Lymph (Tissue Fluid)

As blood flows through the tiny capillaries within the tissues, it does not stay entirely enclosed. Some of its water, together with many small water-soluble substances, seeps out of the capillaries into the spaces between the tissue cells. The larger proteins and most of the formed elements are too big to leave and remain behind inside the blood vessels.

Tissue fluid

The fluid that escapes into these intercellular spaces is called the interstitial fluid or tissue fluid. Because it comes from blood plasma, it carries the same distribution of minerals as plasma does. This fluid is the medium through which the actual give-and-take between the blood and the body cells takes place — the exchange of nutrients, gases and similar substances between blood and cells always happens by way of the tissue fluid.

The lymphatic system and lymph

The body does not let this escaped fluid simply accumulate in the tissues. A widespread network of vessels, the lymphatic system, gathers up the tissue fluid and returns it to the major veins, so it is put back into general circulation. The fluid once it is inside the lymphatic vessels is called lymph.

Lymph has these features:

• It is a colourless fluid.
• It contains specialised lymphocytes, the cells that carry out the body's immune responses.
• It also serves as a transport medium, carrying nutrients, hormones and similar substances. …