Tissue fluid, also known as interstitial fluid, is a fluid that surrounds the cells of human beings and other animals. The fluid has a variety of functions related to the body's homeostasis. It is formed around the capillaries due to osmotic pressure, and is constantly recycled through the lymphatic system. If the body cannot process the fluid, debilitating and possibly life-threatening conditions can occur.
Many functions necessary for the body's cells to survive and work together rely on tissue fluid. The first is support. Cells suspended in fluid are protected from damage caused by the vibrations of an animal's movement. More importantly, it acts as a medium for cells to send chemical messages to one another. A related function is that the fluid acts as a waste disposal system for every cell it surrounds.
Tissue fluid is created through osmotic pressure in the body's capillaries. In osmosis, water will naturally flow from an area of low solute concentration to high solute concentration if a permeable membrane separates the two areas. Blood capillaries are a perfect membrane for interstitial fluid. A constantly flowing bloodstream along with a high concentration of solutes in the blood ensures that osmotic pressure is maintained between blood and the tissue fluid just outside the capillary.
Just like the blood it comes from, tissue fluid is circulated throughout the body. The lymphatic system, a main component of the body's immune system, circulates tissue fluid back into the bloodstream where the cycle begins again later on. This system relies on an animal maintaining proper hydration.
Though caused by a parasite, elephantiasis is an excellent example of what occurs when the lymphatic system can not constantly recycle interstitial fluid. Elephantiasis presents as extreme swelling of the legs and genitals. Only these areas are affected as gravity pulls fluid to the lowest point. The condition is treatable through antibiotics. Though swelling reduces after treatment, long-term cosmetic and functional side effects are possible depending on how long a patient waits before treatment.