How tissue fluid forms
At the arterial end of a capillary, blood pressure forces water and small dissolved substances out of the blood, forming tissue fluid that bathes the cells. Cells take up oxygen and nutrients from it and release wastes into it. Most of this fluid seeps back into the capillaries at the venous end, but some is left behind. This leftover fluid must be collected and returned to the blood, and that is the job of the lymphatic system.
Structure and lymph
The system is a one-way network of lymphatic vessels that begin as blind-ended capillaries in the tissues. The fluid inside them is called lymph — it is similar to tissue fluid but also carries lymphocytes (a type of white blood cell). Along the vessels lie lymph nodes, which filter the lymph and trap bacteria and other pathogens. The vessels finally empty their lymph back into large veins near the neck (the subclavian veins), returning the fluid to the blood.
Example
In the small intestine, special lymphatic vessels called lacteals absorb digested fats. The milky fluid they carry is called chyle, which shows the lymphatic system also has a role in nutrient transport.
No pump — how lymph moves
Unlike blood, lymph has no heart to pump it. It is pushed slowly along by the squeezing of nearby skeletal muscles during body movement, helped by valves that stop it flowing backwards. This is why gentle exercise and movement help drainage. The system therefore has no closed loop of its own: fluid travels in one direction only, from the tissues towards the neck, where it rejoins the blood and the cycle of fluid exchange can continue.
Remember
Three roles of the lymphatic system: return tissue fluid to the blood, absorb fats from the intestine, and defend the body by filtering pathogens in lymph nodes.