the fluid is called lymph which isn't considered blood because it doesn't have haemoglobin, and isn't considered tissue fluid because it lacks certain substances... so they call it lymph and it gets mixed with blood in the end anyway...
Lymph is a tissue that diffuses into the lymphatic capillaries
Tissue Fluid
Lymphatic vessels originate as blind-ended capillaries in the tissue spaces. These capillaries merge to form larger vessels that eventually join the lymphatic ducts, which return lymph fluid back to the bloodstream.
lymph nodes
The lymphatic capillaries are made up of endothelial cells, which are thin-walled cells that line the inside of the vessels. These cells have small openings that allow fluid and molecules to enter the capillaries.
Tissue fluid is returned to circulation through the lymphatic system, where it is collected in lymph capillaries and transported back to the bloodstream through lymphatic vessels.
The sequence of fluid flow is blood capillaries.... interstitial space or interstitial fluid..... lymphatic capillaries.... lymphatic vessels.... lymphatic ducts..... junction of the internal jugular and subclavian veins. Then what helps this whole sequence is the skeletal muscle pump and the respiratory pump.
The capillaries are the starting point of the one-way lymphatic system. Lymph capillaries originate in tissues and drain any excess tissue fluid that has not been reabsorbed into the bloodstream and move it into lymph veins. This prevents swelling or edema (collection of fluid in the tissues).
Lymphatic system
Most gas exchange between blood and tissues takes place in the capillaries. This is where oxygen diffuses from the blood into the tissues, and where carbon dioxide diffuses from the tissues into the blood. The thin walls of the capillaries allow for efficient exchange of gases.
Small spherical bodies made up of lymphatic tissue are called lymph nodes.
While surrounding tissues it is called interstitial fluid. Once it enters the lymphatic system it is called lymphatic fluid.