Lymph : Lymph is a colourless fluid. It lacks RBC, platelets and large plasma proteins, but has more number of leucocytes. It is chiefly composed of plasma and lymphocytes. When compared to the tissue fluid, it contains very small amounts of nutrients and oxygen, but has abundant CO2 and other metabolites. The most important site of formation of lymph is interstitial space.As blood passes through the blood capillaries, some portion of blood that includes water, solutes and proteins of low molecular weight passes through the walls of capillaries, into the interstitial spaces due to hydrostatic pressure at the arteriolar ends. This fluid forms the interstitial fluid(tissue fluid). Most of the interstitial fluid is returned directly to the capillaries due to osmotic pressure at the venular ends. Little amount of this tissue fluid passes through a system of lymphatic capillaries (lymph capillaries of the intestinal villi are called 'lacteals'), vessels, ducts and finally reach the blood through the subclavian veins. The extracellular 'tissue fluid' that passes into the lymph capillaries and lymph vessels is called 'lymph'. Lymphatic system represents an 'accessory route'by which interstitial fluid flows from tissue spaces into blood.
