Final answer:
Interstitial fluid is the leaked fluid picked up by the lymphatic vessels and returned to the bloodstream.
Step-by-step explanation:
Lymph fluid that leaks out of capillaries into spaces between cells and circulates in the vessels of the lymphatic system is called interstitial fluid. The lymphatic vessels primarily function to pick up this leaked fluid and return it to the bloodstream. Lymph flows from tissue spaces, where it leaks out of blood vessels, to major veins in the upper chest, where it is returned to the cardiovascular system. Once the interstitial fluid enters the lymphatic vessels, it is called lymph.