Lymph (Tissue Fluid)

Blood does not deliver its dissolved nutrients and gases directly to the cells of most tissues; instead, as blood flows through the network of capillaries, some of its plasma is continuously filtered out through the thin, single-cell-thick capillary walls into the narrow spaces between the surrounding tissue cells, forming what is called interstitial fluid or tissue fluid. It is this tissue fluid, once it has been collected into the blind-ending lymphatic capillaries that run alongside the ordinary blood capillaries throughout the body and drained onward through the larger lymphatic vessels, that is properly called lymph.

In composition, lymph closely resembles blood plasma, since it is derived directly from plasma by filtration, but it differs from plasma in two important respects. First, because only the smaller plasma proteins can cross the capillary wall efficiently, lymph contains a considerably lower concentration of protein than plasma does. Second, lymph normally contains no red blood cells at all, since red blood cells are too large to pass through the capillary wall along with the filtered fluid; the only cellular component regularly found in lymph is a population of lymphocytes, the white blood cells specialised for immune defence, picked up as the lymph passes through lymph nodes along its route. …