Interstitial fluid

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) of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal.

Production and removal
Plasma and interstitial fluid are very similar. Plasma, the major component in blood, communicates freely with interstitial fluid through pores and intercellular clefts in capillary endothelium.

Formation of tissue fluid
Hydrostatic pressure is generated by the systolic force of the heart. It pushes water out of the capillaries.

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.

Removal of 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 causes swelling, and can often be seen around the feet and ankles, for example Elephantiasis. The position of swelling is due to the effects of gravity.

Composition
Interstitial fluid consists of a water solvent containing amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, 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 a part of the interstitial fluid. 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
Interstitial fluid bathes the cells of the tissues. This provides a means of delivering materials to the cells, intercellular communication, as well as removal of metabolic waste.