As the blood passes through the capillaries in tissues, some water along with many small water-soluble substances move out into the spaces between the cells of tissues leaving the larger proteins and most of the formed elements (RBC, WBC and Platelets) in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as that in plasma. Exchange of nutrients, gases, etc., between the blood and the cells, always occur through this fluid. An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called the lymph. Hence lymph acts as the middleman.