Some tissue fluid present in the intercellular spaces enters the blind lymphatic capillaries (lacteals). After the tissue fluid has entered the lymphatic vessels, it is now called lymph.
The excess tissue fluid is returned to the blood through the lymphatic system. This system consists of an extensive network of thin-walled vessels, the lymph capillaries. The lymphatic system is quite distinct from the blood vascular system.
The lymph capillaries have valves is their walls which allow the tissue fluid to enter the lymph capillaries but prevent its flow out of them. These capillaries join together to form larger vessels called the lymph vessels forming the lymphatic system.
The lymphatic system serves as an accessory drainage system of the body by removing the excess fluid and dissolved substances that leak from the blood capillaries. The larger lymphatic vessels join the subclavian veins and pour their fluid into them. This is how the lymph is returned to the circulatory system. The lymphatic system transports fats from the intestine to blood stream. It also helps is defending against the attacks of bacteria and viruses by filtering or destroying them at special sites called lymph nodes.