Interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid. The interstitial fluid is found in the interstitial spaces, also known as the tissue spaces.
On average, a person has about 11 litres (2.4 imperial gallons or ~2.9 US gal) of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal.
Production and removal 
The water potential is created due to the ability of small solutes to pass through the walls of capillaries. This buildup of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.
The balance between the two forces differs at different points on the capillaries. At the arterial end of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement (see net flux) favors water and other solutes being passed into the tissue fluid. At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid.
To prevent a build up of tissue fluid surrounding the cells in the tissue, the lymphatic system plays a part in the transport of tissue fluid. Tissue fluid can pass into the surrounding lymph vessels, and eventually ends up rejoining the blood.
Sometimes the removal of tissue fluid does not function correctly, and there is a build-up. This can cause swelling, often around the feet and ankles, which is generally known as edema. The position of swelling is due to the effects of gravity.
Interstitial fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, as well as waste products from the cells.
The composition of tissue fluid depends upon the exchanges between the cells in the biological tissue and the blood. This means that tissue fluid has a different composition in different tissues and in different areas of the body.
Not all of the contents of the blood pass into the tissue, which means that tissue fluid and blood are not the same. Red blood cells, platelets, and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is, in essence, blood plasma without the plasma proteins. Tissue fluid also contains some types of white blood cell, which help combat infection.
Lymph is considered to be extracellular fluid until it enters the lymphatic vessels where it is then considered to be lymph. The lymphatic system returns protein and excess interstitial fluid to the circulation.
The ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs-Donnan effect. This causes a slight difference in the concentration of cations and anions between the two fluid compartments.
Physiological function 
Structure of the lymphatic system 
The lymphatic system is a collection system which starts in the tissue space as initial lymph collectors that have fenestrated openings to allow fluid and particles to enter. These initial lymph collectors are valveless vessels and go on to form the precollector vessels which have rudimentary valves which are not considered to be fully functional. These structures go on to form increasingly larger lymphatic vessels which form co-laterals and have lymph-angions (lymph hearts). The lymphatic system, once thought to be passive is now known to be an active pumping system with active pumping segments with a function similar to that of peristalsis. Lymph hearts have stretch receptors and smooth muscle tissue embedded in their walls. The lymphatic vessels make their way to the lymph nodes and from the lymph nodes the vessels form into trunks which connect to the internal jugular group of veins in the neck.
See also 
- Marieb, Elaine N. (2003). Essentials of Human Anatomy & Physiology (Seventh Edition ed.). San Francisco: Benjamin Cummings. ISBN 0-8053-5385-2.