Although blood and lymph do not provide mechanical support or structure to other tissues, they connect different organs, deliver nutrients, remove waste, and defend the body from foreign invaders. Thus, blood and lymph are called fluid connective tissues. Blood contains cells such as erythrocytes, phagocytes, lymphocytes, and platelets suspended in a fluid matrix called plasma. The red color of blood comes from the oxygen-binding protein hemoglobin in erythrocytes. Phagocytes and lymphocytes are part of the immune system and help fight infection and other diseases. Platelets, along with fibrous proteins called fibrins, induce blood coagulation. Blood flows through capillaries and reaches peripheral tissues, surrounded by a network of thin-walled tubes called lymphatic vessels. The hydrostatic pressure forces surplus fluid out of the capillary walls. This fluid occupies the space around the surrounding tissue cells and is called interstitial fluid. When it drains back into the lymphatic vessels, it forms the lymph. Lymph resembles plasma but contains fewer proteins and more immune cells.