The fluid that bathes the body cells of animals, consisting of water, ions, and dissolved gases and food substances. It is formed when blood is ultrafiltered (see ultrafiltration) from the capillaries into the intercellular spaces. The pressure in the arterial capillaries causes most components of the blood to pass across the capillary walls; blood cells and most of the plasma proteins are retained in the capillaries. The tissue fluid surrounds the body cells, facilitating the exchange of nutrients and waste materials. At the venous end of the capillaries, the tissue fluid is drawn into the capillaries by osmosis. In animals with an open circulation, such as insects and other arthropods, the haemolymph serves also as the tissue fluid.