Body fluid that fills the spaces between the cells.
The aqueous substance found in the small spaces between cells in the body is referred to as tissue fluid. It is a part of the extracellular fluid, which consists of all bodily fluids located outside of cells, such as blood and lymph.
Oxygen and nutrients need to move from the blood vessels into the tissue fluid to be accessible to cells. Conversely, carbon dioxide and other waste materials travel from the cells to the tissue fluid, and from there, they enter the bloodstream.
Besides nutrients and waste products, tissue fluid also has ions in it. This fluid has a higher concentration of sodium ions and a lower concentration of potassium ions compared to the fluid inside cells. These differing ion levels facilitate the osmotic movement of water into and out of cells. They also contribute to the conduction of electrical signals through nerves and muscles.
Tissue fluid is created when liquid filters out through the walls of the initial sections of blood capillaries, closest to an arteriole, pushed by high blood pressure. In the capillary sections nearest to a venule, the blood pressure is significantly lower, allowing the tissue fluid to re-enter the capillaries. Some of this fluid also gets drained into the lymphatic vessels. This creates a continuous cycle, maintaining a stable amount of tissue fluid. Certain conditions, like congestive heart failure, can disturb this equilibrium between the creation and drainage of tissue fluid, resulting in an abnormal buildup of fluid in the tissues, a condition known as edema.