Lymph is formed when interstitial fluid comes into contact with blood. Initially, it is a watery liquid with the same consistency as interstitial fluid, according to the Journal of Lipid Research. However, lymph thickens as it accumulates additional material from blood, such as proteins and lymphocytes.
Interstitial fluid forms in the ends of the capillaries nearest the heart, known medically as the arterial end. This fluid accumulation is the result of the difference in blood pressure between the arterial end of the capillaries and the other end, medically known as the venous end. Gray’s Anatomy on theodora.com reports that most of the interstitial fluid returns to the venous ends of the capillaries, although a minority of this fluid flows into the lymph capillaries to become lymph.
Lymph allows blood and interstitial fluid to remain in equilibrium with each other through the exchange of constituents. Interstitial fluid comprises the immediate environment of the cells in the body because it fills the spaces between cells. These cells and the blood are continually adding and removing constituents from interstitial fluid, which changes the specific composition of interstitial fluid. These constituents diffuse through the gaps between the capillary walls as the interstitial fluid becomes lymph, according to Gray’s Anatomy.