Tissue fluid

Interstitial fluid (ISF) or tissue fluid is a solution that bathes and surrounds the tissue 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 interstices - the spaces between cells (also known as the tissue spaces). On average, a person has about 10 litres (2.2 imperial gallons or ~2.4 US gal) of interstitial fluid (making up 16% of the total body weight), providing the cells of the body with nutrients and a means of waste removal.

Plasma and interstitial fluid are very similar. This similarity exists because water, ions, and small solutes are continuously exchanged between plasma and interstitial fluids across the walls of capillaries. Plasma, the major component in blood, communicates freely with interstitial fluid through and intercellular clefts in capillary endothelium.

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 inability of certain blood proteins (mostly serum albumin) to pass through the walls of capillaries. The build-up of these proteins within the capillaries 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.

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