MAVO Year 4 · Transport and immune defence · Section 13.5

Tissue fluid and lymph

Tissue fluid

Fluid is filtered from blood capillaries into the spaces around tissue cells, influenced by pressure differences and the properties of vessel walls. This tissue fluid, or interstitial fluid, forms a medium for exchange. Oxygen and nutrients move towards cells, while carbon dioxide and other products move away. White blood cells can also enter the tissues.

Water and dissolved substances exchange between capillaries and tissues. A major route returning excess filtered fluid and escaped proteins to the blood is the lymphatic system. It should not be assumed that almost all filtered fluid simply re-enters at the venous end of every capillary.

Formation and composition of lymph

Once tissue fluid enters lymphatic vessels, it is called lymph. It contains water, dissolved substances, proteins and immune cells. Its composition varies with the tissue it drains. Lymph from the intestine can also carry absorbed fats.

Valves limit backward flow. Muscle activity and movements associated with breathing help propel lymph through progressively larger vessels. Major ducts, including the thoracic duct and right lymphatic duct, return it to veins near the collarbones.

Lymph nodes and defence

The lymphatic system includes vessels, lymph nodes and other lymphoid tissues. It returns excess tissue fluid to circulation and contributes to immune defence. Nodes contain immune cells that detect antigens, engulf some material and activate specific responses, including antibody production. Clusters occur in areas such as the neck, armpits and groin.

Lymph nodes do not remove every harmful substance, and lymph is not a separate pump-driven circulation equivalent to the blood system.