Final answer:
The fluid in lymphatic vessels is known as lymph, which is interstitial fluid that has entered the lymphatic system and includes white blood cells.
Step-by-step explanation:
The fluid found in lymphatic vessels is known as lymph. This is the term used to describe interstitial fluid once it has entered the lymphatic system. Lymph originates as plasma that filters out of the capillaries into the interstitial space of the tissues. Most of this plasma is reabsorbed directly by the blood vessels, but some becomes interstitial fluid, providing nutrients and waste removal for cells. The part that is not reabsorbed enters the lymphatic vessels as lymph, which also contains white blood cells that help combat infection. The lymph is then returned to the bloodstream through a series of vessels, trunks, and ducts.