does the lymphatic system differ in structure from the veins?
permiability of the lymphatic vessels compared to the blood vessels
I know it sounds like a physiologic difference, but the bonus permeability that the lymphatic system possesses lies in the structure of the lymphatic veins. They have some uni-directional openings that allow them to collect interstitial fluid, including proteins that got out of the blood vessels due to osmotic pressure
( they can't get back in the blood vessels due to their huge molecule size )